The structure of the nucleus of an atom is chemistry. The structure of the nucleus of an atom

In 1911, E. Rutherford, on the basis of his experiments, substantiated the presence of a positively charged nucleus in the atom. You see Rutherford's experience scheme in the figure. Cylinder 1 contained a radioactive substance that emitted a stream of α-particles 2. They fell on thin gold foil 3 and, interacting with it, hit the screen 4. Flashes of light 5 appeared on the screen at the points of impact of the particles.

The fact that some α-particles significantly changed the direction of flight contradicted Thomson's "loose" model of the atom (see § 15-b). Rutherford reasoned like this. If some α-particles fly back, then there is a strong positive charge in the atoms of the foil. But since most of the α-particles fly through the foil, almost without deviating, this positive charge takes up a small part of the atom. They called her core atom.

Counting α-particles deviating at different angles, we determined the size of the nucleus of an atom: about 10 -14 m. This is approximately 10,000 times smaller than the atom itself. Therefore, in his nuclear model of the atom, Rutherford had to “fill” all the space in the atom that was not occupied by the nucleus with electrons. He wrote: "The atom consists of a central electric charge concentrated at a point and surrounded by a uniform spherical distribution of opposite electricity of equal magnitude."

Rutherford did not indicate exactly how the electrons are located in the atom and whether they move. His experiment could not give an answer to this question, but it was born assumption that electrons move around the nucleus like planets around the sun. However, then the electrons would have centripetal acceleration (see § 12-k). And, like any accelerated charges, they would radiate electromagnetic waves (see § 11-h). Over time, losing energy, all the electrons would inevitably "fall" on the nucleus. But the size of any atoms does not decrease. So the hypothesis about the movement of electrons is wrong?

The first step towards removing the contradiction between planetary model of the atom and its durability was made in 1913 by the Dane N. Bohr. He developed Planck's ideas about the quantization of energy (see § 15-a) and suggested that quanta are emitted by atoms, not electrons. According to Bohr, an atomic system (nucleus and electrons) can be in energetically stable states, during transitions between which radiation quanta are emitted or absorbed that transfer energy.

Bohr succeeded in deriving a formula describing the positions of lines in the spectra of hydrogen and other monovalent atoms (see § 14th). The calculated positions of the spectral lines coincided with the observed spectra. Bohr's theory easily explained their origin. discontinuous nature of the admissible values ​​of the energy of the atom.

The following steps to understand the reasons for the stability of atoms led to the rejection of the concept of the electron orbit in favor of the concepts electronic levels and sublevels. Therefore, since the middle of the 20th century, the planetary model has not been used in physics. In spite of this, quantization of the energy of an atomic system is one of the main principles of physics to this day.

Let's move on to consider the structure of the nucleus of an atom. In 1919, E. Rutherford, bombarding nitrogen atoms with nuclei of helium atoms, discovered the appearance of nuclei of hydrogen atoms. A similar bombardment of sodium, aluminium, neon and other elements also generated the nuclei of hydrogen atoms. They were called protons and concluded that they are part of the nuclei of all atoms. In 1932, the English physicist D. Chadwick discovered neutron- a particle of the nucleus with a mass equal to the mass of a proton, and without an electric charge.

It is currently considered that the atomic nucleus is made up of protons and neutrons, collectively referred to as nucleons(lat. "nucleus" - the core). Nuclei are strong due to the presence of special nuclear forces acting between all nucleons. These forces are about 100 times more intense than electric ones, but they act only at distances up to 10 -15 m, that is, within the core.

The content of the article

NUCLEAR STRUCTURE. The nucleus is the central part of the atom. It contains a positive electric charge and the bulk of the mass of an atom; compared to the radius of electron orbits, the dimensions of the nucleus are extremely small: 10–15–10–14 m. The nuclei of all atoms consist of protons and neutrons, which have almost the same mass, but only the proton carries an electric charge. The total number of protons is called the atomic number Z an atom that matches the number of electrons in a neutral atom. Nuclear particles (protons and neutrons), called nucleons, are held together by very strong forces; by their nature, these forces cannot be either electric or gravitational, and in magnitude they are many orders of magnitude greater than the forces that bind electrons to the nucleus.

The first idea of ​​the true dimensions of the nucleus was given by Rutherford's experiments on the scattering of alpha particles in thin metal foils. The particles penetrated deeply through the electron shells and were deflected as they approached the charged nucleus. These experiments clearly indicated the small size of the central nucleus and pointed to a method for determining the nuclear charge. Rutherford found that alpha particles approach the center of a positive charge at a distance of about 10–14 m, and this allowed him to conclude that this is the maximum possible radius of the nucleus.

Based on such assumptions, Bohr built his quantum theory of the atom, which successfully explained discrete spectral lines, the photoelectric effect, X-rays, and the periodic table of elements. However, in Bohr's theory, the nucleus was considered as a positive point charge.

The nuclei of most atoms turned out to be not only very small - they were not affected by such means of excitation of optical phenomena as an arc spark discharge, flame, etc. An indication of the presence of some internal structure of the nucleus was the discovery in 1896 by A. Becquerel of radioactivity. It turned out that uranium, and then radium, polonium, radon, etc. emit not only short-wavelength electromagnetic radiation, X-rays and electrons (beta rays), but also heavier particles (alpha rays), and they could only come from the massive part of the atom. Rutherford used the alpha particles of radium in his scattering experiments, which served as the basis for the formation of ideas about the nuclear atom. (At the time, it was known that alpha particles were helium atoms stripped of their electrons; but the question of why some heavy atoms spontaneously emit them was not yet answered, nor was there an accurate idea of ​​the size of the nucleus.)

Discovery of isotopes.

Measurements of the masses of "channel rays", carried out by J. Thomson, F. Aston and other researchers using more advanced mass spectrometers and with greater accuracy, gave the key to the structure of the nucleus, as well as the atom as a whole. For example, measurement of the charge-to-mass ratio showed that the charge of the hydrogen nucleus, apparently, is a unit positive charge, numerically equal to the charge of the electron, and the mass m p = 1837me, where me is the mass of the electron. Helium could produce ions with a double charge, but its mass was 4 times that of hydrogen. Thus, the hypothesis expressed earlier by V. Prout that all atoms are built from hydrogen atoms was seriously shaken.

Comparing the mass of the neon atom with the known masses of other elements on his mass spectrograph, Thomson unexpectedly discovered in 1912 that two parabolas correspond to neon instead of one. Calculations of particle masses showed that one of the parabolas corresponds to particles with a mass of 20, and the other - with a mass of 22. This was the first evidence that the atoms of a certain chemical element can have different mass numbers. Since the measured (average) mass number turned out to be 20.2, Thomson suggested that neon consists of two types of atoms, 90% with a mass of 20 and 10% with a mass of 22. Since both types of atoms in nature exist in the form of a mixture and they cannot be chemically separated, the mass number of neon is 20.2.

The presence of two types of neon atoms suggested that other elements could also be mixtures of atoms. Subsequent mass spectrometric measurements showed that most natural elements are mixtures of two to ten different kinds of atoms. Atoms of the same element with different masses are called isotopes. Some elements have only one isotope, which required a theoretical explanation, as well as the fact of different abundances of elements, as well as the existence of radioactivity only in certain substances.

In connection with the discovery of isotopes, the problem of standardization arose, since chemists had previously chosen "oxygen" (16.000000 atomic mass units) as the standard, which turned out to be a mixture of four isotopes. As a result, it was decided to establish a "physical" mass scale, in which the most common isotope of oxygen was assigned a value of 16.000000 a.m.u. However, in 1961 an agreement was reached between chemists and physicists, according to which 12.00000 a.m.u. were assigned to the most common isotope carbon-12. Since the number of atoms in 1 mole of an isotope is equal to Avogadro's number N 0 , we get

Note that the atomic mass unit includes the mass of one electron, and the mass of the lightest isotope of hydrogen is almost 1% greater than 1 amu.

Discovery of the neutron.

The discovery of isotopes did not clarify the question of the structure of the nucleus. By this time, only protons were known - hydrogen nuclei and electrons, and therefore it was natural to try to explain the existence of isotopes by various combinations of these positively and negatively charged particles. One might think that the nuclei contain A protons, where A is the mass number, and A-Z electrons. In this case, the total positive charge coincides with the atomic number Z.

Such a simple picture of a homogeneous nucleus at first did not contradict the conclusion about the small size of the nucleus, which followed from Rutherford's experiments. "Natural radius" of an electron r 0 = e 2 /mc 2 (which is obtained by equating the electrostatic energy e 2 /r 0 charge distributed over the spherical shell, electron self-energy mc 2) is r 0 = 2.82 x 10 -15 m. Such an electron is small enough to be inside a nucleus with a radius of 10 -14 m, although it would be difficult to place a large number of particles there. In 1920, Rutherford and other scientists considered the possibility of a stable combination of a proton and an electron, reproducing a neutral particle with a mass approximately equal to that of a proton. However, due to the lack of an electrical charge, such particles would be difficult to detect. It is unlikely that they could also knock out electrons from metal surfaces, like electromagnetic waves during the photoelectric effect.

It was not until a decade later, after natural radioactivity had been thoroughly investigated and radioactive radiation began to be widely used to cause artificial transformation of atoms, that the existence of a new constituent of the nucleus was reliably established. In 1930, W. Bothe and G. Becker from the University of Giessen irradiated lithium and beryllium with alpha particles and, using a Geiger counter, recorded the resulting penetrating radiation. Since this radiation was not affected by electric and magnetic fields and it had a high penetrating power, the authors concluded that hard gamma radiation was emitted. In 1932, F. Joliot and I. Curie repeated experiments with beryllium, passing such penetrating radiation through a paraffin block. They found that protons with unusually high energy were emitted from the paraffin and concluded that the gamma radiation passing through the paraffin produced protons as a result of scattering. (In 1923 it was found that X-rays scatter on electrons, giving the Compton effect.)

J. Chadwick repeated the experiment. He also used paraffin and, using an ionization chamber (Fig. 1), in which the charge generated when electrons were knocked out of atoms, was collected, he measured the range of recoil protons.

Chadwick also used gaseous nitrogen (in a cloud chamber where water droplets condense along the trail of a charged particle) to absorb radiation and measure the range of nitrogen recoil atoms. Applying the laws of conservation of energy and momentum to the results of both experiments, he came to the conclusion that the detected neutral radiation is not gamma radiation, but a stream of particles with a mass close to that of a proton. Chadwick also showed that known sources of gamma radiation do not knock out protons.

This confirmed the existence of a new particle, which is now called the neutron. The splitting of metallic beryllium proceeded as follows:

Alpha particles 4 2 He (charge 2, mass number 4) collided with beryllium nuclei (charge 4, mass number 9), resulting in carbon and a neutron.

The discovery of the neutron was an important step forward. The observed characteristics of nuclei could now be interpreted by considering neutrons and protons as constituents of nuclei. On fig. 2 schematically shows the structure of several light nuclei.

The neutron is now known to be 0.1% heavier than the proton. Free neutrons (outside the nucleus) undergo radioactive decay, turning into a proton and an electron. This is reminiscent of the original hypothesis of a compound neutral particle. However, inside a stable nucleus, neutrons are bound to protons and do not spontaneously decay.

Nuclear connection.

Prout's original suggestion that all atomic masses must be integer multiples of the mass of the hydrogen atom is very close to the truth, particularly when applied to isotopes. Deviations are extremely small, always no more than 1%, and in most cases no more than 0.1%. A detailed study of the masses of isotopes has been brought to the highest degree of perfection: the measurement error at present, as a rule, does not exceed several millionths.

It has been established that the number of neutrons approximately coincides with the number of protons in an atom, i.e.

In fact, there is some excess of neutrons in heavier nuclei. Because the neutron is uncharged, the forces that hold neutrons and protons in the nucleus are not inherently electrostatic; In addition, like charges repel each other. The fact that nuclei are very difficult to split indicates the existence of large forces of nuclear attraction. Despite the small distances, the gravitational attraction between nucleons is still too weak to ensure the stability of the nucleus.

According to Einstein, the total energy of an isolated system is conserved, and mass is a form of energy: E = mc 2. In order to split such a bound system as the nucleus of a stable atom into its constituent neutrons and protons, it must be given energy. This means that the mass of neutrons and protons exceeds the mass of the nucleus by an amount

D M = ZM p + NM n – M A,Z,

where Mp and M n are the masses of the free proton and neutron, and M A,Z is the mass of the nucleus with charge Z and mass number A. This mass difference, expressed in units of energy, is called the binding energy. The conversion factor is:

1 amu = 931.14 MeV,

where 1 MeV = 10 6 eV. So the bond energy E B= D Mc 2 is the energy needed to split the nucleus into individual neutrons and protons.

The average binding energy per nucleon is E B/A, changes quite regularly with an increase in the number of nucleons in the nucleus (Fig. 3). The lightest nucleus after the proton is the deuteron 2 1 H, the splitting of which requires an energy of 2.2 MeV, i.e. 1.1 MeV per nucleon. The alpha particle 4 2 He is bound much stronger than its neighbors: its binding energy is 28 MeV. For nuclei with a mass number greater than 20, the average binding energy per nucleon remains almost constant at about 8 MeV.

The binding energy of nuclei is many orders of magnitude greater than the binding energy of valence electrons in an atom and atoms in a molecule. To remove its only electron from a hydrogen atom, an energy of 13.5 eV is sufficient; to remove internal electrons in lead, which are most strongly bound, an energy equal to 0.1 MeV is required. Consequently, all nuclear processes are associated with energies much higher than those we deal with in ordinary chemical reactions or at ordinary temperatures and pressures.

natural radioactivity.

Nuclear physics began with the phenomenon of natural radioactivity. The alpha, beta and gamma radiation emitted by uranium are of nuclear origin, while the optical and x-ray spectra correspond to the electronic structure of the atom. The alpha particles turned out to be helium nuclei. Beta particles are identical in charge and mass to the shell electrons of an atom, but their nuclear origin has been clearly demonstrated by the change in the charge of the decaying nucleus. In addition, the energy of gamma radiation significantly exceeds the energy that electrons can emit from the outer shell of an atom, therefore, this penetrating radiation is of nuclear origin.

Some naturally occurring high atomic number elements (uranium, thorium, actinium) have radioactive isotopes that decay to other radioactive isotopes (such as radium) and eventually stable lead. The lifetime of the "parent" isotope in each case is comparable to the age of the Earth, which is estimated at 10 billion years. It is assumed that a large number of radioactive substances existed during the formation of the Earth, but short-lived elements have long since turned into stable end products. It is possible that some of the isotopes that are called "stable" actually decay, but their decay periods ("lifetimes") are so long that they cannot be measured by existing methods.

An important role of radioactivity in nuclear physics is related to the fact that radioactive radiation carries information about the types of particles and energy levels of the nucleus. For example, the emission of alpha particles from the nucleus and the relative stability of the formation of two protons and two neutrons indirectly indicate the possibility of the existence of alpha particles inside the nucleus.

The distinction between natural and artificially induced radioactivity is not very important for understanding the structure of the nucleus, but the study of natural radioactive series made it possible to draw important conclusions about the age of the Earth and use such elements as sources of bombarding particles long before particle accelerators were invented.

Artificial transformations of nuclei.

Experiments with naturally radioactive elements have shown that the rate of radioactive decay cannot be influenced by conventional physical means: heat, pressure, etc. Thus, at first it seemed that there was no effective method for studying the structure of naturally stable isotopes. However, in 1919 Rutherford discovered that nuclei could be split by bombarding them with alpha particles. The first split element was nitrogen, which filled the cloud chamber as a gas. Alpha particles emitted by the thorium source collided with nitrogen nuclei and were absorbed by them, as a result of which fast protons were emitted. At the same time there was a reaction

As a result of this reaction, a nitrogen atom is converted into an oxygen atom. In this example, the binding energies of the nuclei are similar to the heat that is released during a chemical reaction, although they significantly exceed it. Subsequently, similar results were obtained with many other elements. Using various methods, it is possible to measure the energies and escape angles of the emitted charged particles, which allows quantitative experiments to be carried out.

The next step was the discovery made by J. Cockcroft and E. Walton in 1932. They found that artificially accelerated proton beams with an energy of 120 keV (i.e., much less than that of alpha particles in Rutherford's experiments) are capable of causing the splitting of lithium atoms in progress

Two helium nuclei (alpha particles) simultaneously fly out in opposite directions. The reason this reaction takes place at low energy is because of the strong bonding of the alpha particles; when a proton is added to the mass of the 7 Li nucleus, an energy is reported that is almost equivalent to the masses of two alpha particles. The rest of the energy required for the reaction to proceed is drawn from the kinetic energy of the bombarding protons.

All known elements and naturally occurring isotopes can be “artificially” converted into neighboring elements. All of these new isotopes turn out to be radioactive, but as a result of subsequent decay, they turn into stable isotopes. New elements were received, up to the element with the serial number 103; they all turned out to be radioactive with relatively short half-lives. Currently, over 1000 isotopes are known.

Energy levels of nuclei and nuclear models.

The study of nuclear reactions has convincingly demonstrated the existence of nuclear energy levels. These levels are states of the nucleus with a certain energy, to which certain quantum numbers are assigned, just like the energy levels of an atom. By analogy with optical spectroscopy, the study of radiation emitted by a nucleus during transitions between energy levels is called nuclear spectroscopy. However, as can be seen from Fig. 4, the distance between the energy levels of nuclei is much greater than between the electronic levels of atoms, and nuclear radiation, in addition to electromagnetic radiation, also includes radiation from electrons, protons, alpha particles and particles of other types.

The existence of discrete energy levels in the nucleus is evidenced by the fact that the excitation of the nucleus, leading to the emission of radiation, occurs only at certain energies of the bombarding particles, and also by the fact that the energies of the emitted particles correspond to transitions between certain levels. For example, one can measure the number of protons produced by bombarding boron-10 with monoenergetic deuterons in the reaction

and determine their momenta by deflection in a magnetic field. The recorded spectrum of protons from a target containing boron with impurities of carbon, nitrogen, and silicon is shown in Fig. 4. Sharp, sharp peaks clearly show that the energy of the nucleus is quantized like the energy of an atom.

On fig. Figure 5 shows the diagram of the energy levels of the boron-11 nucleus (11 V), with the excitation energies expressed in MeV. The uneven distribution of nuclear energy levels, which is not typical for the distribution of atomic energy levels, is due to a denser packing of nuclei and a stronger interaction of particles inside the nucleus. From excited levels corresponding to a 10 V nucleus bombarded by deuterons with an energy of 1.51 MeV, transitions can occur to any of the levels located below, accompanied by the emission of protons. If, after the emission of a proton, the 11B nucleus remains in an excited state, it can then decay, passing into the lowest, "ground" state with the emission of one or more gamma quanta.

Currently, there is no consistent and unified explanation of the causes of the emergence of nuclear energy levels, but there are a number of theories that can explain some phenomena. One of them is the "shell model", which, borrowing from atomic physics the concept of the shell structure of the atom, applied it to the analysis of the configurations of neutrons and protons inside the nucleus.

In 1932, J. Bartlett noticed that all stable nuclei located between 4 He and 16 O belong to the sequence

4 He+ n+p+n+p +...,

while between 16 O and 36 Ar the analogous sequence takes the form

16O+ n+n+p+p+n+n +....

He suggested that these changes in the sequence reflect the order in which the shells are filled with neutrons and protons. The Pauli exclusion principle operates in the case of nuclear particles in exactly the same way as in the case of electrons, and in the shell model it leads to the fact that the first shell can contain only two protons and two neutrons, and the second shell can contain six of both particles (filled at 16 O) and on the third by ten (filled at 36 Ar). The presence of periodicity in the structure of nuclei manifests itself further, although with some deviations. The existence of certain "magic numbers" (2, 8, 20, 28, 50, 82 and 126) of neutrons and protons in nuclei, which correspond to the peaks of the binding energy curve, can be explained on the basis of a modified shell model (called the model of independent particles), which allows correctly predict the spins and magnetic moments of nuclei. For example, the spins of nuclei with filled shells, as predicted by this model, are equal to zero. However, despite many advantages, the available versions of the shell model still do not explain all nuclear phenomena, which is not surprising in view of the complex structure of the nucleus.

Compound nucleus and drop model.

In heavier nuclei, the number of nucleons is so large that many of the observed patterns in the behavior of these nuclei are best reproduced by the drop model. This model was proposed in 1936 by N. Bohr to explain the long lifetimes of excited nuclei formed during the capture of slow neutrons. (In this case, the lifetime is understood as the time from the moment the nucleus is excited to the moment it loses the excitation energy as a result of the emission of radiation.) The lifetimes turned out to be a million times longer than the time required for a neutron to cross the nucleus (10–22 s). This indicates that the excited nucleus is a kind of system (“compound nucleus”), the lifetime of which is much longer than the time of its formation.

Bohr suggested that a nuclear reaction proceeds in two stages. At the first stage, the incident particle enters the target nucleus, forming a "compound nucleus", where in numerous collisions it loses its initial energy, distributing it among other nucleons of the nucleus. As a result, none of the particles has the energy necessary to escape from the nucleus. The second stage, the decay of the compound nucleus, occurs after some time, when the energy is accidentally concentrated on one of the particles or lost in the form of gamma radiation. It is believed that the second stage does not depend on the details of the mechanism of formation of the compound nucleus. The type of decay is determined only by the play of possible variants.

As a simple analogy to this picture of a nuclear reaction, Bohr proposed to consider the behavior of a drop. Forces act between the molecules of such a drop, connecting them with each other and preventing evaporation until heat is supplied from the outside. The appearance of another molecule with additional kinetic energy leads, as a result of its statistical redistribution, to an increase in the temperature of the drop as a whole. After some time, a random concentration of energy on any molecule can lead to its evaporation. Bohr's theory was developed in detail and made it possible to build a consistent picture of various nuclear reactions, including reactions under the action of neutrons and charged particles of intermediate energies (up to 100 MeV). The concepts of nuclear temperature, specific heat, and evaporation of particles, introduced by analogy, turned out to be useful. For example, the angular distribution of "evaporated" particles turned out to be independent of the direction of the incident particle, i.e. isotropic, since all information about the original direction is lost at the stage of existence of the compound nucleus.

The droplet model turned out to be especially valuable in explaining the phenomenon of nuclear fission, when the absorption of one slow neutron is sufficient to break up a uranium nucleus into two approximately equal parts with a large release of energy. The electrostatic repulsion of protons causes some instability in the nucleus, which is usually offset by the nuclear forces providing the binding energy. But with an increase in the nuclear temperature of a spherical “drop”, oscillations can arise in it, as a result of which the drop is deformed into an ellipsoid. If the deformation of the nucleus continues, then the electrostatic repulsion of its two positively charged halves can prevail, and then its fission will occur.

The size and shape of the nucleus.

For the first time, the size of the nucleus was correctly estimated by Rutherford, using the scattering of alpha particles for this purpose. His first experiments showed that the dimensions of the charged part of the nucleus were of the order of 10–14 m. Later and more accurate experiments made it possible to establish that the radius of the nucleus is approximately proportional to A 1/3 and, therefore, the density of the nuclear substance is almost constant. (It is colossal: 100,000 t/mm 3 .)

With the discovery of the neutron, it became clear that it is an ideal tool for studying the nucleus, since neutral particles, passing at a considerable distance from the nucleus, are not deflected by the nuclear charge. In other words, the neutron collides with the nucleus if the distance between their centers is less than the sum of their radii, otherwise it is not deflected. Experiments on neutron beam scattering showed that the radius of the nucleus (assuming a spherical shape) is equal to:

R = r 0 A 1/3 ,

r 0 » 1.4h 10 -15 m.

Thus, the radius of the nucleus of uranium-238 is equal to 8.5 × 10 -15 m. The obtained value corresponds to the radius of action of nuclear forces; it characterizes the distance from the center of the nucleus at which the outer neutral nucleon begins to "feel" its influence for the first time. Such a radius of the nucleus is comparable to the distance from the center of the nuclei, at which alpha particles and protons are scattered.

The scattering of alpha particles, protons and neutrons by nuclei is due to the action of nuclear forces; consequently, such measurements of the radii of nuclei give an estimate of the radius of action of nuclear forces. The interaction of electrons with nuclei is almost completely determined by electric forces. Therefore, electron scattering can be used to study the shape of the charge distribution in the nucleus. Experiments with electrons of very high energies, carried out by R. Hofstadter at Stanford University, gave detailed information about the distribution of positive charge along the radius of the nucleus. On fig. 6 shows the angular distribution of electrons scattered by gold nuclei with an energy of 154 MeV. The upper curve characterizes the angular distribution calculated assuming that the positive charge is concentrated at the point; Obviously, the experimental data do not correspond to this assumption. Much better agreement is achieved under the assumption of a uniform distribution of protons over the volume of the nucleus (lower curve). However, the "charge radius" turns out to be approximately 20% smaller than the "nuclear force" radius obtained from neutron scattering data. This may mean that the distribution of protons in the nucleus differs from the distribution of neutrons.

Nuclear forces and mesons.

The small radius of action of nuclear forces was first clearly revealed already in experiments on Rutherford scattering. Alpha particles approaching the center of the nucleus up to 10–14 m experienced the action of forces whose sign and magnitude differed from the usual electrostatic repulsion. More recent experiments using neutrons have shown that large short-range forces exist between all nucleons. These forces are different from the well-known electrostatic and gravitational forces, which do not disappear even at very large distances. Nuclear forces are forces of attraction, which directly follows from the fact of the existence of stable nuclei, despite the electrostatic repulsion of the protons in them. The nuclear forces between any pair of nucleons (neutrons and protons) are the same; this shows a comparison of the energy levels of "mirror nuclei", which differ from each other in that in them protons are replaced by neutrons and vice versa. Within its radius of action, nuclear forces reach a very large value. The electrostatic potential energy of two protons located at a distance of 1.5 × 10 -15 m from each other is only 1 MeV, which is 40 times less than the nuclear potential energy. Nuclear forces also saturate, since a given nucleon is only able to interact with a limited number of other nucleons. Hence the rapid initial growth (with increasing A) the average binding energy per nucleon (Fig. 3), and the relative constancy of this energy in the future. (If each nucleon interacted with all the nucleons in the nucleus, then the binding energy per nucleon would increase proportionally all the time A.)

So far, there is no satisfactory theory of nuclear forces, and the problem is being intensively studied experimentally and theoretically. However, many of the ideas underlying the "meson theory of nuclear forces" published in 1935 by H. Yukawa turned out to be in agreement with the experimental facts. Yukawa hypothesized that the attraction that holds nucleons inside the nucleus arises due to the presence of "quanta" of a certain field, similar to photons (light quanta) of the electromagnetic field and ensuring the interaction of electric charges. It follows from quantum field theory that the radius of action of a force is inversely proportional to the mass of the corresponding quantum; in the case of an electromagnetic field, the mass of quanta - photons - is equal to zero, and the range of forces is infinite. The mass of nuclear field quanta (called "mesons"), calculated from the experimentally measured range of nuclear forces, turned out to be about 200 times the mass of an electron.

The position of Yukawa's theory was strengthened after K. Anderson and S. Neddermeyer discovered in 1936 a new particle with a mass of about 200 electron masses (now called a muon), which they discovered using a cloud chamber in cosmic rays. (In 1932, Anderson discovered the "positron," the positive electron.) At first, it seemed that the quanta of nuclear forces had been found, but later experiments revealed a discouraging circumstance: the "key to nuclear forces" does not interact with nuclei! This confusing situation became clear only after S. Powell discovered in 1947 a particle with a suitable mass that interacts with nuclei. This particle (called the pi-meson, or pion) turned out to be unstable and spontaneously decayed, turning into a muon. The pi-meson fit the role of the Yukawa particle, and its properties were studied in great detail by physicists who used cosmic rays and modern accelerators for this purpose.

Although the existence of pi-mesons encouraged supporters of the Yukawa theory, it turned out to be very difficult to correctly predict such detailed properties of nuclear forces as their saturation, binding energies, and nuclear level energies on its basis. Difficulties of a mathematical nature did not allow us to establish exactly what this theory predicts. The situation has become even more complicated after the discovery of new types of mesons, which are believed to be relevant to nuclear forces.

The only stable atom that does not contain neutrons in the nucleus is light hydrogen (protium).

The atomic nucleus, considered as a class of particles with a certain number of protons and neutrons, is commonly called nuclide.
In some rare cases, short-lived exotic atoms can be formed, in which other particles serve as the nucleus 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 table  (Periodic system 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 a nucleus is called its 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 varieties 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, indicated in the periodic table. Nuclides with the same mass number but different proton-neutron composition are 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.

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The scattering of charged particles can be explained by assuming 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 such a structure of the atom, α- and β-particles, when they pass at a close distance from the center of the atom, experience large deviations, although the probability of such a deviation is small.

Thus, Rutherford discovered the atomic nucleus, from that moment nuclear physics began, studying 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 a structural particle of all nuclei. Since 1920, the nucleus of the hydrogen atom has had an official term - 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 the 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 process of development of physics, various hypotheses were put forward for the structure of the atomic nucleus; 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:

• Drop model nucleus - proposed in 1936 by Niels Bohr.
• Shell model nucleus - proposed in the 30s of the XX century.
• Generalized Bohr-Mottelson model
• Cluster kernel model
• Model of nucleon associations
• Superfluid core model
• Statistical model of the nucleus

Nuclear physics

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

A (\displaystyle a) and b (\displaystyle b)- permanent.

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

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 units mass ( a. eat.), for 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, it is necessary 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, in nuclear physics, the energy equivalent mass is often used. According to the Einstein relation, each mass value M (\displaystyle M) corresponds to the total energy:

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

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:

where is a constant.

Since the radius of the nucleus is not a purely geometric characteristic and is associated primarily with the radius of action of nuclear forces, the value r 0 (\displaystyle r_(0)) depends on the process in the analysis of which the value is obtained R (\displaystyle R), average value r 0 = 1 , 23 ⋅ 10 − 15 (\displaystyle r_(0)=1.23\cdot 10^(-15)) m, thus the core radius in meters:

R = 1 , 23 ⋅ 10 − 15 A 1 / 3 (\displaystyle R=1,23\cdot 10^(-15)A^(1/3)).

Kernel moments

Like the nucleons that make it up, 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 moment of momentum of each nucleon. Orbital moments take only integer values ℏ (\displaystyle \hbar )(constant Dirac). All mechanical moments of nucleons, both spins and orbital, are summed 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 cancel each other out, that is, pairs always interact with antiparallel spins. The total orbital momentum of a pair is also always zero. As a result, nuclei consisting of an even number of protons and an even number of neutrons do not have a mechanical momentum. Non-zero spins exist only for nuclei that have unpaired nucleons in their composition, the spin of such a nucleon is added to its own orbital momentum and has some half-integer value: 1/2, 3/2, 5/2. Nuclei of odd-odd composition have integer spins: 1, 2, 3, etc. .

Magnetic moment

The measurements of spins became possible due to the presence of magnetic moments directly related to them. They are measured in magnetons and for different nuclei they are from -2 to +5 nuclear magnetons. Due to the relatively large mass of nucleons, the magnetic moments of nuclei are very small compared to those of electrons, so measuring them is much more difficult. Like spins, magnetic moments are measured by spectroscopic methods, the most accurate being the nuclear magnetic resonance method.

The magnetic moment of even-even pairs, like the spin, is equal to 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 with a spin greater than or equal to unity have non-zero quadrupole moments, indicating that they are not exactly spherical. The quadrupole moment has a plus sign if the nucleus is extended 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.

Bond energy

Core Stability

From the fact that the average binding energy decreases for nuclides with mass numbers greater than 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)- the process of division. At present, both of these processes, leading to the release of energy, have been carried out, the latter being the basis of modern nuclear energy, and the former being under development.

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

If we consider the table of stable nuclides found in nature, we can pay attention to their distribution by even and odd values. 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 the isobars with odd A, as a rule, only one is stable. In the case of even A (\displaystyle A) often there are two, three or more stable isobars, therefore, the most stable are even-even, the least - odd-odd. This phenomenon indicates that both neutrons and protons tend to cluster in pairs with antiparallel spins, which leads to a violation of 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, which differ 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 attractive forces that act only at small distances. They have saturation properties, in connection with which the nuclear forces are assigned an exchange character (with the help of pi-mesons). Nuclear forces are spin dependent, independent of electric charge, and are not central forces.

Kernel levels

Unlike free particles, for which the energy can take on any value (the so-called continuous spectrum), bound particles (that is, particles whose kinetic energy is less than the absolute value of the potential), according to quantum mechanics, can only be in states with certain discrete energy values , 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 main. If energy is transferred to the nucleus, it will turn into excited state.

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

D (\displaystyle D)- average distance between levels,

Atom is the smallest particle of a chemical element. The chemical properties of an element depend on the structure of the atom, in particular, its ability to combine with atoms of other elements to form molecules of complex substances.

An atom consists of a nucleus and electrons revolving around it in certain orbits. An atomic nucleus carries an electrical charge q= Ze, whereZ – the ordinal number of the element in the periodic table, and e- so-called elementary electric charge, which cannot be divided into parts: e\u003d 1.6 10 -19 C (coulomb). The charge of an elementary particle of an electron is - e, and the number of electrons in the orbits of the atom is Z, so their total charge q e = – Ze in absolute value it is equal to the charge of the nucleus, but opposite in sign, therefore the atom as a whole is electrically neutral.

The sizes of atoms of all elements are approximately the same, and their radii are approximately ≈ 10 -8 cm.

1.1.2. The structure and properties of the atomic nucleus

atomic nucleus- the central part of an atom, in which almost all of its mass is concentrated. The atomic nucleus consists of elementary particles - nucleons, which have two varieties called protons (p) and neutrons (n) . All the main characteristics of protons and neutrons - sizes, masses, and others - are almost the same, and their main difference lies in the electric charge: the proton charge is + e, and the neutron charge is equal to zero, i.e. the neutron is electrically neutral.

Different atomic nuclei contain different numbers of nucleons of each kind. Number of protons in the nucleus Z coincides with the serial number of the chemical element and determines the electric charge of the nucleus (see above). Number of neutrons N does not affect the charge of the nucleus, and, consequently, the belonging of the atom to one or another element. Therefore, the nuclei of atoms of one element have the same Z , but may have different N. Varieties of the same element with different numbers of neutrons in their nuclei are called isotopeami. Since the masses of protons and neutrons are almost the same, the mass of the nucleus is determined in the first approximation by the total number of all nucleons N + Z = A . Therefore the number A called mass number. When designating isotopes, the mass number is indicated at the top left of the chemical element symbol. So, for example, hydrogen isotopes are known: ordinary hydrogen, the nucleus of which is a single proton - 1 H, heavy hydrogen (deuterium), in the nucleus of which one neutron is added to the proton - 2 H, and superheavy hydrogen (tritium) 3 H, the nuclei of which are one proton and two neutrons. Deuterium and tritium are sometimes denoted by the symbols D and T, respectively. All elements have isotopes, and in some cases their number reaches two or three dozen. For natural uranium ( Z\u003d 92) there are three isotopes: 234 U, 235 U and 238 U, and in addition to this, several more isotopes are artificially obtained: 232 U, 233 U, 236 U, 239 U and others. All isotopes of one element have the same chemical properties - they enter into the same chemical reactions, form the same chemical compounds, etc., but their nuclear properties can vary greatly. For example, 235 U nuclei are fissile by slow neutrons, while 238 U nuclei are not (see below).

Nuclei with the same number of all nucleons, and therefore with the same mass numbers A, called isobarami, i.e. kernels of approximately the same weight (from the Greek word baros - weight). Isobars are, for example, the nuclei 3 H and 3 He, or 58 Fe and 58 Ni. Sometimes nuclei are isolated isotones, containing the same number of neutrons, and nuclei isomers, which will be discussed in more detail below.

Possibility of various combinations of numbers Z and N leads to the possibility of the existence of a huge number of different types of nuclei. Each type of kernels with certain values Z and N called nuclide. There are about 300 different nuclides in nature and over 2000 more can be produced artificially.

Like electric charges of protons repel each other according to the laws of electrostatics, trying to break the nucleus apart. Nevertheless, it is known that the nuclei of many nuclides are extremely durable objects that can exist almost forever without any changes. This fact indicates that some powerful forces of attraction act between nucleons in the nucleus, much greater than the forces of electrostatic repulsion. These forces are called nuclear forces. Nuclear forces have a number of specific properties that sharply distinguish them from all other forces in nature. With their large size, the huge reserves of energy contained in atomic nuclei are associated.

Sizes of atomic nuclei extremely small - about 10 -12 cm. This means that the nucleus is 10,000 times smaller than the atom itself. But it is in these nuclei that more than 99.9% of the mass of all matter and huge reserves of energy are concentrated. Experiments show that the radii of all nuclei depend on the number of nucleons in the nucleus and are expressed by a simple formula:

R=1.4 10 -13 A 1/3 cm.

The mass of the nucleus. In the beginning, we note that in nuclear physics it is customary to deal not with the masses of nuclei, but with the masses of atoms, since they are easier to measure, and if necessary, the mass of the nucleus can always be easily found by subtracting from the mass of the atom M the total mass of electrons Zm e, because the mass of an electron is well known: m e\u003d 9.108 10 -28 g. To express the masses of atoms in nuclear physics, a special unit is adopted, which is called atomic mass unit(amu) and is defined as one twelfth of the mass of an atom of the main carbon isotope 12 C. 1 amu = 1.66 10 -27 kg \u003d 1.66 10 -24 g. Expressed in in these units, the mass of an atom is called atomic mass M. The unit of atomic mass was chosen specifically in such a way that atomic masses, rounded to whole numbers, would coincide with atomic numbers, i.e. with the number of nucleons in the nucleus. For instance:

M(1 N) = 1.007825 amu,

M(238 U)=238.05076 amu

The difference between atomic mass and mass number is called excess or mass decrement: δ = M - A. It is these quantities that are usually given in tables so as not to clutter them with unnecessary numbers, and knowing the decrement, you can always find the exact value of the mass of the atom M=A +δ.

Atomic masses are measured using special instruments. mass spectrographs and mass spectrometers, the principle of operation of which is based on the deflection of ion beams in electric and magnetic fields: the heavier the ion, the less it deflects when passing through such fields. Therefore, the mass of the ion can be determined from the magnitude of the deviation.

Different physical methods of their separation are also based on the difference in the masses of atoms of different isotopes, because chemical methods of separating substances are completely unsuitable for separating isotopes.

Core binding energy. It is possible to overcome the effect of nuclear forces by introducing a sufficient amount of energy into the nucleus. The amount of energy required to break the nucleus into individual nucleons is called nuclear binding energy. The same amount of energy would be released during the formation of a nucleus from individual nucleons, leaving the system in the form of emitted gamma quanta. The binding energy of any nucleon or groups of nucleons is determined similarly, for example: the binding energy of a neutron in a nucleus is the energy that must be expended to pull out one neutron from the nucleus.

Specific binding energy of nucleons in the nucleusV . This is the name of the fraction of the total binding energy of the nucleus, which falls on average per nucleon in the nucleus. From this definition follows: B = E St. /A. Value V depends on the number of nucleons in the nucleus A(Fig. 1): with growth A values V first increase sharply, and then, having passed a smooth maximum, gradually decrease. At the same time, for most nuclei (except for the lightest ones), the values V do not differ much from 8 MeV. The shape of the specific binding energy curve in Fig. 1.1 indicates that the nuclei with average values ​​are the most tightly bound A. That is why how fission processes of heavy nuclei, so fusion processes of light nuclei, leading to the formation of nuclei with average mass numbers are "energetically favorable", i.e. accompanied by the release of enormous energy. Therefore, these processes are based on two well-known methods for obtaining "nuclear" energy - this is the fission of heavy nuclei and the synthesis of light nuclei (thermonuclear), respectively.

mass defect. According to the theory of relativity, any change in the energy of a system is accompanied by a change in its mass: Δ E =Δ M s 2 . Since binding energy is released from individual nucleons during the formation of a nucleus, according to the last relation, such a process should lead to a decrease in the mass of the system. Therefore, the mass of the nucleus always turns out to be less than the sum of the masses of the individual nucleons that make up this nucleus:

Δ M=ZM H + (A – Z)m n – M(A,Z) .

Fig.1.1. Dependence of the specific binding energy on the mass number of the nuclide.

This decrease in mass during the formation of a nucleus is called mass defect(here M H is the mass of a hydrogen atom, m n is the neutron mass, M(A, Z) is the mass of the atom in question. Recall that although the masses of atoms appear in this expression, the atom (A, Z) contains exactly the same number of electrons as Z hydrogen atoms, so the masses of electrons are reduced, and the mass defect actually expresses the difference in the masses of individual nucleons and the nucleus in question.

It follows from the above that the mass defect determines the binding energy of the nucleus: E St. =Δ M s 2 . This expression can be substantially simplified if, using the relation E= M s 2 find the amount of energy corresponding to one atomic mass unit: 1a.u.m = 931.5 MeV. Then, having calculated the value Δ M in atomic mass units, one can easily find the value of the binding energy in MeV: E sv (MeV) = 931.5 Δ M(a.u.m.).

The expression for the mass defect can also be simplified by expressing all the masses included in it in terms of the corresponding decrements: M H= 1 + δ(Н), m n = 1 + δ n , M(A, Z) = A +δ( A, Z), which, after canceling like terms, gives:

Δ M= Zδ(N) +(A-Z) δ n – δ( A,Z).

Energy states of nuclei. Nucleons and atomic nuclei consisting of them, like all other elementary particles, obey the laws of quantum mechanics, which differ in many respects from the laws of classical physics. In particular, energy in the microcosm can change only in certain portions (quanta), and not continuously, as in classical mechanics. Accordingly, the nucleus can only be in states with certain values ​​of energy, and intermediate states are impossible. These states are usually denoted in the diagrams by dashes, which are called energy levels(Fig. 1.2). Energy in such schemes is deposited from the bottom up. The state with the lowest possible energy is called main, other - excited. Usually, all nuclei are in their ground states, but having received a sufficient portion of energy, they can go into one of the excited states. Energy E i , necessary for the transition of the nucleus to i-th state, indicated on the level diagrams next to the corresponding level (the energy of the ground state is taken as 0). Caught on i-th level, the core can go to any To th level with less energy. With such a transition, the energy difference is released, which is carried away by the gamma-ray quantum emitted from the nucleus: E γ = E i – E To. After several such transitions, called cascade, the core comes to the ground state. The time the nucleus spends in an excited state is called lifetime corresponding level and is denoted by the letter τ. For the lower excited levels, the values ​​of τ are usually of the order of 10 -10 - 10 -12 s, for the upper levels - even less, of the order of 10 -15 - 10 -17 s. However, some nuclei have excited levels with anomalously long lifetimes from a few seconds to millions of years. Such long-lived levels are called metastable levels, and the phenomenon as a whole - nuclear isomerism.

Fig.1.2. Diagram of nuclear levels

In addition to energy, each level is characterized by a number of other quantities, including angular momentum. In quantum mechanics, the angular momentum is the quantity P=ћ√ I(I+1), where I- so-called. quantum number of angular momentum. Since the value P uniquely determined by the number I, then usually, speaking of the moment of momentum, only this number is called. According to the laws of quantum mechanics, for nuclei with an even number of nucleons, the values I can only be integers: 0, 1, 2, 3, ...., and for nuclei with an odd number of nucleons - only half-integers: 1/2, 3/2, 5/2, etc. Each excited level has its own number value I, usually determined experimentally. Numbers I strongly influence the probability of transitions of the nucleus between levels: the greater the difference between the values I between initial and final levels Δ I= I i - I k, the less likely the transition.

The atomic nucleus is the central part of the atom, in which its main mass is concentrated (more than 99.9%). The nucleus is positively charged, the charge of the nucleus determines the chemical element to which the atom is assigned. The dimensions of the nuclei of various atoms are several femtometers, which is more than 10 thousand times smaller than the size of the atom itself.

The atomic nucleus, considered as a class of particles with a certain number of protons and neutrons, is commonly called a nuclide. The number of protons in the nucleus is called its charge number - this number is equal to the ordinal number of the element to which the atom belongs in the table (Periodic system 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 a nucleus is called its isotopic number. Nuclei with the same number of protons and different numbers of neutrons are called isotopes.

In 1911, Rutherford, in his report "Scattering of α- and β-rays and the structure of the atom" in the Philosophical Society of Manchester, stated:

The scattering of charged particles can be explained by assuming 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 such a structure of the atom, α- and β-particles, when they pass at a close distance from the center of the atom, experience large deviations, although the probability of such a deviation is small.

Thus, Rutherford discovered the atomic nucleus, from that moment nuclear physics began, studying 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 a structural particle of all nuclei. Since 1920, the nucleus of the hydrogen atom has been officially termed 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 the 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.

Radioactivity

Radioactive decay (from Latin radius "beam" and āctīvus "effective") is a spontaneous change in the composition (charge Z, mass number A) or the internal structure of unstable atomic nuclei by emitting elementary particles, gamma quanta and / or nuclear fragments. The process of radioactive decay is also called radioactivity, and the corresponding nuclei (nuclides, isotopes and chemical elements) are radioactive. Substances containing radioactive nuclei are also called radioactive.

The law of radioactive decay is a law discovered experimentally by Frederick Soddy and Ernest Rutherford and formulated in 1903. The modern wording of the law:

which means that the number of decays in a time interval t in an arbitrary substance is proportional to the number N of radioactive atoms of a given type present in the sample.

In this mathematical expression, λ is the decay constant, which characterizes the probability of radioactive decay per unit time and has the dimension c −1 . The minus sign indicates a decrease in the number of radioactive nuclei over time. The law expresses the independence of the decay of radioactive nuclei from each other and from time: the probability of decay of a given nucleus in each subsequent unit of time does not depend on the time that has elapsed since the beginning of the experiment, and on the number of nuclei remaining in the sample.

The solution to this differential equation is:

Or , where T is the half-life equal to the time during which the number of radioactive atoms or the activity of the sample decreases by 2 times.

12. Nuclear reactions.

A nuclear reaction is the process of interaction of an atomic nucleus with another nucleus or elementary particle, accompanied by a change in the composition and structure of the nucleus. The consequence of the interaction may be the fission of the nucleus, the emission of elementary particles or photons. The kinetic energy of the newly formed particles can be much higher than the initial one, and one speaks of the release of energy by a nuclear reaction.

Types of nuclear reactions

Nuclear fission reaction - the process of splitting an atomic nucleus into two (rarely three) nuclei with similar masses, called fission fragments. As a result of fission, other reaction products can also appear: light nuclei (mainly alpha particles), neutrons and gamma quanta. Fission can be spontaneous (spontaneous) and forced (as a result of interaction with other particles, primarily with neutrons). The fission of heavy nuclei is an exoenergetic process, as a result of which a large amount of energy is released in the form of the kinetic energy of the reaction products, as well as radiation.

Nuclear fission serves as a source of energy in nuclear reactors and nuclear weapons.

Nuclear fusion reaction - the process of fusion of two atomic nuclei with the formation of a new, heavier nucleus.

In addition to the new nucleus, in the course of the fusion reaction, as a rule, various elementary particles and (or) quanta of electromagnetic radiation are also formed.

Without the supply of external energy, the fusion of nuclei is impossible, since positively charged nuclei experience electrostatic repulsion forces - this is the so-called "Coulomb barrier". To synthesize nuclei, it is necessary to bring them closer to a distance of about 10 −15 m, at which the strong interaction will exceed the electrostatic repulsion forces. This is possible if the kinetic energy of the approaching nuclei exceeds the Coulomb barrier.

photonuclear reaction

When a gamma quantum is absorbed, the nucleus receives an excess of energy without changing its nucleon composition, and a nucleus with an excess of energy is a compound nucleus. Like other nuclear reactions, the absorption of a gamma-quantum by the nucleus is possible only if the necessary energy and spin ratios are met. If the energy transferred to the nucleus exceeds the binding energy of the nucleon in the nucleus, then the decay of the resulting compound nucleus occurs most often with the emission of nucleons, mainly neutrons.

Recording nuclear reactions

the method of writing formulas for nuclear reactions is similar to writing formulas for chemical reactions, that is, the sum of the initial particles is written on the left, the sum of the resulting particles (reaction products) is written on the right, and an arrow is placed between them.

Thus, the reaction of radiative capture of a neutron by a cadmium-113 nucleus is written as follows:

We see that the number of protons and neutrons on the right and left remains the same (the baryon number is preserved). The same applies to electric charges, lepton numbers and other quantities (energy, momentum, angular momentum, ...). In some reactions where the weak interaction is involved, protons can turn into neutrons and vice versa, but their total number does not change.