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 physical Chemical properties substances. 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 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 element symbol, either to the right or to the 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), has the greatest efficiency 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 of 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.
A well-known nuclear reaction is the reaction of the interaction of α-particles with a beryllium isotope, 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 sum of phantom Po particles before and after the reaction
The numbers of phantom Po particles before and after the reaction are equal.
Long before the emergence of reliable data on the internal structure of all things, Greek thinkers imagined matter in the form of the smallest fiery particles that were in constant motion. Probably, this vision of the world order of things was derived from purely logical conclusions. Despite some naivety and absolute lack of evidence for this statement, it turned out to be true. Although scientists were able to confirm a bold guess only twenty-three centuries later.
The structure of atoms
At the end of the 19th century, the properties of a discharge tube through which a current was passed were investigated. Observations have shown that two streams of particles are emitted:
The negative particles of the cathode rays were called electrons. Subsequently, particles with the same charge-to-mass ratio were found in many processes. Electrons seemed to be universal constituents of various atoms, quite easily separated by the bombardment of ions and atoms.
Particles carrying a positive charge were represented by fragments of atoms after they lost one or more electrons. In fact, the positive rays were groups of atoms devoid of negative particles, and therefore having a positive charge.
Thompson model
On the basis of experiments, it was found that positive and negative particles represented the essence of the atom, were its constituents. The English scientist J. Thomson proposed his theory. According to him, the structure of the atom and atomic nucleus were a mass of negative charges crammed into a positively charged ball like raisins in a cake. Charge compensation made the cake electrically neutral.
Rutherford model
The young American scientist Rutherford, analyzing the tracks left after alpha particles, came to the conclusion that the Thompson model is imperfect. Some alpha particles were deflected by small angles - 5-10 o . In rare cases, alpha particles were deflected at large angles of 60-80 o , and in exceptional cases, the angles were very large - 120-150 o . Thompson's model of the atom could not explain such a difference.
Rutherford proposes a new model that explains the structure of the atom and the atomic nucleus. The physics of processes states that an atom must be 99% empty, with a tiny nucleus and electrons revolving around it, which move in orbits.
He explains the deviations during impacts by the fact that the particles of the atom have their own electric charges. Under the influence of bombarding charged particles, atomic elements behave like ordinary charged bodies in the macrocosm: particles with the same charges repel each other, and with opposite charges they attract.
State of atoms
At the beginning of the last century, when the first particle accelerators were launched, all theories explaining the structure of the atomic nucleus and the atom itself were waiting for experimental verification. By that time, the interactions of alpha and beta rays with atoms had already been thoroughly studied. Until 1917, it was believed that atoms were either stable or radioactive. Stable atoms cannot be split, the decay of radioactive nuclei cannot be controlled. But Rutherford managed to refute this opinion.
First proton
In 1911, E. Rutherford put forward the idea that all nuclei consist of the same elements, the basis for which is the hydrogen atom. This idea was prompted by an important conclusion of previous studies of the structure of matter: the masses of all chemical elements are divided without a trace by the mass of hydrogen. The new assumption opened up unprecedented possibilities, allowing us to see the structure of the atomic nucleus in a new way. Nuclear reactions had to confirm or disprove the new hypothesis.
Experiments were carried out in 1919 with nitrogen atoms. By bombarding them with alpha particles, Rutherford achieved an amazing result.
The N atom absorbed the alpha particle, then turned into an oxygen atom O 17 and emitted a hydrogen nucleus. This was the first artificial transformation of an atom of one element into another. Such an experience gave hope that the structure of the atomic nucleus, the physics of existing processes make it possible to carry out other nuclear transformations.
The scientist used in his experiments the method of scintillation - flashes. From the frequency of flashes, he drew conclusions about the composition and structure of the atomic nucleus, about the characteristics of the particles born, about their atomic mass and serial number. The unknown particle was named by Rutherford the proton. It had all the characteristics of a hydrogen atom stripped of its single electron - a single positive charge and a corresponding mass. Thus it was proved that the proton and the nucleus of hydrogen are the same particles.
In 1930, when the first large accelerators were built and launched, Rutherford's model of the atom was tested and proved: each hydrogen atom consists of a lone electron, the position of which cannot be determined, and a loose atom with a lone positive proton inside. Since protons, electrons, and alpha particles can fly out of an atom when bombarded, scientists thought that they were the constituents of any atom's nucleus. But such a model of the nucleus atom seemed unstable - the electrons were too large to fit in the nucleus, in addition, there were serious difficulties associated with the violation of the law of momentum and conservation of energy. These two laws, like strict accountants, said that the momentum and mass during the bombardment disappear in an unknown direction. Since these laws were generally accepted, it was necessary to find explanations for such a leak.
Neutrons
Scientists around the world set up experiments aimed at discovering new constituents of the nuclei of atoms. In the 1930s, German physicists Becker and Bothe bombarded beryllium atoms with alpha particles. In this case, an unknown radiation was registered, which it was decided to call G-rays. Detailed studies revealed some features of the new beams: they could propagate strictly in a straight line, did not interact with electric and magnetic fields, had a high penetrating power. Later, the particles that form this type of radiation were found in the interaction of alpha particles with other elements - boron, chromium and others.
Chadwick's hypothesis
Then James Chadwick, a colleague and student of Rutherford, gave a short report in Nature magazine, which later became well known. Chadwick drew attention to the fact that the contradictions in the conservation laws are easily resolved if we assume that the new radiation is a stream of neutral particles, each of which has a mass approximately equal to the mass of a proton. Considering this assumption, physicists significantly supplemented the hypothesis explaining the structure of the atomic nucleus. Briefly, the essence of the additions was reduced to a new particle and its role in the structure of the atom.
Properties of the neutron
The discovered particle was given the name "neutron". The newly discovered particles did not form electromagnetic fields around themselves and easily passed through matter without losing energy. In rare collisions with light nuclei of atoms, the neutron is able to knock out the nucleus from the atom, losing a significant part of its energy. The structure of the atomic nucleus assumed the presence of a different number of neutrons in each substance. Atoms with the same nuclear charge but different numbers of neutrons are called isotopes.
Neutrons have served as an excellent replacement for alpha particles. Currently, they are used to study the structure of the atomic nucleus. Briefly, their significance for science cannot be described, but it was thanks to the bombardment of atomic nuclei by neutrons that physicists were able to obtain isotopes of almost all known elements.
The composition of the nucleus of an atom
At present, the structure of the atomic nucleus is a collection of protons and neutrons held together by nuclear forces. For example, a helium nucleus is a lump of two neutrons and two protons. Light elements have an almost equal number of protons and neutrons, while heavy elements have a much larger number of neutrons.
This picture of the structure of the nucleus is confirmed by experiments at modern large accelerators with fast protons. The electric forces of repulsion of protons are balanced by vigorous forces that act only in the nucleus itself. Although the nature of nuclear forces is not yet fully understood, their existence is practically proven and fully explains the structure of the atomic nucleus.
Relationship between mass and energy
In 1932, a cloud chamber captured an amazing photograph proving the existence of positive charged particles, with the mass of an electron.
Prior to this, positive electrons were theoretically predicted by P. Dirac. A real positive electron was also discovered in cosmic radiation. The new particle was called the positron. When colliding with its twin - an electron, annihilation occurs - the mutual annihilation of two particles. This releases a certain amount of energy.
Thus, the theory developed for the macrocosm was fully suitable for describing the behavior of the smallest elements of matter.
Investigating the passage of an α-particle through a thin gold foil (see Section 6.2), E. Rutherford came to the conclusion that an atom consists of a heavy positively charged nucleus and electrons surrounding it.
core called the center of the atom,in which almost all the mass of an atom and its positive charge is concentrated.
IN composition of the atomic nucleus includes elementary particles : protons And neutrons (nucleons from the Latin word nucleus- core). Such a proton-neutron model of the nucleus was proposed by the Soviet physicist in 1932 D.D. Ivanenko. The proton has a positive charge e + = 1.06 10 -19 C and a rest mass m p\u003d 1.673 10 -27 kg \u003d 1836 me. Neutron ( n) is a neutral particle with rest mass m n= 1.675 10 -27 kg = 1839 me(where the mass of the electron me, is equal to 0.91 10 -31 kg). On fig. 9.1 shows the structure of the helium atom according to the ideas of the late XX - early XXI century.
Core charge equals Ze, Where e is the charge of the proton, Z- charge number equal to serial number chemical element in Mendeleev's periodic system of elements, i.e. the number of protons in the nucleus. The number of neutrons in a nucleus is denoted N. Usually Z > N.
Nuclei with Z= 1 to Z = 107 – 118.
Number of nucleons in the nucleus A = Z + N called mass number . nuclei with the same Z, but different A called isotopes. Kernels, which, at the same A have different Z, are called isobars.
The nucleus is denoted by the same symbol as the neutral atom, where X is the symbol for a chemical element. For example: hydrogen Z= 1 has three isotopes: – protium ( Z = 1, N= 0), is deuterium ( Z = 1, N= 1), – tritium ( Z = 1, N= 2), tin has 10 isotopes, and so on. In the vast majority of isotopes of the same chemical element, they have the same chemical and close physical properties. In total, about 300 stable isotopes and more than 2000 natural and artificially obtained are known. radioactive isotopes.
The size of the nucleus is characterized by the radius of the nucleus, which has a conditional meaning due to the blurring of the nucleus boundary. Even E. Rutherford, analyzing his experiments, showed that the size of the nucleus is approximately 10–15 m (the size of an atom is 10–10 m). There is an empirical formula for calculating the core radius:
| , | (9.1.1) |
Where R 0 = (1.3 - 1.7) 10 -15 m. From this it can be seen that the volume of the nucleus is proportional to the number of nucleons.
The density of the nuclear substance is on the order of 10 17 kg/m 3 and is constant for all nuclei. It greatly exceeds the density of the densest ordinary substances.
Protons and neutrons are fermions, because have spin ħ /2.
The nucleus of an atom has own angular momentum – nuclear spin :
 ,
|
(9.1.2) |
Where I – internal(complete)spin quantum number.
Number I accepts integer or half-integer values 0, 1/2, 1, 3/2, 2, etc. Kernels with even A have integer spin(in units ħ ) and obey the statistics Bose–Einstein(bosons). Kernels with odd A have half-integer spin(in units ħ ) and obey the statistics Fermi–Dirac(those. nuclei are fermions).
Nuclear particles have their own magnetic moments, which determine the magnetic moment of the nucleus as a whole. The unit for measuring the magnetic moments of nuclei is nuclear magneton μ poison:
| . | (9.1.3) |
Here e is the absolute value of the electron charge, m p is the mass of the proton.
Nuclear magneton in m p/me= 1836.5 times smaller than the Bohr magneton, hence it follows that the magnetic properties of atoms are determined magnetic properties its electrons .
There is a relationship between the spin of the nucleus and its magnetic moment:
| , | (9.1.4) |
where γ poison - nuclear gyromagnetic ratio.
The neutron has a negative magnetic moment μ n≈ – 1.913μ poison because the direction of the neutron spin and its magnetic moment are opposite. Magnetic moment 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 in general case asymmetrically. The measure of deviation of this distribution from spherically symmetric is quadrupole electric moment of the nucleus Q. If the charge density is assumed to be the same everywhere, then Q determined only by the shape of the nucleus. So, for an ellipsoid of revolution
 ,
|
(9.1.5) |
Where b is the semiaxis of the ellipsoid along the spin direction, A- axis in the perpendicular direction. For a nucleus stretched along the direction of the spin, b > A And Q> 0. For a nucleus oblate in this direction, b < a And Q < 0. Для сферического распределения заряда в ядре b = a And Q= 0. This is true for nuclei with spin equal to 0 or ħ /2.
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An atom is the smallest particle of a chemical element that retains all of its chemical properties. An atom consists of a positively charged nucleus and negatively charged electrons. The charge of the nucleus of any chemical element is equal to the product of Z by e, where Z is the serial number of this element in the periodic system of chemical elements, e is the value of the elementary electric charge.
Electron- this is the smallest particle of a substance with a negative electric charge e=1.6·10 -19 coulombs, taken as an elementary electric charge. Electrons, rotating around the nucleus, are located on the electron shells K, L, M, etc. K is the shell closest to the nucleus. The size of an atom is determined by the size of its electron shell. An atom can lose electrons and become a positive ion, or gain electrons and become a negative ion. The charge of an ion determines the number of electrons lost or gained. The process of turning a neutral atom into a charged ion is called ionization.
atomic nucleus(the central part of the atom) consists of elementary nuclear particles - protons and neutrons. The radius of the nucleus is about a hundred thousand times smaller than the radius of the atom. The density of the atomic nucleus is extremely high. Protons- These are stable elementary particles having a unit positive electric charge and a mass 1836 times greater than the mass of an electron. The proton is the nucleus of the lightest element, hydrogen. The number of protons in the nucleus is Z. Neutron is neutral (having no electrical charge) elementary particle with a mass very close to that of a proton. Since the mass of the nucleus is the sum of the mass of protons and neutrons, the number of neutrons in the nucleus of an atom is A - Z, where A is the mass number of a given isotope (see). The proton and neutron that make up the nucleus are called nucleons. In the nucleus, nucleons are bound by special nuclear forces.
The atomic nucleus has a huge store of energy, which is released during nuclear reactions. Nuclear reactions occur when atomic nuclei interact with elementary particles or with the nuclei of other elements. As a result of nuclear reactions, new nuclei are formed. For example, a neutron can transform into a proton. In this case, a beta particle, i.e., an electron, is ejected from the nucleus.
The transition in the nucleus of a proton into a neutron can be carried out in two ways: either a particle with a mass equal to the mass of an electron, but with a positive charge, called a positron (positron decay), is emitted from the nucleus, or the nucleus captures one of the electrons from the nearest K-shell (K -capture).
Sometimes the formed nucleus has an excess of energy (it is in an excited state) and, passing into the normal state, releases excess energy in the form of electromagnetic radiation with a very short wavelength -. The energy released during nuclear reactions is practically used in various industries.
An atom (Greek atomos - indivisible) is the smallest particle of a chemical element that has its chemical properties. Each element is made up of certain types of atoms. The structure of an atom includes the kernel carrying a positive electric charge, and negatively charged electrons (see), forming its electronic shells. The value of the electric charge of the nucleus is equal to Z-e, where e is the elementary electric charge, equal in magnitude to the charge of the electron (4.8 10 -10 e.-st. units), and Z is the atomic number of this element in the periodic system of chemical elements (see .). Since a non-ionized atom is neutral, the number of electrons included in it is also equal to Z. The composition of the nucleus (see. Atomic nucleus) includes nucleons, elementary particles with a mass approximately 1840 times greater than the mass of an electron (equal to 9.1 10 - 28 g), protons (see), positively charged, and chargeless neutrons (see). The number of nucleons in the nucleus is called the mass number and is denoted by the letter A. The number of protons in the nucleus, equal to Z, determines the number of electrons entering the atom, the structure of the electron shells and the chemical properties of the atom. The number of neutrons in the nucleus is A-Z. Isotopes are called varieties of the same element, the atoms of which differ from each other in mass number A, but have the same Z. Thus, in the nuclei of atoms of different isotopes of one element there are a different number of neutrons with the same number of protons. When designating isotopes, the mass number A is written at the top of the element symbol, and the atomic number at the bottom; for example, isotopes of oxygen are denoted: 
The dimensions of an atom are determined by the dimensions of the electron shells and for all Z are about 10 -8 cm. Since the mass of all the electrons of the atom is several thousand times less than the mass of the nucleus, the mass of the atom is proportional to the mass number. The relative mass of an atom of a given isotope is determined in relation to the mass of an atom of the carbon isotope C 12, taken as 12 units, and is called the isotopic mass. It turns out to be close to the mass number of the corresponding isotope. The relative weight of an atom of a chemical element is the average (taking into account the relative abundance of the isotopes of a given element) value of the isotopic weight and is called the atomic weight (mass).
An atom is a microscopic system, and its structure and properties can only be explained with the help of quantum theory, created mainly in the 20s of the 20th century and intended to describe phenomena on an atomic scale. Experiments have shown that microparticles - electrons, protons, atoms, etc. - in addition to corpuscular ones, have wave properties that manifest themselves in diffraction and interference. In quantum theory, a certain wave field characterized by a wave function (Ψ-function) is used to describe the state of micro-objects. This function determines the probabilities of possible states of a micro-object, i.e., it characterizes the potential possibilities for the manifestation of one or another of its properties. The law of variation of the function Ψ in space and time (the Schrödinger equation), which makes it possible to find this function, plays the same role in quantum theory as Newton's laws of motion in classical mechanics. The solution of the Schrödinger equation in many cases leads to discrete possible states of the system. So, for example, in the case of an atom, the series wave functions for electrons corresponding to different (quantized) energy values. The system of energy levels of the atom, calculated by the methods of quantum theory, has received brilliant confirmation in spectroscopy. The transition of an atom from the ground state corresponding to the lowest energy level E 0 to any of the excited states E i occurs when a certain portion of energy E i - E 0 is absorbed. An excited atom goes into a less excited or ground state, usually with the emission of a photon. In this case, the photon energy hv is equal to the difference between the energies of an atom in two states: hv= E i - E k where h is Planck's constant (6.62·10 -27 erg·sec), v is the frequency of light.
In addition to atomic spectra, quantum theory allowed to explain other properties of atoms. In particular, the valency, nature chemical bond and the structure of molecules, the theory of the periodic system of elements was created.
A feature of radioactive contamination, in contrast to contamination by other pollutants, is that it is not the radionuclide (pollutant) that has a harmful effect on humans and environmental objects, but the radiation that it is the source of.
However, there are cases when a radionuclide is a toxic element. For example, after the accident at the 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 effect of radiation on an object can be assessed on 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 manifest themselves only at very small distances (10 -13 cm), they constitute the potential binding energy of the nucleus, which is partially released during some transformations and 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 permanent 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 P 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.
At present, IUPAC (International Union of Pure and Applied Chemistry) has officially given the name to 109 chemical elements. Of these, only 81 have stable isotopes, the heaviest of which is bismuth. (Z= 83). For the remaining 28 elements, only radioactive isotopes are known, with 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).
