atomic nucleus
Atomic nucleus

atomic nucleus - the central and very compact part of the atom, in which almost all of its mass and all positive electric charge. The nucleus, holding close to itself by the Coulomb forces the electrons in an amount that compensates for its positive charge, forms a neutral atom. Most of the nuclei have a shape close to spherical and a diameter of ≈ 10 -12 cm, which is four orders of magnitude smaller than the diameter of an atom (10 -8 cm). The density of matter in the core is about 230 million tons/cm 3 .
The atomic nucleus was discovered in 1911 as a result of a series of experiments on the scattering of alpha particles by thin gold and platinum foils, carried out in Cambridge (England) under the direction of E. Rutherford. In 1932, after the discovery of the neutron by J. Chadwick, it became clear that the nucleus consists of protons and neutrons
(V. Heisenberg, D.D. Ivanenko, E. Majorana).
To designate the atomic nucleus, the symbol of the chemical element of the atom, which includes the nucleus, is used, and the upper left index of this symbol shows the number of nucleons (mass number) in this nucleus, and the lower left index shows the number of protons in it. For example, a nickel nucleus containing 58 nucleons, of which 28 are protons, is denoted. The same nucleus can also be designated 58 Ni, or nickel-58.

The nucleus is a system of densely packed protons and neutrons moving at a speed of 10 9 -10 10 cm/sec and held by powerful and short-range nuclear forces of mutual attraction (their area of ​​action is limited by distances of ≈ 10 -13 cm). Protons and neutrons are about 10 -13 cm in size and are considered as two different states of a single particle called a nucleon. The radius of the nucleus can be approximately estimated by the formula R ≈ (1.0-1.1)·10 -13 A 1/3 cm, where A is the number of nucleons (the total number of protons and neutrons) in the nucleus. On fig. 1 shows how the density of matter changes (in units of 10 14 g/cm3) inside the nickel nucleus, consisting of 28 protons and 30 neutrons, depending on the distance r (in units of 10 -13 cm) to the center of the nucleus.
Nuclear interaction (interaction between nucleons in the nucleus) occurs due to the fact that nucleons exchange mesons. This interaction is a manifestation of the more fundamental strong interaction between quarks that make up nucleons and mesons (similarly, chemical bonding forces in molecules are a manifestation of more fundamental electromagnetic forces).
The world of nuclei is very diverse. About 3000 nuclei are known, differing from each other either in the number of protons, or in the number of neutrons, or both. Most of them are obtained artificially.
Only 264 cores are stable, ie. do not experience any spontaneous transformations, called decays, over time. The rest are experiencing various forms decay - alpha decay (emission of an alpha particle, i.e. the nucleus of a helium atom); beta decay (simultaneous emission of an electron and an antineutrino or a positron and a neutrino, as well as the absorption of an atomic electron with the emission of a neutrino); gamma decay (photon emission) and others.
Different types of nuclei are often referred to as nuclides. Nuclides with the same number of protons and different numbers of neutrons are called isotopes. Nuclides with the same number of nucleons but different ratios of protons and neutrons are called isobars. Light nuclei contain approximately equal numbers of protons and neutrons. In heavy nuclei, the number of neutrons is about 1.5 times the number of protons. The lightest nucleus is the nucleus of the hydrogen atom, which consists of one proton. The heaviest known nuclei (they are obtained artificially) have a number of nucleons of ≈290. Of these, 116-118 are protons.
Different combinations of the number of protons Z and neutrons correspond to different atomic nuclei. Atomic nuclei exist (i.e. their lifetime t > 10 -23 s) in a rather narrow range of changes in the numbers Z and N. In this case, all atomic nuclei are divided into two large groups - stable and radioactive (unstable). Stable nuclei cluster near the line of stability, which is given by the equation

On fig. 2 shows an NZ diagram of atomic nuclei. Black dots show stable nuclei. The area where stable nuclei are located is usually called the stability valley. On the left side of the stable nuclei are nuclei overloaded with protons (proton-rich nuclei), on the right - nuclei overloaded with neutrons (neutron-rich nuclei). Atomic nuclei currently discovered are highlighted in color. There are about 3.5 thousand of them. It is believed that there should be 7 - 7.5 thousand of them in total. Proton-rich nuclei (crimson color) are radioactive and turn into stable ones mainly as a result of β + decays, the proton that is part of the nucleus turns into a neutron. Neutron-rich nuclei (blue) are also radioactive and become stable as a result of - -decays, with the transformation of a nucleus neutron into a proton.
The heaviest stable isotopes are those of lead (Z = 82) and bismuth (Z = 83). Heavy nuclei, along with the processes of β + and β - decay, are also subject to α decay ( yellow) and spontaneous fission, which become their main decay channels. The dotted line in fig. 2 outlines the region of possible existence of atomic nuclei. The line B p = 0 (B p is the proton separation energy) limits the region of existence of atomic nuclei on the left (proton drip-line). The line B n = 0 (B n is the neutron separation energy) is on the right (neutron drip-line). Outside these boundaries, atomic nuclei cannot exist, since they decay in a characteristic nuclear time (~10 -23 – 10 -22 s) with the emission of nucleons.
When connecting (synthesis) of two light nuclei and fission of a heavy nucleus into two lighter fragments, a lot of energy is released. These two methods of obtaining energy are the most efficient of all known. So 1 gram of nuclear fuel is equivalent to 10 tons of chemical fuel. The fusion of nuclei (thermonuclear reactions) is the source of energy for stars. Uncontrolled (explosive) fusion is carried out when a thermonuclear (or so-called “hydrogen”) bomb is detonated. Controlled (slow) synthesis underlies a promising energy source being developed - a thermonuclear reactor.
Uncontrolled (explosive) fission occurs during the explosion of an atomic bomb. Controlled fission is carried out in nuclear reactors, which are sources of energy in nuclear power plants.
For the theoretical description of atomic nuclei, quantum mechanics and various models are used.
The nucleus can behave both as a gas (quantum gas) and as a liquid (quantum liquid). Cold nuclear liquid has the properties of superfluidity. In a strongly heated nucleus, nucleons decay into their constituent quarks. These quarks interact by exchanging gluons. As a result of such a decay, the set of nucleons inside the nucleus turns into a new state of matter - quark-gluon plasma

An atom is the smallest particle of a chemical element that retains all of its Chemical properties. An atom consists of a positively charged nucleus and negatively charged electrons. The nuclear charge of any chemical element is equal to the product Z to e, where Z is the ordinal number of the given element in periodic system chemical elements, e - the value of the elementary electric charge.

Electron- this is the smallest particle of a substance with a negative electric charge e=1.6·10 -19 coulombs, taken as an elementary electric charge. 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 gain electrons and become a negative ion. The charge of an ion determines the number of electrons lost or gained. The process of turning 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 having a unit 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 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 has a huge store of energy, which 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 into 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, turning into normal condition, releases excess energy in the form electromagnetic radiation with 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 certain types of atoms. The structure of an atom includes the kernel carrying a positive electric charge, and negatively charged electrons (see), forming its electronic shells. The value of the electric charge of the nucleus is equal to Ze, where e is the elementary electric charge, equal in magnitude to the charge of the electron (4.8 10 -10 e.-st. units), and Z is the atomic number of this element in the periodic system of chemical elements (see .). Since a non-ionized atom is neutral, the number of electrons included in it is also equal to Z. The composition of the nucleus (see. Atomic nucleus) 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 chargeless neutrons (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 A-Z. Isotopes are called 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 a different number of neutrons with the same number of protons. When designating isotopes, the mass number A is written at the top of the element symbol, and the atomic number at the bottom; for example, isotopes of oxygen are denoted:

The dimensions of the atom are determined by the dimensions of the electron shells and for all Z are about 10 -8 cm. Since the mass of all the electrons of the atom is several thousand times less than the mass of the nucleus, the mass of the 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 the 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 only be explained with the help of quantum theory, created mainly in the 20s of the 20th century and intended to describe phenomena on an atomic scale. Experiments have shown that microparticles - electrons, protons, atoms, etc. - in addition to corpuscular, have wave properties that manifest themselves in diffraction and interference. In quantum theory, a certain wave field characterized by a wave function (Ψ-function) is used to describe the state of micro-objects. This function determines the probabilities of possible states of the micro-object, i.e., it characterizes the potential possibilities 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 Newton's laws of motion in classical mechanics. 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 series of wave functions for electrons is obtained corresponding to different (quantized) energy values. 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 energy E i - E 0 is absorbed. An excited atom goes 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 an 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 valency, nature chemical bond and the structure of molecules, the theory of the periodic system of elements was created.

Investigating the passage of an α-particle through a thin gold foil (see Section 6.2), E. Rutherford came to the conclusion that an atom consists of a heavy positively charged nucleus and electrons surrounding it.

core called the center of the atom,in which almost all the mass of an atom and its positive charge is concentrated.

IN composition of the atomic nucleus includes elementary particles : protons And neutrons (nucleons from the Latin word nucleus- core). Such a proton-neutron model of the nucleus was proposed by the Soviet physicist in 1932 D.D. Ivanenko. The proton has a positive charge e + = 1.06 10 -19 C and a rest mass m p\u003d 1.673 10 -27 kg \u003d 1836 me. Neutron ( n) is a neutral particle with rest mass m n= 1.675 10 -27 kg = 1839 me(where the mass of the electron me, is equal to 0.91 10 -31 kg). On fig. 9.1 shows the structure of the helium atom according to the ideas of the end of XX - early XXI in.

Core charge equals Ze, where e is the charge of the proton, Z- charge number equal to serial number chemical element in Mendeleev's periodic system of elements, i.e. the number of protons in the nucleus. The number of neutrons in a nucleus is denoted N. Usually Z > N.

Nuclei with Z= 1 to Z = 107 – 118.

Number of nucleons in the nucleus A = Z + N called mass number . nuclei with the same Z, but different BUT called isotopes. Kernels, which, at the same A have different Z, are called isobars.

The nucleus is denoted by the same symbol as the neutral atom, where X is the symbol for a chemical element. For example: hydrogen Z= 1 has three isotopes: – protium ( Z = 1, N= 0), is deuterium ( Z = 1, N= 1), – tritium ( Z = 1, N= 2), tin has 10 isotopes, and so on. In the vast majority of isotopes of the same chemical element, they have the same chemical and close physical properties. In total, about 300 stable isotopes and more than 2000 natural and artificially obtained are known. radioactive isotopes.

The size of the nucleus is characterized by the radius of the nucleus, which has a conditional meaning due to the blurring of the nucleus boundary. Even E. Rutherford, analyzing his experiments, showed that the size of the nucleus is approximately 10–15 m (the size of an atom is 10–10 m). There is an empirical formula for calculating the core radius:

where R 0 = (1.3 - 1.7) 10 -15 m. From this it can be seen that the volume of the nucleus is proportional to the number of nucleons.

The density of the nuclear substance is on the order of 10 17 kg/m 3 and is constant for all nuclei. It greatly exceeds the density of the densest ordinary substances.

Protons and neutrons are fermions, because have spin ħ /2.

The nucleus of an atom has own angular momentum – nuclear spin :

,

(9.1.2)

where I – internal(complete)spin quantum number.

Number I accepts integer or half-integer values ​​0, 1/2, 1, 3/2, 2, etc. Kernels with even BUT have integer spin(in units ħ ) and obey the statistics Bose–Einstein(bosons). Kernels with odd BUT have half-integer spin(in units ħ ) and obey the statistics Fermi–Dirac(those. nuclei are fermions).

Nuclear particles have their own magnetic moments, which determine the magnetic moment of the nucleus as a whole. The unit for measuring the magnetic moments of nuclei is nuclear magneton μ poison:

Here e is the absolute value of the electron charge, m p is the mass of the proton.

Nuclear magneton in m p/me= 1836.5 times smaller than the Bohr magneton, hence it follows that the magnetic properties of atoms are determined magnetic properties its electrons .

There is a relationship between the spin of the nucleus and its magnetic moment:

where γ poison - nuclear gyromagnetic ratio.

The neutron has a negative magnetic moment μ n≈ – 1.913μ poison because the direction of the neutron spin and its magnetic moment are opposite. The magnetic moment of the proton is positive and equal to μ R≈ 2.793μ poison. Its direction coincides with the direction of the proton spin.

The distribution of the electric charge of protons over the nucleus is generally asymmetric. The measure of deviation of this distribution from spherically symmetric is quadrupole electric moment of the nucleus Q. If the charge density is assumed to be the same everywhere, then Q determined only by the shape of the nucleus. So, for an ellipsoid of revolution

,

(9.1.5)

where b is the semiaxis of the ellipsoid along the spin direction, but- axis in the perpendicular direction. For a nucleus stretched along the direction of the spin, b > but And Q> 0. For a nucleus oblate in this direction, b < a And Q < 0. Для сферического распределения заряда в ядре b = a And Q= 0. This is true for nuclei with spin equal to 0 or ħ /2.

To view demos, click on the appropriate hyperlink:

Long before the emergence of reliable data on the internal structure of all things, Greek thinkers imagined matter in the form of the smallest fiery particles that were in constant motion. Probably, this vision of the world order of things was derived from purely logical conclusions. Despite some naivety and absolute lack of evidence for this statement, it turned out to be true. Although scientists were able to confirm a bold guess only twenty-three centuries later.

The structure of atoms

IN late XIX centuries, the properties of a discharge tube through which a current is passed have been investigated. Observations have shown that two streams of particles are emitted:

The negative particles of the cathode rays were called electrons. Subsequently, particles with the same charge-to-mass ratio were found in many processes. Electrons seemed to be universal constituents of various atoms, quite easily separated by the bombardment of ions and atoms.

Particles carrying a positive charge were represented by fragments of atoms after they lost one or more electrons. In fact, the positive rays were groups of atoms devoid of negative particles, and therefore having a positive charge.

Thompson model

On the basis of experiments, it was found that positive and negative particles represented the essence of the atom, were its constituents. The English scientist J. Thomson proposed his theory. In his opinion, the structure of the atom and the atomic nucleus was a kind of mass in which negative charges were squeezed into a positively charged ball, like raisins in a cupcake. Charge compensation made the cake electrically neutral.

Rutherford model

The young American scientist Rutherford, analyzing the tracks left after alpha particles, came to the conclusion that the Thompson model is imperfect. Some alpha particles were deflected by small angles - 5-10 o . In rare cases, alpha particles were deflected at large angles of 60-80 o , and in exceptional cases, the angles were very large - 120-150 o . Thompson's model of the atom could not explain such a difference.

Rutherford proposes a new model that explains the structure of the atom and the atomic nucleus. The physics of processes states that an atom must be 99% empty, with a tiny nucleus and electrons revolving around it, which move in orbits.

He explains the deviations during impacts by the fact that the particles of the atom have their own electric charges. Under the influence of bombarding charged particles, atomic elements behave like ordinary charged bodies in the macrocosm: particles with the same charges repel each other, and with opposite charges they attract.

State of atoms

At the beginning of the last century, when the first particle accelerators were launched, all theories explaining the structure of the atomic nucleus and the atom itself were waiting for experimental verification. By that time, the interactions of alpha and beta rays with atoms had already been thoroughly studied. Until 1917, it was believed that atoms were either stable or radioactive. Stable atoms cannot be split, the decay of radioactive nuclei cannot be controlled. But Rutherford managed to refute this opinion.

First proton

In 1911, E. Rutherford put forward the idea that all nuclei consist of the same elements, the basis for which is the hydrogen atom. This idea of ​​the scientist was prompted by an important conclusion of previous studies of the structure of matter: the masses of all chemical elements are divided without a trace by the mass of hydrogen. The new assumption opened up unprecedented possibilities, allowing us to see the structure of the atomic nucleus in a new way. Nuclear reactions had to confirm or disprove the new hypothesis.

Experiments were carried out in 1919 with nitrogen atoms. By bombarding them with alpha particles, Rutherford achieved an amazing result.

The N atom absorbed the alpha particle, then turned into an oxygen atom O 17 and emitted a hydrogen nucleus. This was the first artificial transformation of an atom of one element into another. Such an experience gave hope that the structure of the atomic nucleus, the physics of existing processes make it possible to carry out other nuclear transformations.

The scientist used in his experiments the method of scintillation - flashes. Based on the frequency of flashes, he drew conclusions about the composition and structure of the atomic nucleus, about the characteristics of the particles born, about their atomic mass and serial number. The unknown particle was named by Rutherford the proton. It had all the characteristics of a hydrogen atom stripped of its single electron - a single positive charge and a corresponding mass. Thus it was proved that the proton and the nucleus of hydrogen are the same particles.

In 1930, when the first large accelerators were built and launched, Rutherford's model of the atom was tested and proved: each hydrogen atom consists of a lone electron, the position of which cannot be determined, and a loose atom with a lone positive proton inside. Since protons, electrons, and alpha particles can fly out of an atom when bombarded, scientists thought that they were the constituents of any atom's nucleus. But such a model of the atom of the nucleus seemed unstable - the electrons were too large to fit in the nucleus, in addition, there were serious difficulties associated with the violation of the law of momentum and conservation of energy. These two laws, like strict accountants, said that the momentum and mass during the bombardment disappear in an unknown direction. Since these laws were generally accepted, it was necessary to find explanations for such a leak.

Neutrons

Scientists around the world set up experiments aimed at discovering new constituents of the nuclei of atoms. In the 1930s, German physicists Becker and Bothe bombarded beryllium atoms with alpha particles. In this case, an unknown radiation was registered, which it was decided to call G-rays. Detailed studies revealed some features of the new beams: they could propagate strictly in a straight line, did not interact with electric and magnetic fields, had a high penetrating power. Later, the particles that form this type of radiation were found in the interaction of alpha particles with other elements - boron, chromium and others.

Chadwick's hypothesis

Then James Chadwick, a colleague and student of Rutherford, gave a short report in Nature magazine, which later became well known. Chadwick drew attention to the fact that the contradictions in the conservation laws are easily resolved if we assume that the new radiation is a stream of neutral particles, each of which has a mass approximately equal to the mass of a proton. Considering this assumption, physicists significantly supplemented the hypothesis explaining the structure of the atomic nucleus. Briefly, the essence of the additions was reduced to a new particle and its role in the structure of the atom.

Properties of the neutron

The discovered particle was given the name "neutron". The newly discovered particles did not form electromagnetic fields around themselves and easily passed through matter without losing energy. In rare collisions with light nuclei of atoms, the neutron is able to knock out the nucleus from the atom, losing a significant part of its energy. The structure of the atomic nucleus assumed the presence of a different number of neutrons in each substance. Atoms with the same nuclear charge but different numbers of neutrons are called isotopes.

Neutrons have served as an excellent replacement for alpha particles. Currently, they are used to study the structure of the atomic nucleus. Briefly, their significance for science cannot be described, but it was thanks to the bombardment of atomic nuclei by neutrons that physicists were able to obtain isotopes of almost all known elements.

The composition of the nucleus of an atom

At present, the structure of the atomic nucleus is a collection of protons and neutrons held together by nuclear forces. For example, a helium nucleus is a lump of two neutrons and two protons. Light elements have practically equal number protons and neutrons, heavy elements have much more neutrons.

This picture of the structure of the nucleus is confirmed by experiments at modern large accelerators with fast protons. The electric forces of repulsion of protons are balanced by vigorous forces that act only in the nucleus itself. Although the nature of nuclear forces is not yet fully understood, their existence is practically proven and fully explains the structure of the atomic nucleus.

Relationship between mass and energy

In 1932, a cloud chamber captured an amazing photograph proving the existence of positive charged particles, with the mass of an electron.

Prior to this, positive electrons were theoretically predicted by P. Dirac. A real positive electron was also discovered in cosmic radiation. The new particle was called the positron. When colliding with its twin - an electron, annihilation occurs - the mutual annihilation of two particles. This releases a certain amount of energy.

Thus, the theory developed for the macrocosm was fully suitable for describing the behavior of the smallest elements of matter.

Proton-electron theory

By the beginning of $1932$, only three elementary particles were known: electron, proton and neutron. For this reason, it was assumed that the nucleus of an atom consists of protons and electrons (proton-electron hypothesis). It was believed that the composition of the nucleus with number $Z$ in Mendeleev's periodic system of elements and mass number $A$ includes $A$ protons and $Z-A$ neutrons. In accordance with this hypothesis, the electrons that were part of the nucleus acted as a “cementing” agent, with the help of which positively charged protons were retained in the nucleus. Supporters of the proton-electron hypothesis of the composition of the atomic nucleus believed that $\beta ^-$ - radioactivity - is a confirmation of the correctness of the hypothesis. But this hypothesis was not able to explain the results of the experiment and was discarded. One of these difficulties was the impossibility to explain the fact that the spin of the nitrogen nucleus $^(14)_7N$ is equal to the unit $(\hbar)$. According to the proton-electron hypothesis, the $^(14)_7N$ nitrogen nucleus should consist of $14$ protons and $7$ electrons. The spin of protons and electrons is equal to $1/2$. For this reason, the nucleus of the nitrogen atom, which according to this hypothesis consists of $21$ particles, must have spin $1/2,\ 3/2,\ 5/2,\dots 21/2$. This discrepancy between the proton-electron theory is called the "nitrogen catastrophe". It was also incomprehensible that in the presence of electrons in the nucleus, its magnetic moment has a small magnetic moment compared to the magnetic moment of the electron.

In $1932$, J. Chadwick discovered the neutron. After this discovery, D. D. Ivanenko and E. G. Gapon put forward a hypothesis about the proton-neutron structure of the atomic nucleus, which was developed in detail by V. Heisenberg.

Remark 1

The proton-neutron composition of the nucleus is confirmed not only by theoretical conclusions, but also directly by experiments on the splitting of the nucleus into protons and neutrons. It is now generally accepted that the atomic nucleus consists of protons and neutrons, which are also called nucleons(from Latin nucleus kernel, grain).

The structure of the atomic nucleus

Core is the central part of the atom, in which the positive electric charge and the main part of the mass of the atom are concentrated. The dimensions of the nucleus, in comparison with the orbits of electrons, are extremely small: $10^(-15)-10^(-14)\ m$. Nuclei are made up of protons and neutrons, which are almost identical in mass, but only the proton carries an electric charge. The total number of protons is called atomic number$Z$ of an atom, which is the same as the number of electrons in a neutral atom. Nucleons are held in the nucleus by large forces, by their nature these forces are neither electrical nor gravitational, and in magnitude they are much greater than the forces that bind electrons to the nucleus.

According to the proton-neutron model of the structure of the nucleus:

• the nuclei of all chemical elements consist of nucleons;
• the charge of the nucleus is due only to protons;
• the number of protons in the nucleus is equal to the ordinal number of the element;
• the number of neutrons is equal to the difference between the mass number and the number of protons ($N=A-Z$)

A proton ($^2_1H\ or\ p$) is a positively charged particle: its charge is equal to the charge of an electron $e=1.6\cdot 10^(-19)\ Cl$, and its rest mass is $m_p=1.627\cdot 10^( -27)\kg$. The proton is the nucleus of the nucleon of the hydrogen atom.

To simplify records and calculations, the mass of the nucleus is often determined in atomic mass units (a.m.u.) or in units of energy (by writing down the corresponding energy $E=mc^2$ instead of mass in electron volts). The atomic mass unit is $1/12$ of the mass of the carbon nuclide $^(12)_6C$. In these units we get:

A proton, like an electron, has its own angular momentum - spin, which is equal to $1/2$ (in units of $\hbar $). The latter, in an external magnetic field, can orient only in such a way that its projection and field directions are equal to $+1/2$ or $-1/2$. The proton, like the electron, is subject to Fermi-Dirac quantum statistics, i.e. belongs to fermions.

The proton is characterized by its own magnetic moment, which for a particle with spin $1/2$, charge $e$ and mass $m$ is equal to

For an electron, its own magnetic moment is equal to

To describe the magnetism of nucleons and nuclei, the nuclear magneton is used ($1836$ times smaller than the Bohr magneton):

At first, it was believed that the magnetic moment of the proton is equal to the nuclear magneton, because. its mass is $1836$ times the mass of an electron. But the measurements showed that in fact the intrinsic magnetic moment of the proton is $2.79$ times greater than that of the nuclear magnetron, has a positive sign, i.e. direction coincides with the spin.

Modern physics explains these disagreements by the fact that protons and neutrons are mutually transformed and for some time remain in a state of dissociation into $\pi ^\pm $ - a meson and another nucleon of the corresponding sign:

The rest mass of the $\pi ^\pm $ - meson is $193.63$ MeV, so its own magnetic moment is $6.6$ times greater than the nuclear magneton. Some effective value of the magnetic moment of the proton and $\pi ^+$ -- of the meson environment appears in the measurements.

Neutron ($n$) -- electrically neutral particle; its rest mass

Although the neutron is devoid of charge, it has a magnetic moment $\mu _n=-1.91\mu _Я$. The "$-$" sign shows that behind the direction the magnetic moment is opposite to the spin of the proton. The magnetism of the neutron is determined by the effective value of the magnetic moment of the particles into which it is able to dissociate.

In the free state, the neutron is an unstable particle and randomly decays (half-life $12$ min): emitting a $\beta $ -- particle and an antineutrino, it turns into a proton. The neutron decay scheme is written in the following form:

In contrast to the intranuclear decay of the $\beta $ neutron -- decay belongs to both internal decay and elementary particle physics.

The mutual transformation of the neutron and proton, the equality of spins, the approximation of masses and properties give grounds to assume that we are talking about two varieties of the same nuclear particle- nucleon. The proton-neutron theory agrees well with experimental data.

As constituents of the nucleus, protons and neutrons are found in numerous fission and fusion reactions.

In arbitrary and piece fission of nuclei, flows of electrons, positrons, mesons, neutrinos and antineutrinos are also observed. The mass $\beta $ of a particle (electron or positron) is $1836$ times less than the mass of a nucleon. Mesons - positive, negative and zero particles - occupy an intermediate place in mass between $\beta $ - particles and nucleons; the lifetime of such particles is very short and amounts to millionths of a second. Neutrinos and antineutrinos are elementary particles whose rest mass is zero. However, electrons, positrons and mesons cannot be constituents of the nucleus. These light particles cannot be localized in a small volume, which is a nucleus with radius $\sim 10^(-15)\ m$.

To prove this, we define the energy of the electrical interaction (for example, an electron with a positron or proton in the nucleus)

and compare it with the self-energy of the electron

Since the energy of the external interaction exceeds the electron's own energy, it cannot exist and retain its own individuality; under the conditions of the nucleus, it will be destroyed. Another situation with nucleons, their own energy is more than $900$ MeV, so they can retain their features in the nucleus.

Light particles are emitted from nuclei in the process of their transition from one state to another.