Nuclear physics composition of the nucleus. Abstract of a physics lesson "The composition of the atomic nucleus. Nuclear forces. The binding energy of atomic nuclei." The connection between mass and energy

Composition and characteristics of the atomic nucleus.

The nucleus of the simplest atom - the hydrogen atom - consists of one elementary particle called a proton. The nuclei of all other atoms consist of two types of elementary particles - protons and neutrons. These particles are called nucleons.

Proton ... Proton (p) has charge + e and mass

m p = 938.28 MeV

For comparison, let us point out that the electron mass is

m e = 0.511 MeV

It follows from the comparison that m p = 1836m e

The proton has a spin equal to half (s =), and its own magnetic moment

A unit of magnetic moment called a nuclear magneton. From a comparison of the masses of the proton and the electron, it follows that μ I is 1836 times smaller than the Bohr magneton μ b. Consequently, the intrinsic magnetic moment of the proton is about 660 times less than the magnetic moment of the electron.

Neutron ... Neutron (n) was discovered in 1932 by an English physicist

D. Chadwick. The electric charge of this particle is zero, and the mass

m n = 939.57 MeV

very close to the mass of a proton. The difference between the masses of a neutron and a proton (m n –m p)

is 1.3 MeV, i.e. 2.5 m e.

The neutron has a spin equal to half (s =) and (despite the absence of an electric charge) its own magnetic moment

μ n = - 1.91 μ i

(the minus sign indicates that the directions of the intrinsic mechanical and magnetic moments are opposite). Explanation of this surprising fact will be given later.

Note that the ratio of the experimental values μ p and μ n with a high degree of accuracy is equal to - 3/2. This was noticed only after such a value was obtained theoretically.

In a free state, a neutron is unstable (radioactive) - it spontaneously decays, turning into a proton and emitting an electron (e -) and another particle called an antineutrino
... The half-life (that is, the time it takes for half of the original number of neutrons to decay) is approximately 12 minutes. The decay scheme can be written as follows:

The rest mass of the antineutrino is zero. The mass of the neutron is more than the mass of the proton by 2.5m e. Consequently, the neutron mass exceeds the total mass of the particles appearing in the right-hand side of the equation by 1.5m e, i.e. by 0.77 MeV. This energy is released during the decay of a neutron in the form of the kinetic energy of the resulting particles.

Characteristics of the atomic nucleus ... One of the most important characteristics of an atomic nucleus is the charge number Z. It is equal to the number of protons that make up the nucleus, and determines its charge, which is equal to + Z e. Number Z defines the ordinal number of a chemical element in the periodic table. Therefore, it is also called the atomic number of the nucleus.

The number of nucleons (i.e. the total number of protons and neutrons) in the nucleus is denoted by the letter A and is called the mass number of the nucleus. The number of neutrons in the nucleus is equal to N = A – Z.

The symbol is used to denote nuclei

where X is the chemical symbol for the element. At the top left is the mass number, at the bottom left is the atomic number (the last sign is often omitted). Sometimes the mass number is written not to the left, but to the right of the symbol of a chemical element.

Kernels with the same Z, but different A are called isotopes... Majority chemical elements has several stable isotopes. So, for example, oxygen has three stable isotopes:

, tin has ten, etc.

Hydrogen has three isotopes:

- ordinary hydrogen, or protium (Z = 1, N = 0),

- heavy hydrogen, or deuterium (Z = 1, N = 1),

- tritium (Z = 1, N = 2).

Protium and deuterium are stable, tritium is radioactive.

Nuclei with the same mass number A are called isobars... An example is
and
... Nuclei with the same number of neutrons N = A - Z are called isotones (
,
Finally, there are radioactive nuclei with the same Z and A, differing in half-life. They're called isomers... For example, there are two isomers of the nucleus
, for one of them the half-decay period is 18 minutes, for the other - 4.4 hours.

About 1500 nuclei are known, differing either Z or A, or both. About 1/5 of these nuclei are stable, the rest are radioactive. Many nuclei were produced artificially using nuclear reactions.

In nature, there are elements with an atomic number Z from 1 to 92, excluding technetium (Tc, Z = 43) and promethium (Pm, Z = 61). Plutonium (Pu, Z = 94), after receiving it artificially, was found in trace amounts in a natural mineral - resin blende. The rest of the transuranic (i.e., sauranium) elements (cZ from 93 to 107) were obtained artificially by means of various nuclear reactions.

The transuranic elements curium (96 Cm), einsteinium (99 Es), fermium (100 Fm) and mendelevium (101 Md) are named after outstanding scientists. II. and M. Curie, A. Einstein, Z. Fermi and D.I. Mendeleev. Lawrence (103 Lw) is named after the inventor of the cyclotron, E. Lawrence. Kurchatoviy (104 Ku) got its name in honor of the outstanding physicist I.V. Kurchatov.

Some transuranium elements, including curchatovium and elements numbered 106 and 107, were obtained at the Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Dubna by a scientist

N.N. Flerov and his staff.

Core sizes ... In the first approximation, the nucleus can be considered a ball, the radius of which is quite accurately determined by the formula

(Fermi is the name of the unit of length used in nuclear physics equal to

10-13 cm). It follows from the formula that the volume of the nucleus is proportional to the number of nucleons in the nucleus. Thus, the density of matter in all nuclei is approximately the same.

Spin the core ... The spins of the nucleons add up to the resulting spin of the nucleus. The nucleon spin is 1/2. Therefore, the quantum number of the spin of the nucleus will be half-integer for an odd number of nucleons A and integer or zero for even A. The spins of the nuclei J do not exceed several units. This indicates that the spins of the majority of nucleons in the nucleus mutually cancel each other out, being located antiparallel. All even-even nuclei (i.e. a nucleus with an even number of protons and an even number of neutrons) have zero spin.

The mechanical moment of the nucleus M J is added with the moment of the electron shell
at the total angular momentum of the atom M F, which is determined by the quantum number F.

The interaction of the magnetic moments of the electrons and the nucleus leads to the fact that the states of the atom corresponding to different mutual orientations M J and
(i.e. different F) have slightly different energies. The interaction of the moments μ L and μ S is responsible for the fine structure of the spectra. Interaction μ J and the hyperfine structure of atomic spectra is determined. The splitting of spectral lines corresponding to the hyperfine structure is so small (on the order of a few hundredths of an angstrom) that it can be observed only with instruments of the highest resolving power.

An atom consists of a positively charged nucleus and electrons surrounding it. Atomic nuclei are approximately 10 -14 ... 10 -15 m in size (the linear dimensions of an atom are 10 -10 m).

The atomic nucleus consists of elementary particles  protons and neutrons. The proton-neutron model of the nucleus was proposed by the Russian physicist D. D. Ivanenko, and later developed by V. Heisenberg.

Proton ( R) has a positive charge equal to the electron charge and rest mass T p = 1.6726 ∙ 10 -27 kg 1836 m e, where m e electron mass. Neutron ( n) Is a neutral particle with rest mass m n= 1.6749 ∙ 10 -27 kg 1839 T e ,. The masses of protons and neutrons are often expressed in other units - in atomic mass units (amu, a unit of mass equal to 1/12 of the mass of a carbon atom

). The masses of the proton and neutron are approximately equal to one atomic mass unit. Protons and neutrons are called nucleons(from lat. nucleus kernel). The total number of nucleons in an atomic nucleus is called the mass number A).

The radii of the nuclei increase with an increase in the mass number in accordance with the ratio R = 1,4A 1/3 10 -13 cm.

Experiments show that nuclei do not have sharp boundaries. There is a certain density of nuclear matter in the center of the nucleus, and it gradually decreases to zero with increasing distance from the center. Due to the absence of a well-defined core boundary, its "radius" is defined as the distance from the center at which the density of nuclear matter is halved. The average distribution of matter density for most nuclei turns out to be not just spherical. Most of the nuclei are deformed. The nuclei are often elongated or flattened ellipsoids.

The atomic nucleus is characterized by chargeZe, where Zcharge number nucleus, equal to the number of protons in the nucleus and coinciding with the ordinal number of a chemical element in Mendeleev's Periodic Table of Elements.

The nucleus is denoted by the same symbol as the neutral atom:

, where Xsymbol of a chemical element, Z Atomic number (number of protons in the nucleus), Amass number (number of nucleons in the nucleus). Mass number A approximately equal to the mass of the nucleus in atomic mass units.

Since the atom is neutral, the charge of the nucleus Z also determines the number of electrons in an atom. Their distribution over states in the atom depends on the number of electrons. The nuclear charge determines the specificity of a given chemical element, that is, it determines the number of electrons in an atom, the configuration of their electron shells, and the magnitude and nature of the intra-atomic electric field.

Nuclei with the same charge numbers Z but with different mass numbers A(i.e., with different numbers of neutrons N = A - Z) are called isotopes, and nuclei with the same A, but different Z - isobars. For example, hydrogen ( Z= l) has three isotopes: H - protium ( Z= l, N = 0), H - deuterium ( Z= l, N= 1), H - tritium ( Z= l, N= 2), tin - ten isotopes, etc. In the overwhelming majority of cases, isotopes of the same chemical element have the same chemical and almost identical physical properties.

E, MeV

Energy levels

and the observed transitions for the nucleus of the boron atom

Quantum theory strictly limits the energies that can be possessed by the constituent parts of nuclei. The aggregates of protons and neutrons in nuclei can only be in certain discrete energy states characteristic of a given isotope.

When an electron goes from a higher to a lower energy state, the energy difference is emitted as a photon. The energy of these photons is of the order of several electron volts. For nuclei, the level energies are in the range from about 1 to 10 MeV. During the transitions between these levels, photons of very high energies (γ-quanta) are emitted. To illustrate such transitions, Fig. 6.1 shows the first five energy levels of the nucleus

The vertical lines indicate the observed transitions. For example, a γ-quantum with an energy of 1.43 MeV is emitted during the transition of a nucleus from a state with an energy of 3.58 MeV to a state with an energy of 2.15 MeV.

An atom is the smallest particle of a chemical element that retains all of it Chemical properties... An atom consists of a nucleus, which has a positive electrical charge, and negatively charged electrons. The charge of the nucleus of any chemical element is equal to the product of Z by e, where Z is the ordinal number of the given element in the periodic table of chemical elements, and e is the value of the elementary electric charge.

Electron is the smallest particle of matter with a negative electric charge e = 1.6 · 10 -19 coulomb, taken as an elementary electric charge. The electrons, rotating around the nucleus, are located on the electron shells K, L, M, etc. K is the shell closest to the nucleus. The size of an atom is determined by the size of its electron shell. An atom can lose electrons and become a positive ion, or attach electrons and become a negative ion. The charge of an ion determines the number of lost or attached electrons. The process of converting a neutral atom into a charged ion is called ionization.

Atomic nucleus(the central part of the atom) consists of elementary nuclear particles - protons and neutrons. The radius of the nucleus is about a hundred thousand times smaller than the radius of the atom. The density of the atomic nucleus is extremely high. Protons- These are stable elementary particles with a single positive electric charge and a mass 1836 times greater than the mass of an electron. The proton is the nucleus of the lightest element, hydrogen. The number of protons in the nucleus is Z. Neutron is a neutral (not having an electric charge) elementary particle with a mass very close to the mass of a proton. Since the mass of the nucleus is the sum of the mass of protons and neutrons, the number of neutrons in the nucleus of an atom is equal to A - Z, where A is the mass number of a given isotope (see). The proton and neutron that make up the nucleus are called nucleons. In the nucleus, nucleons are bound by special nuclear forces.

The atomic nucleus contains a huge amount of energy that is released during nuclear reactions. Nuclear reactions occur when atomic nuclei interact with elementary particles or with the nuclei of other elements. As a result of nuclear reactions, new nuclei are formed. For example, a neutron can transform into a proton. In this case, a beta particle, i.e., an electron, is ejected from the nucleus.

The transition in the nucleus of a proton to a neutron can be carried out in two ways: either a particle with a mass equal to the mass of an electron, but with a positive charge, called a positron (positron decay), is emitted from the nucleus, or the nucleus captures one of the electrons from the nearest K-shell (K - capture).

Sometimes the formed nucleus has an excess of energy (it is in an excited state) and, passing into a normal state, releases excess energy in the form of electromagnetic radiation with a very short wavelength -. The energy released during nuclear reactions is practically used in various industries.

An atom (Greek atomos - indivisible) is the smallest particle of a chemical element that has its chemical properties. Each element is made up of atoms of a certain kind. The composition of the atom includes a nucleus carrying a positive electric charge, and negatively charged electrons (see), which form its electron shells. The magnitude of the electric charge of the nucleus is Ze, where e is an elementary electric charge equal in magnitude to the charge of an electron (4.8 · 10 -10 el. Units), and Z is the atomic number of a given element in the periodic system of chemical elements (see .). Since an unionized atom is neutral, the number of electrons included in it is also equal to Z. The composition of the nucleus (see Nucleus atomic) includes nucleons, elementary particles with a mass approximately 1840 times greater than the mass of an electron (equal to 9.1 10 - 28 g), protons (see), positively charged, and neutrons having no charge (see). The number of nucleons in the nucleus is called the mass number and is denoted by the letter A. The number of protons in the nucleus, equal to Z, determines the number of electrons entering the atom, the structure of the electron shells and the chemical properties of the atom. The number of neutrons in the nucleus is equal to A-Z. Isotopes are varieties of the same element, the atoms of which differ from each other in mass number A, but have the same Z. Thus, in the nuclei of atoms of different isotopes of one element, there are different numbers of neutrons with the same number of protons. When designating isotopes, the mass number A is written above the element symbol, and the atomic number is below; for example, oxygen isotopes are designated:

The dimensions of an atom are determined by the size of the electron shells and for all Z are of the order of 10 -8 cm. Since the mass of all electrons of an atom is several thousand times less than the mass of the nucleus, the mass of an atom is proportional to the mass number. The relative mass of an atom of a given isotope is determined in relation to the mass of an atom of the carbon isotope C 12, taken as 12 units, and is called the isotopic mass. It turns out to be close to the mass number of the corresponding isotope. The relative weight of an atom of a chemical element is the average (taking into account the relative abundance of isotopes of a given element) value of the isotopic weight and is called the atomic weight (mass).

An atom is a microscopic system, and its structure and properties can be explained only with the help of quantum theory, created mainly in the 20s of the 20th century and intended to describe phenomena of an atomic scale. Experiments have shown that microparticles - electrons, protons, atoms, etc., apart from corpuscular ones, have wave properties that manifest themselves in diffraction and interference. In quantum theory, to describe the state of micro-objects, a certain wave field is used, characterized by a wave function (Ψ-function). This function determines the probabilities of possible states of a micro-object, that is, it characterizes the potential for the manifestation of one or another of its properties. The law of variation of the function Ψ in space and time (the Schrödinger equation), which makes it possible to find this function, plays the same role in quantum theory as in classical mechanics Newton's laws of motion. The solution of the Schrödinger equation in many cases leads to discrete possible states of the system. So, for example, in the case of an atom, a number of wave functions for electrons are obtained, corresponding to different (quantized) values of energy. The system of energy levels of the atom, calculated by the methods of quantum theory, has received brilliant confirmation in spectroscopy. The transition of an atom from the ground state corresponding to the lowest energy level E 0 to any of the excited states E i occurs when a certain portion of the energy E i - E 0 is absorbed. An excited atom passes into a less excited or ground state, usually with the emission of a photon. In this case, the photon energy hv is equal to the difference between the energies of the atom in two states: hv = E i - E k where h is Planck's constant (6.62 · 10 -27 erg · sec), v is the frequency of light.

In addition to atomic spectra, quantum theory has made it possible to explain other properties of atoms. In particular, the valence, the nature of the chemical bond and the structure of molecules were explained, the theory of the periodic table of elements was created.

An atomic nucleus, considered as a class of particles with a certain number of protons and neutrons, is usually called nuclide.
In some rare cases, short-lived exotic atoms can be formed, in which other particles serve as nuclei instead of a nucleon.

The number of protons in a nucleus is called its charge number Z (\ displaystyle Z) - this number is equal to the ordinal number of the element to which the atom belongs in the periodic table (Periodic Table of Elements) of Mendeleev. The number of protons in the nucleus determines the structure of the electron shell of a neutral atom and, thus, the chemical properties of the corresponding element. The number of neutrons in the nucleus is called it isotopic number N (\ displaystyle N). Nuclei with the same number of protons and different numbers of neutrons are called isotopes. Nuclei with the same number of neutrons but different numbers of protons are called isotones. The terms isotope and isotone are also used in relation to atoms containing the indicated nuclei, as well as to characterize non-chemical species of one chemical element. The total number of nucleons in a nucleus is called its mass number A (\ displaystyle A) ( A = N + Z (\ displaystyle A = N + Z)) and is approximately equal to the average mass of an atom specified in the periodic table. Nuclides with the same mass number, but different proton-neutron composition are usually called isobars.

Like any quantum system, nuclei can be in a metastable excited state, and in some cases, the lifetime of such a state is calculated in years. Such excited states of nuclei are called nuclear isomers.

Collegiate YouTube

The structure of the atomic nucleus. Nuclear forces

Nuclear forces Binding energy of particles in the nucleus Fission of uranium nuclei Chain reaction

The structure of the atomic nucleus Nuclear forces

Chemistry. Atomic structure: Atomic nucleus. Foxford Online Learning Center

Nuclear reactions

Subtitles

Story

The scattering of charged particles can be explained if we assume an atom that consists of a central electric charge concentrated at a point and surrounded by a uniform spherical distribution of opposite electricity of equal magnitude. With this arrangement of the atom, α- and β-particles, when they pass at a close distance from the center of the atom, experience large deflections, although the probability of such a deflection is small.

Thus, Rutherford discovered the atomic nucleus, and from this moment nuclear physics began, which studies the structure and properties of atomic nuclei.

After the discovery of stable isotopes of elements, the nucleus of the lightest atom was assigned the role of the structural particle of all nuclei. Since 1920, the nucleus of the hydrogen atom has been officially called the proton. After the intermediate proton-electron theory of the structure of the nucleus, which had many obvious shortcomings, first of all, it contradicted the experimental results of measurements of spins and magnetic moments of nuclei, in 1932, James Chadwick discovered a new electrically neutral particle called the neutron. In the same year, Ivanenko and, independently, Heisenberg put forward a hypothesis about the proton-neutron structure of the nucleus. Later, with the development of nuclear physics and its applications, this hypothesis was fully confirmed.

Theories of the structure of the atomic nucleus

In the course of the development of physics, various hypotheses of the structure of the atomic nucleus were put forward; however, each of them is capable of describing only a limited set of nuclear properties. Some models may be mutually exclusive.

The most famous are the following:

Drip model of the nucleus - proposed in 1936 by Niels Bohr.
Shell model of the nucleus - proposed in the 30s of the XX century.
Generalized Bohr - Mottelson model
Cluster core model
Nucleon Association Model
Superfluid core model
Statistical kernel model

Nuclear physical characteristics

For the first time the charges of atomic nuclei were determined by Henry Moseley in 1913. The scientist interpreted his experimental observations as the dependence of the X-ray wavelength on some constant Z (\ displaystyle Z), which changes by one from element to element and is equal to one for hydrogen:

1 / λ = a Z - b (\ displaystyle (\ sqrt (1 / \ lambda)) = aZ-b), where

A (\ displaystyle a) and b (\ displaystyle b) are constants.

From which Moseley concluded that the atomic constant found in his experiments, which determines the wavelength of characteristic X-ray radiation and coincides with the ordinal number of the element, can only be the charge of the atomic nucleus, which became known as Moseley's law .

Weight

Due to the difference in the number of neutrons A - Z (\ displaystyle A-Z) isotopes of an element have different masses M (A, Z) (\ displaystyle M (A, Z)), which is an important characteristic of the kernel. In nuclear physics, the mass of nuclei is usually measured in atomic mass units ( a. eat.), in one a. e. m. take 1/12 of the mass of the 12 C nuclide. It should be noted that the standard mass that is usually given for a nuclide is the mass of a neutral atom. To determine the mass of the nucleus, you need to subtract the sum of the masses of all electrons from the mass of the atom (a more accurate value will be obtained if we also take into account the binding energy of electrons with the nucleus).

In addition, the energy equivalent of mass is often used in nuclear physics. According to Einstein's ratio, each mass M (\ displaystyle M) has a total energy:

E = M c 2 (\ displaystyle E = Mc ^ (2)) where c (\ displaystyle c) is the speed of light in a vacuum.

The relationship between a. e. m. and its energy equivalent in joules:

E 1 = 1.660539 ⋅ 10 - 27 ⋅ (2.997925 ⋅ 10 8) 2 = 1.492418 ⋅ 10 - 10 (\ displaystyle E_ (1) = 1.660539 \ cdot 10 ^ (- 27) \ cdot ( 2.997925 \ cdot 10 ^ (8)) ^ (2) = 1.492418 \ cdot 10 ^ (- 10)), E 1 = 931.494 (\ displaystyle E_ (1) = 931.494).

Radius

Analysis of the decay of heavy nuclei refined Rutherford's estimate and related the radius of the nucleus to the mass number by a simple relationship:

R = r 0 A 1/3 (\ displaystyle R = r_ (0) A ^ (1/3)),

where is a constant.

Since the radius of the nucleus is not a purely geometric characteristic and is primarily related to the radius of action of nuclear forces, the value of r 0 (\ displaystyle r_ (0)) depends on the process, in the analysis of which the value of R (\ displaystyle R) is obtained, the average value r 0 = 1.23 ⋅ 10 - 15 (\ displaystyle r_ (0) = 1.23 \ cdot 10 ^ (- 15)) m, thus the radius of the core in meters:

R = 1.23 ⋅ 10 - 15 A 1/3 (\ displaystyle R = 1.23 \ cdot 10 ^ (- 15) A ^ (1/3)).

Core moments

Like its constituent nucleons, the nucleus has its own moments.

Spin

Since nucleons have their own mechanical moment, or spin, equal to 1/2 (\ displaystyle 1/2), then the nuclei must also have mechanical moments. In addition, nucleons participate in the nucleus in orbital motion, which is also characterized by a certain angular momentum of each nucleon. The orbital moments are only integer values ℏ (\ displaystyle \ hbar) (Dirac's constant). All mechanical moments of nucleons, both spins and orbital, are summed up algebraically and constitute the spin of the nucleus.

Despite the fact that the number of nucleons in a nucleus can be very large, the spins of nuclei are usually small and amount to no more than a few ℏ (\ displaystyle \ hbar), which is explained by the peculiarity of the interaction of nucleons of the same name. All paired protons and neutrons interact only in such a way that their spins are mutually compensated, that is, pairs always interact with antiparallel spins. The total orbital angular momentum of the pair is also always zero. As a result, the nuclei consisting of even number protons and an even number of neutrons do not have a mechanical moment. Nonzero spins exist only for nuclei containing unpaired nucleons, the spin of such a nucleon is summed up with its orbital momentum and has some half-integer value: 1/2, 3/2, 5/2. The odd-odd nuclei have integer spins: 1, 2, 3, etc.

Magnetic moment

Spin measurements became possible due to the presence of directly related magnetic moments. They are measured in magnetons and for different nuclei are equal from −2 to +5 nuclear magnetons. Due to the relatively large mass of nucleons, the magnetic moments of nuclei are very small compared to the magnetic moments of electrons; therefore, their measurement is much more difficult. Like spins, magnetic moments are measured spectroscopically, with nuclear magnetic resonance being the most accurate.

The magnetic moment of even-even pairs, like the spin, is zero. The magnetic moments of nuclei with unpaired nucleons are formed by the intrinsic moments of these nucleons and the moment associated with the orbital motion of the unpaired proton.

Electric quadrupole moment

Atomic nuclei, whose spin is greater than or equal to one, have nonzero quadrupole moments, which indicates that they are not exactly spherical in shape. The quadrupole moment has a plus sign if the nucleus is elongated along the spin axis (fusiform body), and a minus sign if the nucleus is stretched in a plane perpendicular to the spin axis (lenticular body). Nuclei with positive and negative quadrupole moments are known. The absence of spherical symmetry in the electric field created by a nucleus with a nonzero quadrupole moment leads to the formation of additional energy levels of atomic electrons and the appearance of hyperfine structure lines in the spectra of atoms, the distances between which depend on the quadrupole moment.

Communication energy

Stability of nuclei

From the fact of a decrease in the average binding energy for nuclides with mass numbers greater or less than 50-60, it follows that for nuclei with small A (\ displaystyle A), the fusion process is energetically favorable - thermonuclear fusion, leading to an increase in the mass number, and for nuclei with large A (\ displaystyle A) is a fission process. At present, both of these processes leading to the release of energy have been carried out, the latter being the basis of modern nuclear power, and the former being under development.

Detailed studies have shown that the stability of nuclei also significantly depends on the parameter N / Z (\ displaystyle N / Z)- the ratio of the numbers of neutrons and protons. Average for the most stable cores N / Z ≈ 1 + 0.015 A 2/3 (\ displaystyle N / Z \ approx 1 + 0.015A ^ (2/3)), therefore, nuclei of light nuclides are most stable at N ≈ Z (\ displaystyle N \ approx Z), and with an increase in the mass number, the electrostatic repulsion between the protons becomes more and more noticeable, and the stability region shifts towards N> Z (\ displaystyle N> Z)(see explanatory figure).

If you look at the table of stable nuclides found in nature, you can notice their distribution over the even and odd values of Z (\ displaystyle Z) and N (\ displaystyle N). All nuclei with odd values of these quantities are nuclei of light nuclides 1 2 H (\ displaystyle () _ (1) ^ (2) (\ textrm (H))), 3 6 Li (\ displaystyle () _ (3) ^ (6) (\ textrm (Li))), 5 10 B (\ displaystyle () _ (5) ^ (10) (\ textrm (B))), 7 14 N (\ displaystyle () _ (7) ^ (14) (\ textrm (N)))... Among isobars with odd A, as a rule, only one is stable. In the case of even A (\ displaystyle A), there are often two, three or more stable isobars, therefore, even-even are the most stable, and odd-odd are the least stable. This phenomenon suggests that both neutrons and protons tend to bunch in pairs with antiparallel spins, thus breaking the smoothness of the above dependence of the binding energy on A (\ displaystyle A).

Thus, the parity of the number of protons or neutrons creates a certain margin of stability, which leads to the possibility of the existence of several stable nuclides, differing, respectively, in the number of neutrons for isotopes and in the number of protons for isotones. Also, the parity of the number of neutrons in the composition of heavy nuclei determines their ability to fission under the influence of neutrons.

Nuclear forces

Nuclear forces are forces that hold nucleons in the nucleus, which are large forces of attraction that act only at short distances. They possess saturation properties, in connection with which the exchange character (with the help of pi-mesons) is attributed to the nuclear forces. Nuclear forces depend on spin, do not depend on electric charge, and are not central forces.

Kernel levels

Unlike free particles, for which the energy can take any values (the so-called continuous spectrum), bound particles (that is, particles whose kinetic energy is less than the absolute potential value), according to quantum mechanics, can be in states only with certain discrete energies , the so-called discrete spectrum. Since the nucleus is a system of bound nucleons, it has a discrete energy spectrum. It is usually in its lowest energy state, called the main... If you transfer energy to the core, it will go into agitated state.

The location of the energy levels of the nucleus in the first approximation:

D = a e - b E ∗ (\ displaystyle D = ae ^ (- b (\ sqrt (E ^ (*))))), where:

D (\ displaystyle D) is the average distance between levels,

Composition and characteristics of the atomic nucleus.

Proton ... Proton (p) has charge + e and mass

m p = 938.28 MeV

For comparison, let us point out that the electron mass is

m e = 0.511 MeV

It follows from the comparison that m p = 1836m e

The proton has a spin equal to half (s =), and its own magnetic moment

Neutron ... Neutron (n) was discovered in 1932 by an English physicist

D. Chadwick. The electric charge of this particle is zero, and the mass

m n = 939.57 MeV

very close to the mass of a proton. The difference between the masses of a neutron and a proton (m n –m p)

is 1.3 MeV, i.e. 2.5 m e.

The neutron has a spin equal to half (s =) and (despite the absence of an electric charge) its own magnetic moment

μ n = - 1.91 μ i

(the minus sign indicates that the directions of the intrinsic mechanical and magnetic moments are opposite). An explanation of this amazing fact will be given later.

Note that the ratio of the experimental values μ p and μ n with a high degree of accuracy is equal to - 3/2. This was noticed only after such a value was obtained theoretically.

In a free state, a neutron is unstable (radioactive) - it spontaneously decays, turning into a proton and emitting an electron (e -) and another particle called an antineutrino

... The half-life (that is, the time it takes for half of the original number of neutrons to decay) is approximately 12 minutes. The decay scheme can be written as follows:

The symbol is used to denote nuclei

Kernels with the same Z, but different A are called isotopes... Most chemical elements have several stable isotopes. So, for example, oxygen has three stable isotopes:

, tin has ten, etc.

Hydrogen has three isotopes:

- ordinary hydrogen, or protium (Z = 1, N = 0),

- heavy hydrogen, or deuterium (Z = 1, N = 1),

- tritium (Z = 1, N = 2).

Protium and deuterium are stable, tritium is radioactive.

Nuclei with the same mass number A are called isobars... An example is

and

... Nuclei with the same number of neutrons N = A - Z are called isotones (

,

Finally, there are radioactive nuclei with the same Z and A, differing in half-life. They're called isomers... For example, there are two isomers of the nucleus

, for one of them the half-decay period is 18 minutes, for the other - 4.4 hours.

About 1500 nuclei are known, differing either Z or A, or both. About 1/5 of these nuclei are stable, the rest are radioactive. Many nuclei were produced artificially using nuclear reactions.

N.N. Flerov and his staff.

Core sizes ... In the first approximation, the nucleus can be considered a ball, the radius of which is quite accurately determined by the formula

(Fermi is the name of the unit of length used in nuclear physics equal to

10-13 cm). It follows from the formula that the volume of the nucleus is proportional to the number of nucleons in the nucleus. Thus, the density of matter in all nuclei is approximately the same.

The mechanical moment of the nucleus M J is added with the moment of the electron shell

at the total angular momentum of the atom M F, which is determined by the quantum number F.

The interaction of the magnetic moments of the electrons and the nucleus leads to the fact that the states of the atom corresponding to different mutual orientations M J and

(i.e. different F) have slightly different energies. The interaction of the moments μ L and μ S is responsible for the fine structure of the spectra. Interaction μ J and the hyperfine structure of atomic spectra is determined. The splitting of spectral lines corresponding to the hyperfine structure is so small (on the order of a few hundredths of an angstrom) that it can be observed only with instruments of the highest resolving power.

A feature of radioactive contamination, in contrast to contamination by other pollutants, is that the harmful effect on humans and environmental objects is not caused by the radionuclide (pollutant) itself, but by the radiation it is the source of.

However, there are times when a radionuclide is a toxic element. For example, after the accident at the Chernobyl nuclear power plant in environment plutonium 239, 242 Ru was ejected with particles of nuclear fuel. In addition to the fact that plutonium is an alpha emitter and, when ingested, is a significant hazard, plutonium itself is a toxic element.

For this reason, two groups of quantitative indicators are used: 1) to assess the content of radionuclides and 2) to assess the impact of radiation on an object.
Activity- quantitative measure of the content of radionuclides in the analyzed object. Activity is determined by the number of radioactive decays of atoms per unit of time. The SI unit of activity measurement is Becquerel (Bq) equal to one decay per second (1Bq = 1 dec / s). Sometimes a non-systemic unit of activity measurement is used - Curie (Ki); 1Ci = 3.7 × 1010 Bq.

Radiation dose- a quantitative measure of the effect of radiation on an object.
Due to the fact that the effect of radiation on an object can be estimated at different levels: physical, chemical, biological; at the level of individual molecules, cells, tissues or organisms, etc., several types of doses are used: absorbed, effective equivalent, exposure.

To assess the change in radiation dose over time, the "dose rate" indicator is used. Dose rate is the ratio of dose to time. For example, the dose rate of external exposure from natural sources of radiation is 4-20 μR / h on the territory of Russia.

The main standard for humans - the main dose limit (1 mSv / year) - is introduced in units of the effective equivalent dose. There are standards in units of activity, levels of land contamination, VDU, GWP, SanPiN, etc.

The structure of the atomic nucleus.

An atom is the smallest particle of a chemical element that retains all of its properties. By its structure, the atom is a complex system consisting of a very small positively charged nucleus (10 -13 cm) located in the center of the atom and negatively charged electrons rotating around the nucleus in different orbits. The negative charge of electrons is equal to the positive charge of the nucleus, while in general it turns out to be electrically neutral.

Atomic nuclei are composed of nucleons - nuclear protons ( Z - number of protons) and nuclear neutrons (N is the number of neutrons). "Nuclear" protons and neutrons differ from particles in a free state. For example, a free neutron, unlike one bound in a nucleus, is unstable and turns into a proton and an electron.

The number of nucleons Am (mass number) is the sum of the numbers of protons and neutrons: Am = Z + N.

Proton - elementary particle of any atom, it has a positive charge equal to the charge of an electron. The number of electrons in the shell of an atom is determined by the number of protons in the nucleus.

Neutron - another kind of nuclear particles of all elements. It is absent only in the nucleus of light hydrogen, which consists of one proton. It has no charge and is electrically neutral. In an atomic nucleus, neutrons are stable, and in a free state, they are unstable. The number of neutrons in the nuclei of atoms of the same element can fluctuate, therefore the number of neutrons in the nucleus does not characterize the element.

Nucleons (protons + neutrons) are held inside the atomic nucleus by nuclear forces of attraction. Nuclear forces are 100 times stronger than electromagnetic forces and therefore keep like-charged protons inside the nucleus. Nuclear forces manifest themselves only at very small distances (10 -13 cm), they constitute the potential binding energy of the nucleus, which is partially released during some transformations, transforms into kinetic energy.

For atoms differing in the composition of the nucleus, the name "nuclides" is used, and for radioactive atoms - "radionuclides".

Nuclides call atoms or nuclei with a given number of nucleons and a given nuclear charge (the designation of the nuclide A X).

Nuclides that have the same number of nucleons (Am = const) are called isobars. For example, the nuclides 96 Sr, 96 Y, 96 Zr belong to a series of isobars with the number of nucleons Am = 96.

Nuclides with the same number of protons (Z = const) are called isotopes. They differ only in the number of neutrons, therefore they belong to the same element: 234 U , 235 U, 236 U , 238 U .

Isotopes- nuclides with the same number of neutrons (N = Am -Z = const). Nuclides: 36 S, 37 Cl, 38 Ar, 39 K, 40 Ca belong to a series of isotopes with 20 neutrons.

Isotopes are usually designated as Z X M, where X is a symbol of a chemical element; M - mass number, equal to the sum the number of protons and neutrons in the nucleus; Z is the atomic number or charge of the nucleus, equal to the number of protons in the nucleus. Since each chemical element has its own constant atomic number, it is usually omitted and limited to writing only the mass number, for example: 3 H, 14 C, 137 Cs, 90 Sr, etc.

Nuclear atoms that have the same mass numbers, but different charges and, consequently, different properties are called "isobars", so for example one of the phosphorus isotopes has a mass number of 32-15 P 32, the same mass number has one of the sulfur isotopes - 16 S 32.

Nuclides can be stable (if their nuclei are stable and do not decay) and unstable (if their nuclei are unstable and undergo changes that ultimately lead to an increase in the stability of the nucleus). Unstable atomic nuclei capable of spontaneously decaying are called radionuclides. The phenomenon of spontaneous disintegration of an atomic nucleus, accompanied by the emission of particles and (or) electromagnetic radiation, is called radioactivity.

As a result of radioactive decay, both a stable and a radioactive isotope can be formed, which in turn spontaneously decays. Such chains of radioactive elements, connected by a series of nuclear transformations, are called radioactive families.

Currently, IURAC (International Union of Pure and Applied Chemistry) has officially named 109 chemical elements. Of these, only 81 have stable isotopes, the heaviest of which is bismuth (Z= 83). For the remaining 28 elements, only radioactive isotopes are known, and uranium (U ~ 92) is the heaviest element found in nature. The largest of natural nuclides has 238 nucleons. In total, the existence of about 1700 nuclides of these 109 elements has now been proven, and the number of isotopes known for individual elements ranges from 3 (for hydrogen) to 29 (for platinum).

The composition of the atomic nucleus
The total number of nucleons in a given nucleus
called the mass number, denoted
The number of protons in the nucleus is called
charge number, denoted
(it is equal to the number of the chemical element)
The number of neutrons in the nucleus is denoted
The nucleus of an atom is denoted in the same way as
the corresponding chemical element,
putting in front of it at the top - the mass number,
and below is the charge number
207
For example: 235
Pb
82
92
U

Proton-neutron nuclear model
1
1
p
proton
+
Core
Z is the number of protons in the nucleus
N is the number of neutrons in the nucleus
m p mN 1a.m.
me core
neutron 1
0
n
A = Z + N - mass number
A = M (rounded to the nearest whole number)
How many protons and neutrons are in the nucleus of uranium isotopes?
A) 235
92
U
A = 235
B) 238
Z = 92
92
N = A-Z = 235-92 = 143
U
A = 238
Z = 92
N = A-Z = 238-92 = 146

Isotopes

Have the same chemical element
there are atoms with different masses
nuclei.
Nuclei with the same charge, but different masses
called isotopes.
Isotopes (from the Greek words isos - the same and topos
- place) have the same serial number in
periodic table
Isotopes have the same number of protons, but different
the number of neutrons.
Isotopes
physical
3 properties
1 have different
2
For instance:
hydrogen1 has three isotopes
1
1
H
protium
N
N
deuterium
tritium

99,985%
0,015%
Natural isotopic composition of H
10 15 10 16%

17
Since 1906 it is known
35
17
Cl
Cl
37
17
M = 35.457
Cl
92
U
239
92
U
234
92
U
235
92
U
238
92
U
M = 238.0289

What forces ensure the stability of the atomic nucleus?

Answer option: Gravitational forces
The answer is incorrect, since these forces
significantly less electrostatic forces
repulsion between protons.
Modern scientists to explain
core stability use the concept
nuclear forces
Nuclear forces are forces acting
between nucleons in the nucleus and providing
existence of stable nuclei
Nuclear forces are strong
interaction

Properties of nuclear forces

Nuclear forces are gravitational forces as they
keep nucleons inside the nucleus (at a very strong
approaching nucleons, the nuclear forces between them have
the nature of repulsion).
Nuclear forces are not electrical forces, as they
act not only between protons, but also between non
having charges of neutrons, and not gravitational,
which are too small to explain nuclear effects.
Study of the degree of bonding of nucleons in different nuclei
show that nuclear forces have the property
saturation, analogous to the valence of chemical forces.
In accordance with this property of nuclear forces, one
the same nucleon does not interact with all
the rest of the nucleons of the nucleus, but with only a few
neighboring.

Properties of nuclear forces

The most important property of nuclear forces is their charge
independence, that is, the identity of the three types
nuclear interaction: between two protons, between
a proton and a neutron, and between two neutrons.
The area of action of nuclear forces is negligible.
The radius of their action is 10 -13 m. At long distances
there is no nuclear interaction between particles.
Nuclear forces (in the area where they operate) are very
intense. Their intensity is much higher
intensity of electromagnetic forces, since nuclear forces
keep inside the nucleus like charged protons,
repelling each other with huge
electric forces.
With increasing distance, they decrease very quickly.
(at a distance of 1.4 10 15 m, their action can
neglected)

Solving problems
1. How many nucleons do nuclei contain:
6
3
Li
64
29
108
47
Cu
Ag
207
82
Pb
2. Determine the nucleon composition of nuclei:
4
2
He
16
8
O
79
34
Se
3. Name the chemical element in the atomic nucleus
which contains nucleons:
A). 7p + 7n
14
7
N
B). 18p + 22n 40 Ar
18
V). 33p + 42n
75
33
G). 84p + 126n
210
84
As
Po

RADIOACTIVITY

Discovery of X-rays
gave impetus to new
research. Their study led to new discoveries, one
of which came the discovery of radioactivity.
From about the middle of the XIX
began to appear
experimental facts that questioned
ideas about the indivisibility of atoms. The results of these
experiments suggested that atoms have
complex structure and that they include electrically
charged particles.
The most striking
evidence of a difficult
the structure of the atom was
discovery of the phenomenon
radioactivity made
French physicist Henri
Becquerel in 1896.

Scientists have concluded that
radioactivity is
spontaneous process in atoms
radioactive elements. Now these are phenomena
defined as spontaneous transformation
unstable isotope of one chemical element
into an isotope of another element; at the same time
the emission of electrons, protons, neutrons or
helium nuclei (α-particles).

For 10 years of joint
they did a lot of work
a lot to learn
phenomena
radioactivity.
It was selfless
work in the name of science - in
poorly equipped
laboratories and
lack of necessary
funds.
Maria and Pierre Curie in the laboratory

a - rays
- rays
b - rays

a - particle - the nucleus of a helium atom. a- rays
have the least penetrating
ability. A layer of paper about
0.1 mm is no longer transparent for them. Weakly
deflect in a magnetic field.
A particle for each of the two
elementary charges there are two
atomic mass units. Rutherford
proved that with radioactive a - decay
helium is formed.

β - particles are electrons,
moving at speeds very
close to the speed of light. They are strong
deviate in both magnetic and
electric field. β - rays are much
less absorbed during the passage
through the substance. Aluminum plate
completely delays them only when
a few millimeters thick.

- rays are
electromagnetic waves. According to their
properties very much resemble
x-ray, but only their penetrating
ability is much greater than that of
x-rays. Do not deviate
magnetic field. Have the greatest
penetrating ability. Lead layer
1 cm thick is not for them
insurmountable obstacle. When passing
- rays through such a layer of lead their
the intensity is only halved.

Emitting α - and b - radiation, atoms
the radioactive element changes,
turning into atoms of a new element.
In this sense, the emission of radioactive
radiation is called radioactive decay.
Rules indicating offset
element in the periodic table caused by
decay are called displacement rules.

a - decay
-decay
b -decay

a - decay is called
spontaneous decay of an atomic nucleus into
a - a particle (the nucleus of an atom of helium 24 He) and a nucleus product. The product of a - decay turns out to be
shifted two cells to the beginning
Periodic Table of Mendeleev.
M
Z
X
M 4
Z 2
Y He
4
2

b - decay is called
spontaneous transformation of an atomic
nucleus by emitting an electron. Core -
the beta decay product turns out to be the nucleus
one of the isotopes of an element with ordinal
number in the periodic table per unit
greater than the ordinal number of the original
kernels.
M
Z
X Y e
M
Z 1
0
1

- radiation is not accompanied
charge change; the mass of the nucleus changes
negligible.
M
Z
X Y
M
Z
0
0

Radioactive decay -
radioactive (spontaneous)
transformation of the original (mother) nucleus
into new (child) kernels.
For each radioactive substance
there is a certain interval
time during which
activity decreases by half.

Half-life T is
time during which
half breaks up
available number
radioactive atoms.
N0 is the number of radioactive atoms in
initial moment of time.
N is the number of non-decayed atoms in
any moment in time.

Nucleus structure models.

How to represent the kernel? This is a tricky question, and several kernel models have been proposed. The most popular and used to date are two models: drop and shell.

According to the droplet model, the nucleus is compared with a droplet of liquid, since a drop of liquid and a nucleus have much in common. The main common feature is that the interaction between molecules of a liquid drop, as well as between nucleons of a nucleus, has the saturation property: each molecule is surrounded by only a completely definite number of neighbors. The forces of interaction between molecules in a drop are short-acting. The volume of the drop grows, like that of the nucleus, in proportion to the number of molecules. Comparison of the nucleus with a drop suggests another important idea: a drop of liquid has a surface tension. There is reason to believe that the nucleus-drop also possesses this property. Surface tension pulls the droplet together and makes it spherical. Therefore, the core, one might say, has a spherical shape. There are also differences between a drop of liquid and the nucleus of an atom. The nucleus is charged (protons!), While the drop is usually neutral (although it can be charged on purpose). The main difference is that a drop is a classical system and in it the energy is a continuous quantity, while the nucleus is a typical quantum system and its energy has a discrete spectrum.

In the shell model, the nucleus is compared with the atom, which has a shell structure: the center of the atom, in which the nucleus is concentrated, is surrounded by layers of the electron shell. At first glance, it seems that the nucleus should have nothing in common with the atom, since there is no physically separated center in the nucleus, around which layers of nucleons could be located. However, one must take into account the quantum structure of both the nucleus and the atom. After all, the layers of the electron shell of an atom are created due to the fact that the discrete energy spectrum of atoms is as follows: its energy levels decompose into a number of comparatively close groups, the filling of the levels of which constitutes the layers of the shells of electrons. It turned out that the energy spectra of nuclei in this respect resemble the spectra of atoms: they also constitute groups of closely spaced levels. Therefore, the gradual filling of these groups of levels with nucleons resembles the electronic layers of atoms. This is how the shell model of nuclei is constructed.

Nuclear forces.

In order for atomic nuclei to be stable, protons and neutrons must be held inside nuclei by enormous forces, many times greater than the forces of the Coulomb repulsion of protons.

Nuclear forces - forces acting between nuclear particles - nucleons.

Properties of nuclear forces:

1. These are short-acting forces act at distances between nucleons, of the order of 10–15 m, and sharply decrease with increasing distance; at distances of 1.4 ∙ 10 −15 m, they are already practically equal to 0.

2. These are the most powerful forces of all that nature has., therefore, the interaction of particles in the nucleus is often called strong interactions.

3. Nuclear forces are inherently saturated, those. the nucleon interacts not with all other nucleons, but only with some of the nearest neighbors.

4. Nuclear forces are characterized by charge independence. This means that both charged and uncharged particles are attracted to each other with the same modulus of force, i.e. gravity F pp between two protons is equal to the force of attraction F nn between two neutrons and is equal to the attraction force F rn between a proton and a neutron.

5. Nuclear forces are not central, those. they are not directed along a straight line connecting the centers of these charges.

6. Nuclear forces are so-called exchange forces.

I remind you that there are four types fundamental interactions in nature: strong, electromagnetic, weak and gravitational.

Strong interaction occurs at the level of atomic nuclei and represents the mutual attraction and repulsion of their constituent parts. It acts at a distance of the order of 10 -13 cm. Under certain conditions, strong interaction very firmly binds particles, resulting in the formation of material systems with high binding energy - atomic nuclei. It is for this reason that the nuclei of atoms are very stable, they are difficult to destroy.

Electromagnetic interaction about a thousand times weaker than strong, but much more long-range. This type of interaction is characteristic of electrically charged particles. The carrier of the electromagnetic interaction is a chargeless photon - a quantum of the electromagnetic field. In the process of electromagnetic interaction, electrons and atomic nuclei combine into atoms, atoms - into molecules. In a sense, this interaction is fundamental in chemistry and biology.

Weak interaction possibly between different particles. It extends to a distance of the order of 10 -15 - 10 -22 cm and is mainly associated with the decay of particles, for example, with the transformations of a neutron into a proton, an electron and an antineutrino occurring in an atomic nucleus. In accordance with the current level of knowledge, most particles are unstable precisely because of the weak interaction.

Gravitational interaction- the weakest, not taken into account in the theory of elementary particles, since at their characteristic distances of the order of 10 -13 cm, it gives extremely small effects. However, at ultra-short distances (of the order of 10 -33 cm) and at ultra-high energies, gravity again acquires significant importance. Here begins to manifest unusual properties physical vacuum. Superheavy virtual particles create a noticeable gravitational field around themselves, which begins to distort the geometry of space. On a cosmic scale, gravitational interaction is critical. Its radius of action is not limited.

All four interactions necessary and sufficient to build a diverse world.

Without strong interactions, atomic nuclei would not exist, and stars and the Sun could not generate heat and light due to nuclear energy.

Without electromagnetic interactions, there would be no atoms, no molecules, no macroscopic objects, as well as no heat and light.

Without weak interactions, nuclear reactions in the interior of the Sun and stars would not be possible, supernova explosions would not occur, and heavy elements necessary for life would not be able to propagate in the Universe. Without gravitational interaction, not only would there be no galaxies, stars, planets, but the entire Universe could not evolve, since gravity is a unifying factor that ensures the unity of the Universe as a whole and its evolution.

Modern physics has come to the conclusion that all four fundamental interactions necessary to create a complex and diverse material world from elementary particles can be obtained from one fundamental interaction - superpowers. The most striking achievement was the proof that with very high temperatures(or energies) all four interactions are combined into one.

This assumption is purely theoretical, since it cannot be verified experimentally. These ideas are indirectly confirmed by astrophysical data, which can be considered as experimental material accumulated by the Universe.

Discovery of the neutron and proton.

By the 1920s, physicists no longer doubted that the atomic nuclei discovered by E. Rutherford in 1911, as well as the atoms themselves, have a complex structure. They were convinced of this by numerous experimental facts accumulated by that time: the discovery of radioactivity, the experimental proof of the nuclear model of the nucleus, the measurement of the e / m ratio for an electron, an α-particle and for the so-called H-particle - the nucleus of a hydrogen atom, the discovery of artificial radioactivity and nuclear reactions, measurement of the charges of atomic nuclei, etc. At present, it is firmly established that atomic nuclei of various elements are composed of two particles - protons and neutrons.

The first of these particles is a hydrogen atom from which a single electron has been removed. This particle was already observed in the experiments of J. Thomson (1907), who managed to measure its ratio e / m. In 1919, E. Rutherford discovered the nuclei of the hydrogen atom in the fission products of the nuclei of atoms of many elements. Rutherford called this particle a proton. He suggested that protons are part of all atomic nuclei.

Scheme of Rutherford's experiments on the detection of protons in nuclear fission products. K - lead container with a radioactive source of α-particles, F - metal foil, E - screen covered with zinc sulfide, M - microscope.

Rutherford's device consisted of an evacuated chamber in which a container K with a source of α-particles was located. The chamber window was covered with a metal foil Ф, the thickness of which was chosen so that α-particles could not penetrate through it. Outside the window was screen E, covered with zinc sulfide. With the help of a microscope M, it was possible to observe scintillations at the points where heavy charged particles hit the screen. When the chamber was filled with nitrogen at low pressure, light flashes appeared on the screen, indicating the appearance of a flux of some particles capable of penetrating through the foil F, which almost completely trapped the flux of alpha particles.

Moving E's screen away from the camera window, Rutherford measured mean free path of the observed particles in the air. It turned out to be approximately equal to 28 cm, which coincided with the estimate of the path length of H-particles previously observed by J. Thomson. Studies of the action of electric and magnetic fields on particles knocked out of nitrogen nuclei have shown that these particles have a positive elementary charge and their mass is equal to the mass of the nucleus of a hydrogen atom. Subsequently, the experiment was carried out with a number of other gaseous substances. In all cases, it was found that α-particles knock out H-particles or protons from the nuclei of these substances. According to modern measurements, the positive charge of a proton is exactly equal to elementary charge e = 1.60217733 · 10–19 C, that is, equal in magnitude to the negative charge of an electron. At present, the equality of the charges of a proton and an electron has been verified with an accuracy of 10–22. Such a coincidence of the charges of two dissimilar particles is surprising and remains one of the fundamental mysteries of modern physics.

Proton mass, according to modern measurements, is equal to m p = 1.67262 · 10-27 kg. In nuclear physics, the mass of a particle is often expressed in atomic mass units (amu) equal to 1/12 of the mass of a carbon atom with a mass number of 12:

Therefore, m p = 1.007276 · a. e.m.In many cases, the particle mass is conveniently expressed in equivalent energy values in accordance with formula E = mc 2. Since 1 eV = 1.60218 · 10 -19 J, the proton mass in energy units is equal to 938.272331 MeV. Thus, in the experiment of Rutherford, the phenomenon of the splitting of nuclei of nitrogen and other elements during collisions of fast α-particles was discovered and it was shown that protons are part of the nuclei of atoms. After the discovery of the proton, it was suggested that the nuclei of atoms consist of only protons. However, this assumption turned out to be untenable, since the ratio of the nuclear charge to its mass does not remain constant for different nuclei, as it would be if only protons were included in the composition of the nuclei. For heavier nuclei, this ratio turns out to be less than for light ones, that is, when passing to heavier nuclei, the mass of the nucleus grows faster than the charge. In 1920, Rutherford put forward a hypothesis about the existence in the composition of nuclei of a tightly bound compact proton-electron pair, which is an electrically neutral formation - a particle with a mass approximately equal to the mass of a proton. He even came up with a name for this hypothetical particle - neutron.

It was very beautiful, but, as it turned out later, the wrong idea. An electron cannot be part of the nucleus. A quantum mechanical calculation based on the uncertainty relation shows that an electron localized in a nucleus, that is, a region with a size of R ≈ 10 –13 cm, must have a colossal kinetic energy, many orders of magnitude greater than nuclear binding energy per particle.

The idea of the existence of a heavy neutral particle seemed so attractive to Rutherford that he immediately invited a group of his students led by J. Chadwick to start looking for such a particle. Twelve years later, in 1932, Chadwick experimentally investigated the radiation that occurs when beryllium is irradiated with α-particles, and found that this radiation is a stream of neutral particles with a mass approximately equal to the mass of a proton. This is how the neutron was discovered.

When beryllium is bombarded with α-particles emitted by radioactive polonium, a strong penetrating radiation is generated that can overcome such an obstacle as a layer of lead 10-20 cm thick. This radiation was observed almost simultaneously with Chadwick, the spouses of Joliot-Curie, Irene and Frederic (Irene and Pierre Curie), but they assumed that these were high-energy γ-rays. They found that if a paraffin plate is placed in the path of the radiation of beryllium, then the ionizing ability of this radiation increases sharply. They proved that the radiation of beryllium knocks out protons from the paraffin, which are present in large quantities in this hydrogen-containing substance. From the mean free path of protons in air, they estimated the energy of γ-quanta, capable of imparting the required speed to protons in a collision.

It turned out to be huge - about 50 MeV. J. Chadwick in 1932 performed a series of experiments on a comprehensive study of the properties of radiation arising from the irradiation of beryllium with α-particles. In his experiments, Chadwick used various methods of studying ionizing radiation. In fig. 2 depicts Geiger counter, designed to register charged particles. It consists of a glass tube covered from the inside with a metal layer (cathode) and a thin thread running along the axis of the tube (anode). The tube is filled with an inert gas (usually argon) at low pressure. A charged particle flying through a gas causes ionization of molecules. Free electrons generated as a result of ionization are accelerated electric field between the anode and cathode up to energies at which impact ionization begins. An avalanche of ions arises, and a short discharge current pulse passes through the counter. Another important device for studying particles is the so-called Wilson chamber, in which a fast charged particle leaves a trace (track). The particle trajectory can be observed directly or photographed.

Action Wilson chambers, created in 1912, is based on the condensation of a supersaturated vapor on ions formed in the working volume of the chamber along the trajectory of a charged particle. With the help of the Wilson camera, one can observe the curvature of the trajectory of a charged particle in electric and magnetic fields. J. Chadwick in his experiments observed in the Wilson chamber the tracks of nitrogen nuclei that collided with beryllium radiation. On the basis of these experiments, he made an estimate of the energy of a γ-quantum capable of imparting the experimentally observed velocity to nitrogen nuclei. It turned out to be equal to 100–150 MeV. Such tremendous energy could not be possessed by γ-quanta emitted by beryllium. On this basis, Chadwick concluded that it is not massless γ-quanta that fly out of beryllium under the action of α-particles, but rather heavy particles.

Since these particles had a high penetrating power and did not directly ionize the gas in the Geiger counter, they were therefore electrically neutral. This proved the existence of a neutron - a particle predicted by Rutherford more than 10 years before Chadwick's experiments. Neutron Is an elementary particle. It should not be thought of as a compact proton-electron pair, as Rutherford originally suggested. According to modern measurements, neutron mass m n = 1.67493 · 10-27 kg = 1.008665 amu. e. m. In energy units, the neutron mass is equal to 939.56563 MeV. The mass of the neutron is approximately two electron masses greater than the mass of the proton. Immediately after the discovery of the neutron, the Russian scientist D. D. Ivanenko and the German physicist V. Heisenberg put forward a hypothesis about the proton-neutron structure atomic nuclei, which was fully confirmed by subsequent studies.

The nucleus consists of nucleons: protons and neutrons.

G. Moseley (England) established that the positive charge of the atomic nucleus (in arbitrary units) is equal to the ordinal number of the element in the periodic system of Mendeleev. Each proton has a charge of +1, so the charge on the nucleus is equal to the number of protons.

The mass of the proton, like the mass of the neutron, is approximately 1840 times the mass of the electron. Protons and neutrons are in the nucleus, so the mass of an atom is almost equal to the mass of the nucleus. The mass of a nucleus, like the mass of an atom, is determined by the sum of the number of protons and the number of neutrons. This sum is called the mass number of the atom. Mass number of atom (A) = Number of protons (Z) + Number of neutrons (N) A = Z + N

Protons and neutrons that make up any nucleus are not indivisible elementary particles, but consist of quarks.

Quarks, in turn, interact with each other, continuously exchanging gluons - carriers of a truly strong interaction (it is thousands of times stronger than that which acts between protons and neutrons in the nucleus). As a result, protons and neutrons turn out to be very tightly coupled systems that cannot be broken down into their component parts.

Binding energy of nucleons in the nucleus, mass defect.

The stability of the atomic nucleus is characterized by the binding energy (E St.).

The most accurate measurements show that the rest mass of the nucleus M is always less than the sum of the rest masses of its constituent protons and neutrons: M i< Zm p + Nm n .

Mass defect - the amount by which the mass of all nucleons decreases when an atomic nucleus is formed from them. The mass defect is equal to the difference between the sum of the rest masses of nucleons and the mass of the nucleus M i: ∆M = - M i,where m p, m n are the masses of the proton and neutron, respectively.

Communication energy – the minimum energy that must be spent for the complete splitting of the nucleus into individual nucleons or the energy released during the fusion of free nucleons into the nucleus. Calculated bond energy formula:

E sv = ∆mc 2 = c 2, where c = 3 · 10 8 m / s is the speed of light in vacuum.

If in this formula the masses of the proton, neutron and nucleus are expressed in kilograms, and the speed of light is in meters per second, then the binding energy E sv will be measured in joules. However, in the physics of the atom and atomic nucleus, the energy of nuclei and elementary particles is more often expressed in megaelectron-volts (MeV): 1 MeV = 1.6 10 - 13 J.

Solving the corresponding problems, one can obtain the binding energy in joules, and then, if required, convert it into megaelectron-volts, dividing the resulting number of joules by 1.6 · 10 - 13. But it is much easier to obtain the value of the binding energy in megaelectron-volts, if we leave the masses of the proton, neutron and nucleus expressed in atomic mass units and multiply the mass defect ∆M not by c 2, but by the number 931 . One atomic mass unit corresponds to the binding energy 931MeV.E sv = 931 ∆М or E bv = 931 (Zm p + Nm n - M i) MeV

The binding energy is converted into the energy of γ-quanta emitted during nuclear transformations, which is equal to just E sv , and the mass of which: ∆М = E / s 2.

If as a result of the reaction E = ∆Mc 2> 0, then energy is released, if E = ∆M c 2< 0 - поглощается.

To characterize the strength of the core, a value is used, which is called the specific binding energy ε St.

Specific bond energy - the binding energy per nucleon of the nucleus is equal to the ratio of the binding energy E sv to the mass number of the atomic nucleus A: ε sv = E sv / A, The specific binding energy is determined experimentally.

Nuclear reactions - processes occurring when nuclei or elementary particles collide with other nuclei, as a result of which the quantum state and nucleon composition of the initial nucleus change, and new particles appear among the reaction products.

In this case, it is possible fission reactions, when the nucleus of one atom as a result of the bombardment is divided into two nuclei of different atoms. At synthesis reactions there is a transformation of light nuclei into heavier ones.

ATTENTION: The difference between chemical and nuclear reactions is that in chemical reactions, the total number of atoms of each specific element, as well as the atoms that make up certain substances, remain unchanged. In nuclear reactions, both atoms and elements change.

Isotopes.

Isotopes - these are varieties of atoms of the same chemical element, the atomic nuclei of which have the same number of protons Z and a different number of neutrons n. Isotopes occupy the same place in the periodic table of elements, hence their name. As a rule, isotopes differ significantly in their nuclear properties. The chemical (and almost to the same extent physical) properties of isotopes are the same. This is due to the fact that the chemical properties of an element are determined by the nuclear charge, since it is this charge that affects the structure of the electron shell of the atom.

The exception is isotopes of light elements. Hydrogen isotopes 1 H - protium, 2 N - deuterium, 3 H - tritium so much different in mass that their physical and chemical properties are different. Deuterium is stable (i.e., not radioactive) and is included as a small impurity (1: 4500) in ordinary hydrogen. When deuterium combines with oxygen, heavy water is formed. It boils at 101.2 ° C at normal atmospheric pressure and freezes at 3.8 ° C. Tritium is β-radioactive with a half-life of about 12 years.

All chemical elements have isotopes. Some elements only have unstable (radioactive) isotopes. For all elements, radioactive isotopes are artificially obtained. In the nuclear industry, radioactive isotopes are of increasing value to mankind.

1 MeV = 1.6 10 - 13 J; 1 amu = 1.66 ∙ 10 -27 kg.

« Physics - grade 11 "

The structure of the atomic nucleus. Nuclear forces

Immediately after the neutron was discovered in Chadwick's experiments, Soviet physicist D. D. Ivanenko and the German scientist V. Heisenberg in 1932 proposed a proton-neutron model of the nucleus.
It was confirmed by subsequent studies of nuclear transformations and is now generally accepted.

Proton-neutron nuclear model

According to the proton-neutron model, nuclei consist of elementary particles of two types - protons and neutrons.

Since the atom as a whole is electrically neutral, and the proton charge is equal to the modulus of the electron charge, the number of protons in the nucleus is equal to the number of electrons in the atomic shell.
Therefore, the number of protons in the nucleus is equal to the atomic number of the element Z in the periodic table of elements of D.I.Mendeleev.

The sum of the number of protons Z and the number of neutrons N in the core called massive number and denoted by the letter A:

A = Z + N

The masses of the proton and neutron are close to each other and each of them is approximately equal to the atomic mass unit.
The mass of electrons in an atom is much less than the mass of its nucleus.
Therefore, the mass number of the nucleus is equal to the relative atomic mass of the element, rounded to the nearest integer.
The mass numbers can be determined by roughly measuring the mass of nuclei with instruments that are not very accurate.

Isotopes are nuclei with the same value Z but with different mass numbers A, i.e., with different numbers of neutrons N.

Nuclear forces

Since nuclei are very stable, protons and neutrons must be held inside the nucleus by some kind of force, and very large ones.
It is not gravitational forces that are too weak.
The stability of the nucleus cannot be explained by electromagnetic forces either, since electrical repulsion acts between like-charged protons.
And neutrons have no electrical charge.

This means that between nuclear particles - protons and neutrons, they are called nucleons- there are special forces called nuclear forces.

What are the main properties of nuclear forces? Nuclear forces are about 100 times greater than electrical (Coulomb) forces.
These are the most powerful forces that exist in nature.
Therefore, the interactions of nuclear particles are often called strong interactions.

Strong interactions are manifested not only in the interactions of nucleons in the nucleus.
This is a special type of interactions inherent in most elementary particles along with electromagnetic interactions.

Another important feature of nuclear forces is their short duration.
Electromagnetic forces decay comparatively slowly with increasing distance.
Nuclear forces are noticeably manifested only at distances equal to the dimensions of the nucleus (10 -12 -10 -13 cm), which was already shown by Rutherford's experiments on the scattering of α-particles by atomic nuclei.
A complete quantitative theory of nuclear forces has not yet been developed.
Significant progress in its development has been achieved quite recently - in the last 10-15 years.

Atomic nuclei are made up of protons and neutrons. These particles are held in the nucleus by nuclear forces.

Isotopes

The study of the phenomenon of radioactivity led to an important discovery: the nature of atomic nuclei was clarified.

As a result of the observation of a huge number of radioactive transformations, it was gradually discovered that there are substances that are identical in their chemical properties, but have completely different radioactive properties (that is, they decay in different ways).
They could not be separated in any way by any of the known chemical methods.
On this basis, Soddy in 1911 suggested the possibility of the existence of elements with the same chemical properties, but differing, in particular, in their radioactivity.
These elements must be placed in the same cell of the periodic table of D. I. Mendeleev.
Soddy named them isotopes(i.e., occupying the same places).

Soddy's hypothesis received brilliant confirmation and deep interpretation a year later, when J.J. Thomson made accurate measurements of the mass of neon ions by deflecting them in electric and magnetic fields.
He discovered that neon is a mixture of two kinds of atoms.
Most of them have a relative mass equal to 20.
But there is an insignificant fraction of atoms with a relative atomic mass of 22.
As a result, the relative atomic mass of the mixture was taken equal to 20.2.
Atoms with the same chemical properties differed in mass.

Both types of neon atoms, naturally, occupy the same place in the DI Mendeleev's table and, therefore, are isotopes.
Thus, isotopes can differ not only in their radioactive properties, but also in mass.
That is why isotopes have the same charges of atomic nuclei, which means that the number of electrons in the shells of atoms and, consequently, the chemical properties of isotopes are the same.
But the masses of the nuclei are different.
Moreover, the nuclei can be both radioactive and stable.
Difference of properties radioactive isotopes due to the fact that their nuclei have different masses.

At present, the existence of isotopes has been established for most chemical elements.
Some elements only have unstable (i.e. radioactive) isotopes.
The heaviest element in nature - uranium (relative atomic masses 238, 235, etc.) and the lightest - hydrogen (relative atomic masses 1, 2, 3) have isotopes.

Isotopes of hydrogen are especially interesting, since they differ in mass by 2 and 3 times.
An isotope with a relative atomic mass of 2 is called deuterium.
It is stable (i.e., not radioactive) and is included as a small impurity (1: 4500) in ordinary hydrogen.
When deuterium combines with oxygen, so-called heavy water is formed.
Its physical properties differ markedly from those of ordinary water.
At normal atmospheric pressure, it boils at 101.2 ° C and freezes at 3.8 ° C.

An isotope of hydrogen with an atomic mass of 3 is called tritium.
It is β-radioactive and has a half-life of about 12 years.

The existence of isotopes proves that the charge of an atomic nucleus does not determine all the properties of an atom, but only its chemical properties and those physical properties that depend on the periphery of the electron shell, for example, the size of an atom.
The mass of the atom and its radioactive properties are not determined by the ordinal number in the table of D.I.Mendeleev.

It is noteworthy that with an accurate measurement of the relative atomic masses of the isotopes, it turned out that they are close to whole numbers.
But the atomic masses of chemical elements are sometimes very different from whole numbers.
So, the relative atomic mass of chlorine is 35.5.
This means that in its natural state, a chemically pure substance is a mixture of isotopes in various proportions.
The (approximate) integer of the relative atomic masses of isotopes is very important for clarifying the structure of the atomic nucleus.

Most chemical elements have isotopes.
The charges of the atomic nuclei of the isotopes are the same, but the masses of the nuclei are different.