Atomic nucleus: structure, mass, composition. The structure of the atomic nucleus What is the atomic nucleus in physics
In the 1920s, physicists no longer had any doubts about the complexity of the structure of the atomic nuclei discovered by Rutherford in 1911. On the given fact indicated a large number of different experiments completed by that time, such as:
- discovery of the phenomenon of radioactivity,
- experimental proof of the nuclear model of the atom,
- measurement of the ratio e m for an electron, an α-particle and for an H-particle, which is the nucleus of a hydrogen atom,
- discovery of artificial radioactivity and nuclear reactions,
- measurement of the charges of atomic nuclei and many others.
What particles make up the nuclei of atoms? In our time, it is a fact that the nuclei of atoms of various elements consist of two kinds of particles, that is, neutrons and protons. The second of these particles is a hydrogen atom that has lost its only electron. Such a particle was already noticed in the experiments of J. Thomson in 1907. The scientist was able to measure her e m ratio.
Definition 1
E. Rutherford in 1919 discovered atomic nuclei of hydrogen in the products of fission of atomic nuclei of a significant number of elements. The physicist named the found particle proton. He suggested that the composition of any of the nuclei of atoms includes protons.
The scheme of Rutherford's experiments is illustrated in Figure 6. five . one .
Figure 6. five . one . Scheme of Rutherford's experiments on the detection of protons in nuclear fission products. K is a lead container with a radioactive source of α-particles, F is a metal foil, E is a screen coated with zinc sulfide, M is a microscope.
Rutherford's device consisted of an evacuated chamber with a container TO where the source was α -particles. Metal foil, shown as F, overlapped the camera window. The thickness of the foil was selected in such a way as to prevent penetration through it α -particles. Outside the window was a screen coated with zinc sulfide, in image 6. five . 1 marked with the letter E. Using a microscope M, it was possible to observe light flashes or, as they are also called, scintillations at points, at the points of the screen, at which heavy charged particles hit.
In the process of filling the chamber with nitrogen at low pressure, flashes of light were detected on the screen. This phenomenon pointed to the fact that under the experimental conditions there is a flow of unknown particles that have the ability to penetrate through almost completely blocking the flow α -particle foil F. Time after time, removing the screen from the camera window, E. Rutherford was able to measure the mean free path of the observed particles in the air. The obtained value turned out to be approximately equal to 28 cm, which coincided with the estimate of the path length of H-particles observed earlier by J. Thomson.
With the help of studies of the effect of electric and magnetic fields on particles knocked out of nitrogen nuclei, data were obtained on the positivity of their elementary charge. It was also proved that the mass of such particles is equivalent to the mass of the nuclei of hydrogen atoms.
Subsequently, the experiment was performed with a number of other gaseous substances. In all such experiments carried out, it was found that from their nuclei α -particles knock out H-particles or protons.
According to modern measurements, the positive charge of the proton is absolutely equivalent to the elementary charge e = 1.60217733 10 - 19 K l. In other words, modulo it is equal to the negative charge of the electron. In our time, the equality of the charges of the proton and electron has been verified with an accuracy of 10 - 22. Such a coincidence of the charges of two significantly different particles causes sincere bewilderment and to this day remains one of the fundamental mysteries of modern physics.
Definition 2
Based on modern measurements, we can state that the mass of a proton is equal to mp = 1, 67262 10 - 27 kg. In the conditions of nuclear physics, the mass belonging to particles is often expressed in atomic mass units (a.m.u.), equal to the mass of a carbon atom with mass number 12:
1 a. e.m. = 1.66057 10 - 27 kg
Accordingly, m p \u003d 1, 007276 a. eat.
Quite often, the expression for the mass of a particle is most convenient when using equivalent energy values in accordance with the following formula: E = m c 2 . Due to the fact that 1 e V \u003d 1.60218 10 - 19 J, in energy units the proton mass is 938.272331 M e V.
Consequently, the experiment of Rutherford, who discovered the phenomenon of splitting of nitrogen nuclei and other elements of the periodic table under the conditions of impacts of fast α-particles, also showed that protons are part of atomic nuclei.
As a result of the discovery of protons, some physicists came up with the assumption that new particles are not just part of the nuclei of atoms, but are its only possible elements. However, due to the fact that the ratio of the charge of the nucleus to its mass does not remain constant for different nuclei, as it would be if the nuclei included only protons, this assumption was recognized as untenable. For heavier nuclei, this ratio turns out to be smaller than for light ones, from which it follows that on going to heavier nuclei, the mass of the nucleus increases faster than the charge.
In 1920, E. Rutherford put forward a hypothesis about the presence in the composition of the nuclei of a certain compact rigidly bound pair consisting of an electron and a proton. In the understanding of the scientist, this bundle was an electrically neutral formation as a particle with a mass practically equivalent to the mass of a proton. He also came up with a name for this hypothetical particle, Rutherford wanted to call it a neutron. Unfortunately, this idea, despite its beauty, was erroneous. It was found that an electron cannot be part of a nucleus. A quantum mechanical calculation based on the uncertainty relation shows that an electron localized in the nucleus, i.e., a region of size R ≈ 10–13 cm, must have an incredible kinetic energy, which is many orders of magnitude greater than the binding energy of nuclei per particle.
The idea of the existence of some heavy neutrally charged particle in the nucleus was extremely attractive to Rutherford. The scientist immediately turned to a group of his students, led by J. Chadwick, with a proposal to look for her. After 12 years, in 1932 Chadwick spent pilot study radiation arising under conditions of irradiation of beryllium with α-particles. In the process, he discovered that this radiation is a stream of neutral particles with a mass almost equivalent to that of a proton. Thus the neutron was discovered. Figure 6. five . 2 illustrates a simplified diagram of a setup for detecting neutrons.
Figure 6. five . 2. Scheme of installation for detection of neutrons.
In the process of bombarding beryllium with α-particles emitted by radioactive polonium, powerful penetrating radiation appears, capable of passing through an obstacle in the form of a 10-20 cm layer of lead. This radiation was discovered almost at the same time as Chadwick, the daughter of Marie and Pierre Curie, Irene and Frederic Joliot-Curie, but they suggested that these are γ-rays of high energy. They noticed that if a paraffin plate is installed in the path of beryllium radiation, then the ionizing ability of this radiation increases abruptly. The couple proved that the radiation of beryllium knocks out the protons present in the given hydrogen-containing substance in large quantities from the paraffin. Using the value of the mean free path of protons in air, scientists have estimated the energy of γ-quanta, which have the ability to impart the desired speed to protons under collision conditions. The energy value obtained as a result of the evaluation turned out to be huge - about 50 MeV.
In 1932, J. Chadwick carried out a whole series of experiments aimed at a comprehensive study of the properties of the radiation that arises when beryllium is irradiated with α particles. In his experiments, Chadwick used various methods for studying ionizing radiation.
Definition 3
Figure 6. five . 2 illustrated Geiger counter, an instrument used to detect charged particles.
This device consists of a glass tube coated on the inside with a metal layer (cathode) and a thin thread running along the axis of the tube (anode). The tube is filled with an inert gas, usually argon, at low pressure. A charged particle in the process of moving in a gas causes ionization of molecules.
Definition 4
The free electrons arising as a result of ionization are accelerated by the electric field between the anode and cathode to energies at which the phenomenon of impact ionization begins. An avalanche of ions appears, and a short discharge current pulse passes through the counter.
Definition 5
Another instrument of great importance for the study of particles is the cloud chamber, in which a fast charged particle leaves a trace or, as it is also called, a track.
The particle trajectory can be photographed or observed directly. The foundation of the operation of the cloud chamber created in 1912 is the phenomenon of condensation of supersaturated vapor on ions that form in the working volume of the chamber along the trajectory of a charged particle. Using a cloud chamber, it becomes possible to observe the curvature of the trajectory of a charged particle in electric and magnetic fields.
Proof 1
In his experiments, J. Chadwick observed traces of nitrogen nuclei that had collided with beryllium radiation in a cloud chamber. Based on these experiments, the scientist estimated the energy of the γ-quantum, which is capable of informing the nitrogen nuclei of the speed observed in the experiment. The value obtained was 100 - 150 MeV. The γ-quanta emitted by beryllium could not have such a huge energy. Proceeding from this fact, Chadwick concluded that from beryllium, under the influence of α-particles, not massless γ-quanta fly out, but rather heavy particles. These particles possessed considerable penetrating power and did not directly ionize the gas in the Geiger counter; accordingly, they were electrically neutral. Thus, the existence of the neutron, the particle predicted by Rutherford more than 10 years before Chadwick's experiments, was proved.
Definition 6
Neutron is an elementary particle. Its representation as a compact proton-electron pair, as Rutherford initially assumed, will be erroneous.
Based on the results of modern measurements, we can say that the mass of the neutron m n = 1.67493 10 - 27 kg g = 1.008665 a.u. eat.
In energy units, the mass of a neutron is equivalent to 939.56563 MeV. The mass of a neutron is approximately two electron masses greater than the mass of a proton.
Immediately after the discovery of the neutron, the Russian scientist D. D. Ivanenko, together with the German physicist W. Heisenberg, put forward a hypothesis about the proton-neutron structure of atomic nuclei, which was fully confirmed by subsequent studies.
Definition 7
Protons and neutrons are called nucleons.
A number of notations are introduced to characterize atomic nuclei.
Definition 8
The number of protons that make up the atomic nucleus is denoted by the symbol Z and is called charge number or atomic number(this is the serial number in the periodic table of Mendeleev).
The nuclear charge is Z e , where e is the elementary charge. The number of neutrons is denoted by the symbol N.
Definition 9
The total number of nucleons (i.e., protons and neutrons) is called the nuclear mass number A:
Definition of isotope concept
Nuclei chemical elements denoted by the symbol X Z A , where X is the chemical symbol of the element. For example,
H 1 1 - hydrogen, He 2 4 - helium, C 6 12 - carbon, O 8 16 - oxygen, U 92 238 - uranium.
Definition 10
The number of neutrons in the nuclei of the same chemical element can be different. Such nuclei are called isotopes.
Most of the chemical elements have several isotopes. For example, hydrogen has three of them: H 1 1 - ordinary hydrogen, H 1 2 - deuterium and H 1 3 - tritium. Carbon has 6 isotopes, oxygen has 3.
Chemical elements in natural conditions most often they are a mixture of isotopes. The existence of isotopes determines the value of the atomic mass of a natural element in periodic system Mendeleev. So, for example, the relative atomic mass of natural carbon is 12.011.
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Composition and characteristics of the atomic nucleus.
The nucleus of the simplest atom - the hydrogen atom - consists of one elementary particle called a proton. The nuclei of all other atoms consist of two types of elementary particles - protons and neutrons. These particles are called nucleons.
Proton . Protono (p) has charge +e and mass
m p = 938.28 MeV
For comparison, we indicate that the mass of an electron is equal to
m e = 0.511 MeV
It follows from the comparison that m p = 1836m e
The proton has a spin equal to half (s= ) and its own magnetic moment
A unit of magnetic moment called the nuclear magneton. From a comparison of the proton and electron masses, it follows that μ i is 1836 times smaller than the Bohr magneton μ b. Consequently, the intrinsic magnetic moment of the proton is approximately 660 times less than the magnetic moment of the electron.
Neutron . The neutron (n) was discovered in 1932 by an English physicist
D. Chadwick. The electric charge of this particle is zero, and the mass
m n = 939.57 MeV
very close to the mass of the proton. Neutron and proton mass difference (m n –m p)
is 1.3 MeV, i.e. 2.5 me.
The neutron has a spin equal to half (s= ) and (despite the absence of an electric charge) its own magnetic moment
μ n = - 1.91μ i
(the minus sign indicates that the directions of the intrinsic mechanical and magnetic moments are opposite). Explanation of this amazing fact will be given later.
Note that the ratio of the experimental values of μ p and μ n with a high degree of accuracy is equal to - 3/2. This was noticed only after such a value had been obtained theoretically.
In the free state, the neutron is unstable (radioactive) - it spontaneously decays, turning into a proton and emitting an electron (e -) and another particle called an antineutrino 
. The half-life (i.e., the time it takes for half of the original number of neutrons to decay) is approximately 12 minutes. The decay scheme can be written as follows:
The rest mass of the antineutrino is zero. The mass of a neutron is greater than the mass of a proton by 2.5 m e . Consequently, the mass of the neutron exceeds the total mass of the particles appearing on the right side of the equation by 1.5m e , i.e. by 0.77 MeV. This energy is released during the decay of a neutron in the form of the kinetic energy of the resulting particles.
Characteristics of the atomic nucleus . One of the most important characteristics of the atomic nucleus is the charge number Z. It is equal to the number of protons that make up the nucleus, and determines its charge, which is equal to + Z e . The number Z determines the ordinal number of a chemical element in the periodic table of Mendeleev. Therefore, it is also called the atomic number of the nucleus.
The number of nucleons (that is, the total number of protons and neutrons) in the nucleus is denoted by the letter A and is called the mass number of the nucleus. The number of neutrons in the nucleus is N=A–Z.
The symbol used to designate nuclei
where X is the chemical symbol of the element. At the top left is the mass number, at the bottom left is the atomic number (the last icon is often omitted). Sometimes the mass number is written not to the left, but to the right of the chemical element symbol 
Nuclei with the same Z but different A are called isotopes. Most chemical elements have several stable isotopes. For example, oxygen has three stable isotopes:
, tin has ten, and so on.
Hydrogen has three isotopes:
- ordinary hydrogen, or protium (Z=1, N=0),
- heavy hydrogen, or deuterium (Z=1, N=1),
– tritium (Z=1, N=2).
Protium and deuterium are stable, tritium is radioactive.
Nuclei with the same mass number A are called isobars. An example is 
And 
. Nuclei with the same number of neutrons N = A – Z are called isotons
(
,
). Finally, there are radioactive nuclei with the same Z and A, which differ in half-life. They're called isomers. For example, there are two isomers of the nucleus 
, one of them has a half-life of 18 minutes, the other - 4.4 hours.
About 1500 nuclei are known, differing either in Z, or A, or both. Approximately 1/5 of these nuclei are stable, the rest are radioactive. Many nuclei were obtained artificially using nuclear reactions.
Elements with atomic number Z from 1 to 92 are found in nature, excluding technetium (Tc, Z = 43) and promethium (Pm, Z = 61). Plutonium (Pu, Z = 94), after being obtained artificially, was found in negligible amounts in a natural mineral - resin blende. The rest of the transuranium (i.e., transuranium) elements (cZ from 93 to 107) were obtained artificially through various nuclear reactions.
The transuranium elements curium (96 Cm), einsteinium (99 Es), fermium (100 Fm) and mendelevium (101 Md) were named in honor of prominent scientists II. and M. Curie, A. Einstein, Z. Fermi and D.I. Mendeleev. Lawrencium (103 Lw) is named after the inventor of the cyclotron, E. Lawrence. Kurchatovy (104 Ku) got its name in honor of the outstanding physicist I.V. Kurchatov.
Some transuranium elements, including kurchatovium and elements 106 and 107, were obtained at the Nuclear Reactions Laboratory of the Joint Institute nuclear research scientists in Dubna
N.N. Flerov and his staff.
Core sizes . In the first approximation, the nucleus can be considered a sphere, the radius of which is determined quite accurately by the formula
(fermi is the name of the unit of length used in nuclear physics, equal to
10 -13 cm). It follows from the formula that the volume of the nucleus is proportional to the number of nucleons in the nucleus. Thus, the density of matter in all nuclei is approximately the same.
Spin of the nucleus . The spins of the nucleons add up to the resulting spin of the nucleus. The spin of the nucleon is 1/2. Therefore, the quantum number of the nuclear spin will be half-integer at odd number nucleons A and an integer or zero for even A. The spins of the nuclei do not exceed a few units. This indicates that the spins of most nucleons in the nucleus cancel each other out, being antiparallel. All even-even nuclei (i.e., a nucleus with an even number of protons and an even number of neutrons) have zero spin.
The mechanical moment of the nucleus M J is added to the moment of the electron shell 
in the total angular momentum of the atom M F , which is determined by the quantum number F.
The interaction of the magnetic moments of the electrons and the nucleus leads to the fact that the states of the atom corresponding to different mutual orientations M J and 
(i.e. different F) have slightly different energies. The interaction of the moments μ L and μ S determines the fine structure of the spectra. Interactionμ J and 
the hyperfine structure of atomic spectra is determined. The splitting of spectral lines corresponding to the hyperfine structure is so small (on the order of a few hundredths of an angstrom) that it can only be observed with instruments of the highest resolving power.
A feature of radioactive contamination, in contrast to contamination by other pollutants, is that it is not the radionuclide (pollutant) itself that has a harmful effect on humans and environmental objects, but the radiation, the source of which it is.
However, there are cases when a radionuclide is a toxic element. For example, after an accident Chernobyl nuclear power plant in environment plutonium 239, 242 Pu were thrown out with particles of nuclear fuel. In addition to the fact that plutonium is an alpha emitter and poses a significant danger when it enters the body, plutonium itself is a toxic element.
For this reason, two groups of quantitative indicators are used: 1) to assess the content of radionuclides and 2) to assess the impact of radiation on an object.
Activity- a quantitative measure of the content of radionuclides in the analyzed object. Activity is determined by the number of radioactive decays of atoms per unit time. The SI unit of activity is the Becquerel (Bq) equal to one disintegration per second (1Bq = 1 decay/s). Sometimes an off-system activity measurement unit is used - Curie (Ci); 1Ci = 3.7 × 1010 Bq.
Radiation dose is a quantitative measure of the impact of radiation on an object.
Due to the fact that the impact of radiation on an object can be assessed at different levels: physical, chemical, biological; at the level of individual molecules, cells, tissues or organisms, etc., several types of doses are used: absorbed, effective equivalent, exposure.
To assess the change in the dose of radiation over time, the indicator "dose rate" is used. Dose rate is the ratio of dose to time. For example, the dose rate of external exposure from natural sources of radiation in Russia is 4-20 μR/h.
The main standard for humans - the main dose limit (1 mSv / year) - is introduced in units of the effective equivalent dose. There are standards in units of activity, levels of land pollution, VDU, GWP, SanPiN, etc.
The structure of the atomic nucleus.
An atom is the smallest particle of a chemical element that retains all of its properties. In its structure, an atom is a complex system consisting of a positively charged nucleus of a very small size (10 -13 cm) located in the center of the atom and negatively charged electrons rotating around the nucleus in various orbits. The negative charge of the electrons is equal to the positive charge of the nucleus, while in general it turns out to be electrically neutral.
Atomic nuclei are made up of nucleons - nuclear protons ( Z- number of protons) and nuclear neutrons (N is the number of neutrons). "Nuclear" protons and neutrons differ from particles in a free state. For example, a free neutron, unlike a bound one in a nucleus, is unstable and turns into a proton and an electron.
The number of nucleons Am (mass number) is the sum of the numbers of protons and neutrons: Am = Z + N.
Proton - elementary particle of any atom, it has a positive charge equal to the charge of an electron. The number of electrons in the shell of an atom is determined by the number of protons in the nucleus.
Neutron - another kind of nuclear particles of all elements. It is absent only in the nucleus of light hydrogen, which consists of one proton. It has no charge and is electrically neutral. In the atomic nucleus, neutrons are stable, while in the free state they are unstable. The number of neutrons in the nuclei of atoms of the same element can fluctuate, so the number of neutrons in the nucleus does not characterize the element.
Nucleons (protons + neutrons) are held inside the atomic nucleus by nuclear forces of attraction. nuclear forces 100 times stronger than electromagnetic forces and therefore holds like-charged protons inside the nucleus. Nuclear forces are manifested only at very small distances (10 -13 cm), they constitute the potential binding energy of the nucleus, which is partially released during some transformations, passes into kinetic energy.
For atoms differing in the composition of the nucleus, the name "nuclides" is used, and for radioactive atoms - "radionuclides".
Nuclides call atoms or nuclei with a given number of nucleons and a given charge of the nucleus (nuclide designation A X).
Nuclides having the same number of nucleons (Am = const) are called isobars. For example, the nuclides 96 Sr, 96 Y, 96 Zr belong to a series of isobars with the number of nucleons Am = 96.
Nuclides that have the same number of protons (Z= const) are called isotopes. They differ only in the number of neutrons, therefore they belong to the same element: 234 U , 235 U, 236 U , 238 U .
isotopes- nuclides with the same number of neutrons (N = Am -Z = const). Nuclides: 36 S, 37 Cl, 38 Ar, 39 K, 40 Ca belong to the isotope series with 20 neutrons.
Isotopes are usually denoted as Z X M, where X is the symbol of a chemical element; M is the mass number equal to the sum of the number of protons and neutrons in the nucleus; Z is the atomic number or charge of the nucleus, equal to the number of protons in the nucleus. Since each chemical element has its own constant atomic number, it is usually omitted and limited to writing only the mass number, for example: 3 H, 14 C, 137 Cs, 90 Sr, etc.
Atoms of the nucleus that have the same mass numbers, but different charges and, consequently, different properties are called "isobars", for example, one of the phosphorus isotopes has a mass number of 32 - 15 Р 32, one of the sulfur isotopes has the same mass number - 16 S 32 .
Nuclides can be stable (if their nuclei are stable and do not decay) or unstable (if their nuclei are unstable and undergo changes that eventually increase the stability of the nucleus). Unstable atomic nuclei that can spontaneously decay are called radionuclides. The phenomenon of spontaneous decay of the nucleus of an atom, accompanied by the emission of particles and (or) electromagnetic radiation, is called radioactivity.
As a result of radioactive decay, both a stable and a radioactive isotope can be formed, in turn, spontaneously decaying. Such chains of radioactive elements connected by a series of nuclear transformations are called radioactive families.
Currently, IUPAC (International Union of Pure and Applied Chemistry) has officially named 109 chemical elements. Of these, only 81 have stable isotopes, the heaviest of which is bismuth. (Z= 83). For the remaining 28 elements, only radioactive isotopes, and uranium (u~ 92) is the heaviest element found in nature. The largest of the natural nuclides has 238 nucleons. In total, the existence of about 1700 nuclides of these 109 elements has now been proven, with the number of isotopes known for individual elements ranging from 3 (for hydrogen) to 29 (for platinum).
.
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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History
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. In 1921, Lisa Meitner proposed the first, proton-electron, model of the structure of the atomic nucleus, according to which it consists of protons, electrons and alpha particles: 96 . However, in 1929 there was a "nitrogen catastrophe" - V. Heitler and G. Herzberg established that the nucleus of the nitrogen atom obeys the statistics of Bose - Einstein, and not the statistics of Fermi - Dirac, as predicted by the proton-electron model: 374. Thus, this model came into conflict with the experimental results of measurements of spins and magnetic moments of nuclei. In 1932, James Chadwick discovered a new electrically neutral particle called the neutron. In the same year, Ivanenko and, independently, Heisenberg put forward a hypothesis about the proton-neutron structure of the nucleus. Later, with the development of nuclear physics and its applications, this hypothesis was fully confirmed.
Theories of the structure of the atomic nucleus
In the 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:
1 / λ = a Z − b (\displaystyle (\sqrt (1/\lambda ))=aZ-b), whereA (\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 ( but. 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:
E = M c 2 (\displaystyle E=Mc^(2)), where c (\displaystyle c) is the speed of light in vacuum.The ratio between a. e.m. and its energy equivalent in joules:
E 1 = 1 . 660539 ⋅ 10 − 27 ⋅ (2 . 997925 ⋅ 10 8) 2 = 1 . 492418 ⋅ 10 − 10 (\displaystyle E_(1)=1.660539\cdot 10^(-27)\cdot ( 2.997925\cdot 10^(8))^(2)=1.492418\cdot 10^(-10)), E 1 = 931 , 494 (\displaystyle E_(1)=931,494).Radius
Analysis of the decay of heavy nuclei refined Rutherford's estimate and related the radius of the nucleus to the mass number by a simple relationship:
R = r 0 A 1 / 3 (\displaystyle R=r_(0)A^(1/3)),where is a constant.
Since the radius of the nucleus is not a purely geometric characteristic and is 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, while the former is 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 occurring in nature, we can pay attention to their distribution according to 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 that kinetic energy which 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 = a e − b E ∗ (\displaystyle D=ae^(-b(\sqrt (E^(*))))), where:D (\displaystyle D)- average distance between levels,
E ∗ (\displaystyle E^(*)) is the excitation energy of the nucleus,
A (\displaystyle a) And b (\displaystyle b)- coefficients constant for a given kernel:
A (\displaystyle a)- average distance between the first excited levels (about 1 MeV for light nuclei, 0.1 MeV for heavy nuclei)
The composition of the nucleus of an atom. Calculation of protons and neutrons
According to modern ideas An atom consists of a nucleus and electrons around it. The nucleus of an atom, in turn, consists of smaller elementary particles - from a certain amount protons and neutrons(the common name for which is nucleons), interconnected by nuclear forces.
Number of protons in the nucleus determines the structure of the electron shell of the atom. And the electron shell determines the physicochemical properties of a substance. The number of protons corresponds to the serial number of an atom in Mendeleev's periodic system of chemical elements, also called the charge number, atomic number, atomic number. For example, the number of protons in a Helium atom is 2. In the periodic table, it stands at number 2 and is designated as He 2. The symbol for the number of protons is the Latin letter Z. When writing formulas, the number indicating the number of protons is often located below the symbol of the element or right or left: He 2 / 2 He.
Number of neutrons corresponds to a particular isotope of an element. Isotopes are elements with the same atomic number (the same number of protons and electrons) but different mass numbers. Mass number- the total number of neutrons and protons in the nucleus of an atom (denoted by the Latin letter A). When writing formulas, the mass number is indicated at the top of the element symbol on one of the sides: He 4 2 / 4 2 He (Helium isotope - Helium - 4)
Thus, to find out the number of neutrons in a particular isotope, the number of protons should be subtracted from the total mass number. For example, we know that a Helium-4 He 4 2 atom contains 4 elementary particles, since the mass number of the isotope is 4. At the same time, we know that He 4 2 has 2 protons. Subtracting from 4 (total mass number) 2 (number of protons) we get 2 - the number of neutrons in the nucleus of Helium-4.
THE PROCESS OF CALCULATION OF THE NUMBER OF PHANTOMIC PO PARTICLES IN THE NUCLEAR OF THE ATOM. As an example, we deliberately considered Helium-4 (He 4 2), the nucleus of which consists of two protons and two neutrons. Since the Helium-4 nucleus, called the alpha particle (α particle), is most efficient in nuclear reactions, it is often used for experiments in this direction. It should be noted that in the formulas of nuclear reactions, the symbol α is often used instead of He 4 2 .
It was with the participation of alpha particles that E. Rutherford carried out the first official history physics reaction nuclear transformation. During the reaction, α-particles (He 4 2) “bombarded” the nuclei of the nitrogen isotope (N 14 7), resulting in the formation of an oxygen isotope (O 17 8) and one proton (p 1 1)
This nuclear reaction looks like this:
Let us calculate the number of phantom Po particles before and after this transformation.
TO CALCULATE THE NUMBER OF PHANTOM PARTICLES BY IT IS NECESSARY:
Step 1. Calculate the number of neutrons and protons in each nucleus:
- the number of protons is indicated in the lower indicator;
- we find out the number of neutrons by subtracting the number of protons (lower indicator) from the total mass number (upper indicator).
Step 2. Calculate the number of phantom Po particles in the atomic nucleus:
- multiply the number of protons by the number of phantom Po particles contained in 1 proton;
- multiply the number of neutrons by the number of phantom Po particles contained in 1 neutron;
Step 3. Add the number of phantom particles By:
- add the received amount of phantom Po particles in protons with the received amount in neutrons in nuclei before the reaction;
- add the received amount of phantom Po particles in protons with the received amount in neutrons in nuclei after the reaction;
- compare the number of phantom Po particles before the reaction with the number of phantom Po particles after the reaction.
EXAMPLE OF THE DETAILED CALCULATION OF THE NUMBER OF PHANTOMIC PO PARTICLES IN THE NUCLEI OF ATOMS.
(Nuclear reaction involving an α-particle (He 4 2), carried out by E. Rutherford in 1919)
BEFORE REACTION (N 14 7 + He 4 2)
N 14 7
Number of protons: 7
Number of neutrons: 14-7 = 7
in 1 proton - 12 Po, which means in 7 protons: (12 x 7) \u003d 84;
in 1 neutron - 33 Po, which means in 7 neutrons: (33 x 7) = 231;
Total number of phantom Po particles in the nucleus: 84+231 = 315
He 4 2
Number of protons - 2
Number of neutrons 4-2 = 2
Number of phantom particles By:
in 1 proton - 12 Po, which means in 2 protons: (12 x 2) \u003d 24
in 1 neutron - 33 Po, which means in 2 neutrons: (33 x 2) \u003d 66
Total number of phantom Po particles in the nucleus: 24+66 = 90
Total number of phantom Po particles before the reaction
N 14 7 + He 4 2
315 + 90 = 405
AFTER REACTION (O 17 8) and one proton (p 1 1):
O 17 8
Number of protons: 8
Number of neutrons: 17-8 = 9
Number of phantom particles By:
in 1 proton - 12 Po, which means in 8 protons: (12 x 8) \u003d 96
in 1 neutron - 33 Po, which means in 9 neutrons: (9 x 33) = 297
Total number of phantom Po particles in the nucleus: 96+297 = 393
p 1 1
Number of protons: 1
Number of neutrons: 1-1=0
Number of phantom particles By:
In 1 proton - 12 Po
There are no neutrons.
The total number of phantom Po particles in the nucleus: 12
Total number of phantom particles Po after the reaction
(O 17 8 + p 1 1):
393 + 12 = 405
Let's compare the number of phantom Po particles before and after the reaction:
EXAMPLE OF A REDUCED FORM OF CALCULATION OF THE NUMBER OF PHANTOMIC PO PARTICLES IN A NUCLEAR REACTION.
famous nuclear reaction is the reaction of interaction of α-particles with the isotope of beryllium, in which the neutron was first discovered, which manifested itself as an independent particle as a result of nuclear transformation. This reaction was carried out in 1932 by the English physicist James Chadwick. Reaction formula:
213 + 90 → 270 + 33 - the number of phantom Po particles in each of the nuclei
303 = 303 - total amount phantom Po particles before and after the reaction
The numbers of phantom Po particles before and after the reaction are equal.
