The discovery of the neutron gave impetus to the understanding of how the nuclei of atoms are structured.
In the same year, 1932, when the neutron was discovered, Soviet physicist Dmitry Dmitrievich Ivanenko and German physicist Werner Heisenberg proposed a proton-neutron model of the structure of nuclei, the validity of which was subsequently confirmed experimentally.
Protons and neutrons are called nucleons (from the Latin nucleus - nucleus). Using this term, we can say that atomic nuclei are made up of nucleons.
- The total number of nucleons in a nucleus is called the mass number and is denoted by the letter A
So, for example, for nitrogen the mass number A = 14, for iron A = 56, for uranium A = 235.
It is clear that the mass number A is numerically equal to the mass of the nucleus m, expressed in atomic mass units and rounded to whole numbers (since the mass of each nucleon is approximately equal to 1 amu). For example, for nitrogen m ≈ 14 a. u.m., for iron m ≈ 56 a.m. e.m., etc.
- The number of protons in the nucleus is called the charge number and is denoted by Z
For example, for nitrogen the charge number is Z = 7, for iron Z = 26, for uranium Z = 92, etc.
The charge of each proton is equal to the elementary electric charge. Therefore, the charge number Z is numerically equal to the charge of the nucleus, expressed in elementary electric charges. For each chemical element, the charge number is equal to the atomic (ordinal) number in D.I. Mendeleev’s table.
The nucleus of any chemical element in general view is designated as follows: (X means the symbol of a chemical element).
The number of neutrons in a nucleus is usually denoted by the letter N. Since the mass number A is total number protons and neutrons in the nucleus, then we can write: A = Z + N.
Based on the proton-neutron model of the structure of atomic nuclei, an explanation was given for some experimental facts discovered in the first two decades of the 20th century.
Thus, during the study of the properties radioactive elements It was discovered that the same chemical element contains atoms with nuclei of different masses.
The same charge of the nuclei indicates that they have the same serial number in D.I. Mendeleev’s table, that is, they occupy the same cell, the same place in the table. This is where the name of all varieties of one chemical element comes from: isotopes (from the Greek words isos - identical and topos - place).
- Isotopes are varieties of a given chemical element that differ in the mass of atomic nuclei
Thanks to the creation of the proton-neutron model of the nucleus (i.e., approximately two decades after the discovery of isotopes), it was possible to explain why atomic nuclei with the same charge have different masses. Obviously, the isotope nuclei contain same number protons, but different numbers of neutrons.
For example, there are three isotopes of hydrogen: (protium), . (deuterium) and (tritium). The isotope nucleus has no neutrons at all - it represents one proton. The deuterium nucleus contains two particles: a proton and a neutron. The tritium nucleus consists of three particles: one proton and two neutrons.
The hypothesis that atomic nuclei consist of protons and neutrons has been confirmed by many experimental facts.
But the question arose: why do nuclei not decay into individual nucleons under the influence of electrostatic repulsion forces between positively charged protons?
Calculations show that nucleons cannot be held together due to attractive forces of gravitational or magnetic nature, since these forces are significantly less than electrostatic ones.
In search of an answer to the question of the stability of atomic nuclei, scientists assumed that some special attractive forces act between all nucleons in the nuclei, which significantly exceed the electrostatic repulsive forces between protons. These forces were called nuclear.
The hypothesis about the existence of nuclear forces turned out to be correct. It also turned out that nuclear forces are short-range: at a distance of 10 -15 m they are approximately 100 times greater than the forces of electrostatic interaction, but already at a distance of 10 -14 m they turn out to be negligible. In other words, nuclear forces act at distances comparable to the size of the nuclei themselves.
Questions
- What are protons and neutrons together called?
- What is mass number? What can be said about the numerical value of the mass of an atom (in a.m.u.) and its mass number?
- What can be said about the charge number, the charge of the nucleus (expressed in elementary electric charges) and the serial number in D.I. Mendeleev’s table for any chemical element?
- How are the mass number, charge number and number of neutrons in the nucleus related to each other?
- How can we explain the existence of nuclei with identical charges and different masses within the framework of the proton-neutron model of the nucleus?
- What question arose in connection with the hypothesis that the nuclei of atoms consist of protons and neutrons? What assumption did scientists have to make to answer this question?
- What are the forces of attraction between nucleons in a nucleus called and what are their characteristic features?
Composition and characteristics of the atomic nucleus.
The nucleus of the simplest atom - the hydrogen atom - consists of one elementary particle called a proton. The nuclei of all other atoms consist of two types of elementary particles - protons and neutrons. These particles are called nucleons.
Proton . Proton (p) has charge +e and mass
m p = 938.28 MeV
For comparison, let us point out that the electron mass is equal to
m e = 0.511 MeV
From the comparison it follows that m p = 1836m e
The proton has a spin equal to half (s= ), and its own magnetic moment
A unit of magnetic moment called a nuclear magneton. From a comparison of the masses of a proton and an electron, it follows that μ i is 1836 times less than the Bohr magneton μ b. Consequently, the proton's own magnetic moment 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 a proton. Mass difference between neutron and proton (m n –m p)
is 1.3 MeV, i.e. 2.5 m e.
The neutron has a spin equal to half (s=) and (despite the absence of 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 for this amazing fact will be given later.
Note that the ratio of the experimental values μ p and μ n with a high degree of accuracy is equal to - 3/2. This was noticed only after such a value was obtained theoretically.
In a free state, a neutron is unstable (radioactive) - it spontaneously decays, turning into a proton and emitting an electron (e -) and another particle called an antineutrino 
. The half-life (i.e., the time during which half the original number of neutrons decays) is approximately 12 minutes. The decay scheme can be written as follows:
The rest mass of an antineutrino is zero. The mass of a neutron is 2.5m e greater than the mass of a proton. 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 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 Z number determines the serial 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 (i.e. the total number of protons and neutrons) in the nucleus is denoted by the letter A and is called the mass number of the nucleus. The number of neutrons in the nucleus is N=A–Z.
The symbol used to designate kernels is
where X denotes the chemical symbol of the element. The mass number is placed at the top left, the atomic number at the bottom left (the last icon is often omitted). Sometimes the mass number is written not to the left, but to the right of the symbol of a chemical element 
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:
, for tin - ten, etc.
Hydrogen has three isotopes:
– ordinary hydrogen, or protium (Z=1, N=0),
– heavy hydrogen, or deuterium (Z=1, N=1),
– tritium (Z=1, N=2).
Protium and deuterium are stable, tritium is radioactive.
Nuclei with the same mass number A are called isobars. An example is 
And 
. Nuclei with the same number of neutronsN = A – Z are called isotones
(
,
).Finally, there are radioactive nuclei with the same Z and A, differing in half-life. They're called isomers. For example, there are two isomers of the nucleus 
, 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 in both. Approximately 1/5 of these nuclei are stable, the rest are radioactive. Many nuclei were produced artificially using nuclear reactions.
Elements with atomic numbers 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 quantities in the natural mineral - resin blende. The remaining transuranic (i.e., suburanium) elements (cZ from 93 to 107) were obtained artificially through various nuclear reactions.
The transuranic elements curium (96 Cm), einsteinium (99 Es), fermium (100 Fm) and mendelevium (101 Md) were named in honor of outstanding scientists II. and M. Curie, A. Einstein, Z. Fermi and D.I. Mendeleev. Lawrence (103 Lw) is named after the inventor of the cyclotron, E. Lawrence. Kurchatovy (104 Ku) received its name in honor of the outstanding physicist I.V. Kurchatova.
Some transuranic elements, including Kurchatovium and elements numbered 106 and 107, were obtained at the Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Dubna by scientists
N.N. Flerov and his staff.
Kernel sizes . To a first approximation, the nucleus can be considered a ball, the radius of which is quite accurately determined by the formula
(Fermi is the name of a unit of length used in nuclear physics, equal to
10 -13 cm). From the formula it follows 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.
Nuclear spin . The spins of the nucleons add up to the resulting spin of the nucleus. The spin of a nucleon is 1/2. Therefore, the quantum number of the nuclear spin will be half-integer for an odd number of nucleons A and an integer or zero for an even A. Nuclear spins do not exceed several units. This indicates that the spins of most nucleons in the nucleus cancel each other out, being antiparallel. All even-even nuclei (that is, a nucleus with an even number of protons and an even number of neutrons) have a spin of zero.
The mechanical moment of the core M J is added to the moment of the electron shell 
at the total angular momentum of the atom M F, which is determined by the quantum number F.
The interaction of the magnetic moments of 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 energy. 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 several hundredths of an angstrom) that it can only be observed using instruments of the highest resolution.
By studying the composition of matter, scientists came to the conclusion that all matter consists of molecules and atoms. For a long time the atom (translated from Greek as “indivisible”) was considered the smallest structural unit of matter. However, further research showed that the atom has a complex structure and, in turn, includes smaller particles.
What does an atom consist of?
In 1911, the scientist Rutherford suggested that the atom has a central part with a positive charge. This is how the concept of the atomic nucleus first appeared.
According to Rutherford's scheme, called the planetary model, the atom consists of a nucleus and elementary particles with a negative charge - electrons, moving around the nucleus, just as the planets orbit the Sun.
In 1932, another scientist, Chadwick, discovered the neutron, a particle that has no electrical charge.
According to modern ideas, the nucleus corresponds to the planetary model proposed by Rutherford. The core contains most of the atomic mass. It also has a positive charge. The atomic nucleus contains protons - positively charged particles and neutrons - particles that do not carry a charge. Protons and neutrons are called nucleons. Negatively charged particles - electrons - move in orbit around the nucleus.
The number of protons in the nucleus is equal to those moving in orbit. Therefore, the atom itself is a particle that does not carry a charge. If an atom gains electrons from others or loses its own, it becomes positive or negative and is called an ion.
Electrons, protons and neutrons are collectively called subatomic particles.
Charge of the atomic nucleus
The nucleus has a charge number Z. It is determined by the number of protons that make up the atomic nucleus. Finding out this amount is easy: just contact periodic table Mendeleev. The atomic number of the element to which the atom belongs is equal to the number of protons in the nucleus. Thus, if the chemical element oxygen has an atomic number of 8, then the number of protons will also be eight. Since the number of protons and electrons in an atom is the same, there will also be eight electrons.
The number of neutrons is called the isotopic number and is designated by the letter N. Their number can vary in an atom of the same chemical element.
The sum of protons and electrons in the nucleus is called the mass number of the atom and is denoted by the letter A. Thus, the formula for calculating the mass number looks like this: A = Z + N.
Isotopes
In the case where elements have equal numbers of protons and electrons, but different number neutrons, they are called isotopes of a chemical element. There can be one or more isotopes. They are placed in the same cell of the periodic table.
Isotopes are of great importance in chemistry and physics. For example, an isotope of hydrogen - deuterium - in combination with oxygen gives a completely new substance called heavy water. It has a different boiling and freezing point than normal. And the combination of deuterium with another isotope of hydrogen, tritium, leads to a thermonuclear fusion reaction and can be used to generate huge amounts of energy.
Mass of the nucleus and subatomic particles
The size and mass of atoms are negligible in human perception. The size of the nuclei is approximately 10 -12 cm. The mass of an atomic nucleus is measured in physics in the so-called atomic mass units - amu.
For one amu take one twelfth of the mass of a carbon atom. Using the usual units of measurement (kilograms and grams), mass can be expressed by the following equation: 1 amu. = 1.660540·10 -24 g. Expressed in this way, it is called the absolute atomic mass.
Despite the fact that the atomic nucleus is the most massive component of an atom, its size relative to the electron cloud surrounding it is extremely small.
Nuclear forces
Atomic nuclei are extremely stable. This means that protons and neutrons are held in the nucleus by some force. These cannot be electromagnetic forces, since protons are similarly charged particles, and it is known that particles with the same charge repel each other. Gravitational forces are too weak to hold nucleons together. Consequently, particles are held in the nucleus by another interaction - nuclear forces.
Nuclear force is considered the strongest of all existing in nature. Therefore, this type of interaction between the elements of the atomic nucleus is called strong. It is present in many elementary particles, just like electromagnetic forces.
Features of nuclear forces
- Short action. Nuclear forces, unlike electromagnetic ones, appear only at very small distances, comparable to the size of the nucleus.
- Charge independence. This feature is manifested in the fact that nuclear forces act equally on protons and neutrons.
- Saturation. The nucleons of the nucleus interact only with a certain number of other nucleons.
Nuclear binding energy
Another thing closely related to the concept of strong interaction is the binding energy of nuclei. Nuclear bond energy refers to the amount of energy required to split an atomic nucleus into its constituent nucleons. It equals the energy required to form a nucleus from individual particles.
To calculate the binding energy of a nucleus, it is necessary to know the mass of subatomic particles. Calculations show that the mass of a nucleus is always less than the sum of its constituent nucleons. A mass defect is the difference between the mass of a nucleus and the sum of its protons and electrons. Using the relationship between mass and energy (E = mc 2), one can calculate the energy generated during the formation of a nucleus.
The strength of the binding energy of a nucleus can be judged by the following example: the formation of several grams of helium produces the same amount of energy as the combustion of several tons of coal.
Nuclear reactions
The nuclei of atoms can interact with the nuclei of other atoms. Such interactions are called nuclear reactions. There are two types of reactions.
- Fission reactions. They occur when heavier nuclei, as a result of interaction, decay into lighter ones.
- Synthesis reactions. The reverse process of fission: nuclei collide, thereby forming heavier elements.
All nuclear reactions are accompanied by the release of energy, which is subsequently used in industry, the military, the energy sector, and so on.
Having familiarized ourselves with the composition of the atomic nucleus, we can draw the following conclusions.
- An atom consists of a nucleus containing protons and neutrons, and electrons around it.
- The mass number of an atom is equal to the sum of the nucleons in its nucleus.
- Nucleons are held together by strong interactions.
- The enormous forces that give stability to the atomic nucleus are called nuclear binding energies.
Studying the passage of an alpha particle through thin gold foil (see section 6.2), E. Rutherford came to the conclusion that the atom consists of a heavy positively charged nucleus and electrons surrounding it.
Core called the central part of the atom,in which almost the entire mass of the atom and its positive charge are concentrated.
IN composition of the atomic nucleus includes elementary particles : protons And neutrons (nucleons from the Latin word nucleus- core). Such a proton-neutron model of the nucleus was proposed by the Soviet physicist in 1932 D.D. Ivanenko. The proton has a positive charge e + = 1.06 10 –19 C and a rest mass m p= 1.673·10 –27 kg = 1836 m e. Neutron ( n) – neutral particle with rest mass m n= 1.675·10 –27 kg = 1839 m e(where is the electron mass m e, equal to 0.91·10 –31 kg). In Fig. 9.1 shows the structure of the helium atom according to the ideas of the late XX - beginning of the XXI V.
Core charge equals Ze, Where e– proton charge, Z– charge number, equal serial number chemical element in Mendeleev’s periodic table of elements, i.e. number of protons in the nucleus. The number of neutrons in the nucleus is denoted N. Usually Z > N.
Currently known kernels with Z= 1 to Z = 107 – 118.
Number of nucleons in a nucleus A = Z + N called mass number . Cores with the same Z, but different A are called isotopes. Cores that, with the same A have different Z, are called isobars.
The nucleus is denoted by the same symbol as the neutral atom, where X– symbol of a chemical element. For example: hydrogen Z= 1 has three isotopes: – protium ( Z = 1, N= 0), – deuterium ( Z = 1, N= 1), – tritium ( Z = 1, N= 2), tin has 10 isotopes, etc. In the overwhelming majority of isotopes of one chemical element they have the same chemical and similar physical properties. In total, about 300 stable isotopes and more than 2000 natural and artificially obtained ones are known. radioactive isotopes.
The size of the nucleus is characterized by the radius of the nucleus, which has a conventional meaning due to the blurring of the boundary of the nucleus. Even E. Rutherford, analyzing his experiments, showed that the size of the nucleus is approximately 10–15 m (the size of an atom is 10–10 m). There is an empirical formula for calculating the radius of the core:
| , | (9.1.1) |
Where R 0 = (1.3 – 1.7)·10 –15 m. This shows that the volume of the nucleus is proportional to the number of nucleons.
The density of nuclear matter is of the order of magnitude 10 17 kg/m 3 and is constant for all nuclei. It significantly exceeds the densities of the densest ordinary substances.
Protons and neutrons are fermions, because have spin ħ /2.
The nucleus of an atom has intrinsic angular momentum – nuclear spin :
 ,
|
(9.1.2) |
Where I – internal(complete)spin quantum number.
Number I accepts integer or half-integer values 0, 1/2, 1, 3/2, 2, etc. Cores with even A have integer spin(in units ħ ) and obey statistics Bose–Einstein(bosons). Cores with odd A have half-integer spin(in units ħ ) and obey statistics Fermi–Dirac(those. nuclei - fermions).
Nuclear particles have their own magnetic moments, which determine the magnetic moment of the nucleus as a whole. The unit of measurement for the magnetic moments of nuclei is nuclear magneton μ poison:
| . | (9.1.3) |
Here e– absolute value of the electron charge, m p– proton mass.
Nuclear magneton in m p/m e= 1836.5 times less than the Bohr magneton, it follows that the magnetic properties of atoms are determined magnetic properties its electrons .
There is a relationship between the spin of a nucleus and its magnetic moment:
| , | (9.1.4) |
where γ poison – nuclear gyromagnetic ratio.
The neutron has a negative magnetic moment μ n≈ – 1.913μ poison since the direction of the neutron spin and its magnetic moment are opposite. The magnetic moment of the proton is positive and equal to μ R≈ 2.793μ poison. Its direction coincides with the direction of the proton spin.
The distribution of the electric charge of protons over the nucleus is generally asymmetrical. The measure of deviation of this distribution from spherically symmetric is quadrupole electric moment of the nucleus Q. If the charge density is assumed to be the same everywhere, then Q determined only by the shape of the nucleus. So, for an ellipsoid of revolution
 ,
|
(9.1.5) |
Where b– semi-axis of the ellipsoid along the spin direction, A– semi-axis in the perpendicular direction. For a nucleus elongated along the spin direction, b > A And Q> 0. For a core flattened in this direction, b < a And Q < 0. Для сферического распределения заряда в ядре b = a And Q= 0. This is true for nuclei with spin equal to 0 or ħ /2.
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