What is the diameter of an atom? What is the size and mass of an atom. The world is beautiful

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. Figure 9.1 shows the structure of the helium atom according to the ideas of the late 20th - early 21st centuries.

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, isotopes of one chemical element 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:

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:

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 an atom are determined by the magnetic properties of its electrons .

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

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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ATOM, the smallest particle of a substance that can enter into chemical reactions. Each substance has a unique set of atoms. At one time it was believed that the atom was indivisible, however, it consists of a positively charged NUCLEUS, around which negatively charged electrons rotate. The nucleus (the presence of which was established in 1911 by Ernst RUTHERFORD) consists of densely packed protons and neutrons. It occupies only a small part of the space inside the atom, however, it accounts for almost the entire mass of the atom. In 1913, Niels BOR proposed that electrons move in fixed orbits. Since then, research in QUANTUM MECHANICS has led to a new understanding of orbits: according to the Heisenberg UNCERTAINTY PRINCIPLE, the exact position and MOMENTUM of a subatomic particle cannot be known simultaneously. The number of electrons in an atom and their arrangement determine the chemical properties of the element. When one or more electrons are added or taken away, an ion is created.

The mass of an atom depends on the size of the nucleus. It accounts for the largest fraction of the weight of an atom, since electrons weigh nothing. For example, the uranium atom is the heaviest atom found in nature. It has 146 neutrons, 92 protons and 92 electrons. On the other hand, the lightest atom is the hydrogen atom, which has 1 proton and an electron. However, the uranium atom, although 230 times heavier than the hydrogen atom, is only three times larger in size. The weight of an atom is expressed in units of atomic mass and is denoted as u. Atoms are made up of even smaller particles called subatomic (elementary) particles. The main ones are protons (positively charged), neutrons (electrically neutral) and electrons (negatively charged). Clusters of electrons and neutrons form a nucleus in the center of all atoms (except hydrogen, which has only one proton). "Electrons" spinning around! nuclei at some distance from it, commensurate with the dimensions of the atom. | (If, for example, the nucleus of a helium atom were the size of a tennis ball, then the electrons would be at a distance of 6 km from it. There are 112 different types of atoms, the same number elements on the periodic table. Atoms of elements differ in atomic number and atomic mass. ATOMIC NUCLEUS The mass of an atom is derived mainly from the relatively dense nucleus. I (rotons and neutrons have a mass approximately 1K4() times greater than electrons. Because the runs are charged positive, and neutrons are neutral, the nucleus of an atom is always positively charged. 11since opposite charges attract each other, the nucleus holds electrons in their orbits. Progons and neutrons consist of even smaller particles, quarks. ELECTRONS determines its chemical gnonstia H oshichis from the planets of the Solar system, nemrops revolve around the core randomly, oiMiiMi nor at a fixed distance from the core, form-IVH "aboutSyulochki". The more energy the elek-ipon has. li"M further it can move away, overcoming the attraction of a positively charged nucleus. In a neutral atom, the positive charge of the electrons balances the positive charge of the protons of the nucleus. 11 Therefore, the removal or addition of one electron in the agom leads to the appearance of a charged ion. The electron shells are located at fixed distances from the nucleus, depending on their energy level. Each shell is numbered starting from the nucleus. There are no more than seven shells in an agoma, and each of them can contain only a certain number of electrons. If there is enough energy, an electron can jump from one shell to another, higher one. When it hits the lower shell again, it emits radiation in the form of a photon. The electron belongs to a class of particles called leptons, and its antiparticle is called a positron.

NUCLEAR CHAIN ​​REACTION. In a nuclear explosion, such as a nuclear explosion, a neutron strikes a uranium 23b nucleus (that is, a nucleus with a total number of protons and neutrons equal to ? 35). When the neutron is absorbed, uranium 236 is created. It is very unstable and splits into two smaller nuclei, which releases a huge amount of energy and several neutrons. Each of these neutrons can, in turn, hit another uranium nucleus. If created in this way called critical conditions (the amount of uranium-235 exceeds the critical mass), then the number of neutron collisions will be sufficient for the reaction to develop at lightning speed, i.e. a chain reaction occurs. In a nuclear reactor, the heat released during the process is used to heat steam, which drives a turbine generator that produces electricity.

Scientific and technical encyclopedic dictionary.

See what "ATOM" is in other dictionaries:

atom- atom, and... Russian spelling dictionary

- (Greek atomos, from a negative part., and tome, tomos department, segment). An infinitely small indivisible particle, the totality of which makes up any physical body. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. ATOM Greek ... Dictionary of foreign words of the Russian language

atom- a m. atome m. 1. The smallest indivisible particle of matter. Atoms cannot be eternal. Cantemir About nature. Ampere believes that every indivisible particle of matter (atom) contains an integral amount of electricity. OZ 1848 56 8 240. Let there be... ... Historical Dictionary of Gallicisms of the Russian Language

- (from the Greek atomos - indivisible) the smallest constituent particles of matter from which everything that exists is composed, including the soul, formed from the finest atoms (Leucippus, Democritus, Epicurus). Atoms are eternal, they neither arise nor disappear, being in constant... ... Philosophical Encyclopedia

Atom- Atom ♦ Atome Etymologically, an atom is an indivisible particle, or a particle subject only to speculative division; indivisible element (atomos) of matter. Democritus and Epicurus understand the atom in this sense. Modern scientists are well aware that this... ... Sponville's Philosophical Dictionary

- (from the Greek atomos indivisible) the smallest particle of a chemical element that retains its properties. At the center of the atom there is a positively charged nucleus, in which almost the entire mass of the atom is concentrated; electrons move around, forming electron... Big Encyclopedic Dictionary

Male, Greek indivisible; substance in the extreme limits of its divisibility, an invisible speck of dust, from which all bodies are supposedly composed, every substance, as if from grains of sand. | An immeasurable, infinitesimal speck of dust, an insignificant amount. | Chemists have their word... Dahl's Explanatory Dictionary

Cm … Synonym dictionary

ATOM- (from the Greek atomos indivisible). The word A. is used in modern science in different senses. In most cases, A. is called the maximum amount of chemical. element, further fragmentation of the element leads to a loss of individuality of the element, i.e. to a sharp... ... Great Medical Encyclopedia

atom- atom Atom is a part of speech, which is the smallest bearer of the chemical powers of a single chemical element. There are many types of atoms, as well as chemical elements and isotopes. Electrically neutral, composed of nuclei and electrons. Atomic radius... ... Girnichy encyclopedic dictionary

Books

• The hydrogen atom and non-Euclidean geometry, V.A. Fok. This book will be produced in accordance with your order using Print-on-Demand technology. Reproduced in the original author's spelling of the 1935 edition (publishing house "Publishing House...
• The hydrogen atom is the simplest of atoms. Continuation of Niels Bohr's theory. Part 5. The frequency of photon radiation coincides with the average frequency of electron radiation in the transition, A. I. Shidlovsky. Bohr's theory of the hydrogen atom was continued ("parallel" to the quantum mechanical approach) along the traditional path of development of physics, where observable and unobservable quantities coexist in the theory. For…

An atom is the smallest particle of a chemical substance that can retain its properties. The word "atom" comes from the ancient Greek "atomos", meaning "indivisible". Depending on how many and what particles are in an atom, a chemical element can be determined.

Briefly about the structure of the atom

How can you briefly list the basic information about is a particle with one nucleus, which is positively charged. Around this nucleus is a negatively charged cloud of electrons. Each atom in its normal state is neutral. The size of this particle can be entirely determined by the size of the electron cloud that surrounds the nucleus.

The nucleus itself, in turn, also consists of smaller particles - protons and neutrons. Protons are positively charged. Neutrons do not carry any charge. However, protons and neutrons are combined into one category and are called nucleons. If basic information about the structure of the atom is needed briefly, then this information can be limited to the listed data.

First information about the atom

The ancient Greeks suspected that matter could consist of small particles. They believed that everything that exists is made of atoms. However, such a view was purely philosophical in nature and cannot be interpreted scientifically.

The first to obtain basic information about the structure of the atom was an English scientist. It was this researcher who was able to discover that two chemical elements can enter into different ratios, and each such combination will represent a new substance. For example, eight parts of the element oxygen give rise to carbon dioxide. Four parts oxygen is carbon monoxide.

In 1803, Dalton discovered the so-called law of multiple ratios in chemistry. Using indirect measurements (since not a single atom could then be examined under the microscopes of that time), Dalton made a conclusion about the relative weight of atoms.

Rutherford's research

Almost a century later, basic information about the structure of atoms was confirmed by another English chemist - the Scientist proposed a model of the electron shell of the smallest particles.

At that time, Rutherford's "Planetary Model of the Atom" was one of the most important steps that chemistry could take. Basic information about the structure of the atom indicated that it was similar to the solar system: electron particles rotate around the nucleus in strictly defined orbits, just as planets do.

Electronic shell of atoms and formulas of atoms of chemical elements

The electron shell of each atom contains exactly as many electrons as there are protons in its nucleus. This is why the atom is neutral. In 1913, another scientist obtained basic information about the structure of the atom. Niels Bohr's formula was similar to that obtained by Rutherford. According to his concept, electrons also revolve around the nucleus located at the center. Bohr refined Rutherford's theory and brought harmony to its facts.

Even then, formulas for some chemical substances were compiled. For example, schematically the structure of the nitrogen atom is denoted as 1s 2 2s 2 2p 3, the structure of the sodium atom is expressed by the formula 1s 2 2s 2 2p 6 3s 1. Through these formulas you can see how many electrons move in each of the orbitals of a particular chemical substance.

Schrödinger model

However, later this atomic model also became outdated. Basic information about the structure of the atom, known to science today, largely became available thanks to the research of the Austrian physicist

He proposed a new model of its structure - a wave model. By this time, scientists had already proven that the electron is endowed not only with the nature of a particle, but also has the properties of a wave.

However, the Schrödinger and Rutherford model also has general provisions. Their theories are similar in that electrons exist at certain levels.

Such levels are also called electronic layers. Using the level number, the electron energy can be characterized. The higher the layer, the more energy it has. All levels are counted from bottom to top, so the level number corresponds to its energy. Each of the layers in the electron shell of an atom has its own sublevels. In this case, the first level may have one sublevel, the second - two, the third - three, and so on (see the above electronic formulas for nitrogen and sodium).

Even smaller particles

At the moment, of course, even smaller particles have been discovered than the electron, proton and neutron. It is known that the proton consists of quarks. There are even smaller particles of the universe - for example, the neutrino, which is a hundred times smaller in size than a quark and a billion times smaller than a proton.

A neutrino is such a small particle that it is 10 septillion times smaller than, for example, a tyrannosaurus rex. The tyrannosaurus itself is as many times smaller in size than the entire observable Universe.

Basic information about the structure of the atom: radioactivity

It has always been known that no chemical reaction can transform one element into another. But in the process of radioactive radiation this happens spontaneously.

Radioactivity is the ability of atomic nuclei to transform into other nuclei - more stable ones. When people received basic information about the structure of atoms, isotopes, to a certain extent, could serve as the embodiment of the dreams of medieval alchemists.

As isotopes decay, radioactive radiation is emitted. This phenomenon was first discovered by Becquerel. The main type of radioactive radiation is alpha decay. When it occurs, an alpha particle is released. There is also beta decay, in which a beta particle is ejected from the nucleus of an atom.

Natural and artificial isotopes

Currently, about 40 natural isotopes are known. Most of them are located in three categories: uranium-radium, thorium and actinium. All these isotopes can be found in nature - in rocks, soil, air. But besides them, about a thousand artificially derived isotopes are also known, which are produced in nuclear reactors. Many of these isotopes are used in medicine, especially in diagnostics..

Proportions within an atom

If we imagine an atom whose dimensions are comparable to the dimensions of an international sports stadium, then we can visually obtain the following proportions. The electrons of an atom in such a “stadium” will be located at the very top of the stands. Each one will be smaller than the head of a pin. Then the core will be located in the center of this field, and its size will be no larger than the size of a pea.

Sometimes people ask what an atom actually looks like. In fact, it literally does not look like anything - not for the reason that the microscopes used in science are not good enough. The dimensions of an atom are in those areas where the concept of “visibility” simply does not exist.

Atoms are very small in size. But how small are these sizes really? The fact is that the smallest grain of salt, barely visible to the human eye, contains about one quintillion atoms.

If we imagine an atom of such a size that could fit in a human hand, then next to it there would be viruses 300 meters long. Bacteria would be 3 km long, and the thickness of a human hair would be 150 km. In a supine position, he would be able to go beyond the boundaries of the earth's atmosphere. And if such proportions were valid, then a human hair could reach the Moon in length. This is such a complex and interesting atom, which scientists continue to study to this day.

DEFINITION

Atom– the smallest chemical particle.

The variety of chemical compounds is due to the different combinations of atoms of chemical elements into molecules and non-molecular substances. The ability of an atom to enter into chemical compounds, its chemical and physical properties are determined by the structure of the atom. In this regard, for chemistry, the internal structure of the atom and, first of all, the structure of its electronic shell are of paramount importance.

Atomic structure models

At the beginning of the 19th century, D. Dalton revived the atomic theory, relying on the fundamental laws of chemistry known by that time (constancy of composition, multiple ratios and equivalents). The first experiments were carried out to study the structure of matter. However, despite the discoveries made (atoms of the same element have the same properties, and atoms of other elements have different properties, the concept of atomic mass was introduced), the atom was considered indivisible.

After obtaining experimental evidence (late 19th - early 20th century) of the complexity of the structure of the atom (photoelectric effect, cathode and x-rays, radioactivity), it was found that the atom consists of negatively and positively charged particles that interact with each other.

These discoveries gave impetus to the creation of the first models of atomic structure. One of the first models was proposed J. Thomson(1904) (Fig. 1): the atom was imagined as a “sea of ​​positive electricity” with electrons oscillating in it.

After experiments with α-particles, in 1911. Rutherford proposed the so-called planetary model atomic structure (Fig. 1), similar to the structure of the solar system. According to the planetary model, at the center of the atom there is a very small nucleus with a charge Z e, the dimensions of which are approximately 1,000,000 times smaller than the dimensions of the atom itself. The nucleus contains almost the entire mass of the atom and has a positive charge. Electrons move around the nucleus in orbits, the number of which is determined by the charge of the nucleus. The external trajectory of the electrons determines the external dimensions of the atom. The diameter of an atom is 10 -8 cm, while the diameter of the nucleus is much smaller -10 -12 cm.

Rice. 1 Models of atomic structure according to Thomson and Rutherford

Experiments on studying atomic spectra have shown the imperfection of the planetary model of the structure of the atom, since this model contradicts the line structure of atomic spectra. Based on Rutherford's model, Einstein's doctrine of light quanta and Planck's quantum theory of radiation Niels Bohr (1913) formulated postulates, which consists theory of atomic structure(Fig. 2): an electron can rotate around the nucleus not in any, but only in some specific orbits (stationary), moving along such an orbit it does not emit electromagnetic energy, radiation (absorption or emission of a quantum of electromagnetic energy) occurs during a transition (jump-like) electron from one orbit to another.

Rice. 2. Model of the structure of the atom according to N. Bohr

The accumulated experimental material characterizing the structure of the atom has shown that the properties of electrons, as well as other micro-objects, cannot be described on the basis of the concepts of classical mechanics. Microparticles obey the laws of quantum mechanics, which became the basis for the creation modern model of atomic structure.

The main theses of quantum mechanics:

- energy is emitted and absorbed by bodies in separate portions - quanta, therefore, the energy of particles changes abruptly;

- electrons and other microparticles have a dual nature - they exhibit the properties of both particles and waves (wave-particle duality);

— quantum mechanics denies the presence of certain orbits for microparticles (for moving electrons it is impossible to determine the exact position, since they move in space near the nucleus, you can only determine the probability of finding an electron in different parts of space).

The space near the nucleus in which the probability of finding an electron is quite high (90%) is called orbital.

Quantum numbers. Pauli's principle. Klechkovsky's rules

The state of an electron in an atom can be described using four quantum numbers.

n– main quantum number. Characterizes the total energy reserve of an electron in an atom and the number of the energy level. n takes on integer values ​​from 1 to ∞. The electron has the lowest energy when n=1; with increasing n – energy. The state of an atom when its electrons are at such energy levels that their total energy is minimal is called ground state. States with higher values ​​are called excited. Energy levels are indicated by Arabic numerals according to the value of n. Electrons can be arranged in seven levels, therefore, n actually exists from 1 to 7. The main quantum number determines the size of the electron cloud and determines the average radius of an electron in an atom.

l– orbital quantum number. Characterizes the energy reserve of electrons in the sublevel and the shape of the orbital (Table 1). Accepts integer values ​​from 0 to n-1. l depends on n. If n=1, then l=0, which means that there is a 1st sublevel at the 1st level.

m e– magnetic quantum number. Characterizes the orientation of the orbital in space. Accepts integer values ​​from –l through 0 to +l. Thus, when l=1 (p-orbital), m e takes on the values ​​-1, 0, 1 and the orientation of the orbital can be different (Fig. 3).

Rice. 3. One of the possible orientations in space of the p-orbital

s– spin quantum number. Characterizes the electron's own rotation around its axis. Accepts values ​​-1/2(↓) and +1/2(). Two electrons in the same orbital have antiparallel spins.

The state of electrons in atoms is determined Pauli principle: an atom cannot have two electrons with the same set of all quantum numbers. The sequence of filling the orbitals with electrons is determined Klechkovsky rules: the orbitals are filled with electrons in increasing order of the sum (n+l) for these orbitals, if the sum (n+l) is the same, then the orbital with the smaller n value is filled first.

However, an atom usually contains not one, but several electrons, and to take into account their interaction with each other, the concept of effective nuclear charge is used - an electron in the outer level is subject to a charge that is less than the charge of the nucleus, as a result of which the internal electrons screen the external ones.

Basic characteristics of an atom: atomic radius (covalent, metallic, van der Waals, ionic), electron affinity, ionization potential, magnetic moment.

Electronic formulas of atoms

All the electrons of an atom form its electron shell. The structure of the electron shell is depicted electronic formula, which shows the distribution of electrons across energy levels and sublevels. The number of electrons in a sublevel is indicated by a number, which is written to the upper right of the letter indicating the sublevel. For example, a hydrogen atom has one electron, which is located in the s-sublevel of the 1st energy level: 1s 1. The electronic formula of helium containing two electrons is written as follows: 1s 2.

For elements of the second period, electrons fill the 2nd energy level, which can contain no more than 8 electrons. First, electrons fill the s-sublevel, then the p-sublevel. For example:

5 B 1s 2 2s 2 2p 1

Relationship between the electronic structure of the atom and the position of the element in the Periodic Table

The electronic formula of an element is determined by its position in the Periodic Table D.I. Mendeleev. Thus, the period number corresponds to In elements of the second period, electrons fill the 2nd energy level, which can contain no more than 8 electrons. First, electrons fill In elements of the second period, electrons fill the 2nd energy level, which can contain no more than 8 electrons. First, electrons fill the s-sublevel, then the p-sublevel. For example:

5 B 1s 2 2s 2 2p 1

In atoms of some elements, the phenomenon of electron “leap” from the outer energy level to the penultimate one is observed. Electron leakage occurs in atoms of copper, chromium, palladium and some other elements. For example:

24 Cr 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1

an energy level that can contain no more than 8 electrons. First, electrons fill the s-sublevel, then the p-sublevel. For example:

5 B 1s 2 2s 2 2p 1

The group number for elements of the main subgroups is equal to the number of electrons in the outer energy level; such electrons are called valence electrons (they participate in the formation of a chemical bond). Valence electrons for elements of side subgroups can be electrons of the outer energy level and the d-sublevel of the penultimate level. The group number of elements of secondary subgroups III-VII groups, as well as for Fe, Ru, Os, corresponds to the total number of electrons in the s-sublevel of the outer energy level and the d-sublevel of the penultimate level

Tasks:

Draw the electronic formulas of the phosphorus, rubidium and zirconium atoms. Indicate the valence electrons.

Answer:

15 P 1s 2 2s 2 2p 6 3s 2 3p 3 Valence electrons 3s 2 3p 3

37 Rb 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 5s 1 Valence electrons 5s 1

40 Zr 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 2 5s 2 Valence electrons 4d 2 5s 2

ATOM [French atome, from Latin atomus, from Greek?τομος (ουσ?α) - indivisible (essence)], a particle of matter, the smallest part of a chemical element, which is the bearer of its properties. The atoms of each element are individual in structure and properties and are designated by the chemical symbols of the elements (for example, the hydrogen atom - H, iron - Fe, mercury - Hg, uranium - U, etc.). Atoms can exist both in a free state and in a bound state (see Chemical bond). The whole variety of substances is due to different combinations of atoms with each other. The properties of gaseous, liquid and solid substances depend on the properties of their constituent atoms. All physical and chemical properties of an atom are determined by its structure and obey quantum laws. (For the history of the development of the doctrine of the atom, see the article Atomic Physics.)

General characteristics of the structure of atoms. An atom consists of a heavy nucleus with a positive electrical charge and light electrons with negative electrical charges surrounding it, forming the electron shells of the atom. The dimensions of an atom are determined by the dimensions of its outer electron shell and are large compared to the dimensions of the atomic nucleus. The characteristic orders of diameters, cross-sectional areas and volumes of an atom and nucleus are:

Atom 10 -8 cm 10 -16 cm 2 10 -24 cm 3

Core 10 -12 cm 10 -24 cm 2 10 -36 cm 3

The electron shells of an atom do not have strictly defined boundaries, and the sizes of an atom depend to a greater or lesser extent on the methods for determining them.

The nuclear charge is the main characteristic of an atom, which determines its belonging to a certain element. The charge of the nucleus is always an integer multiple of the positive elementary electric charge, equal in absolute value to the charge of the electron -e. The charge of the nucleus is +Ze, where Z is the atomic number (atomic number). Z= 1, 2, 3,... for atoms of successive elements in the periodic table of chemical elements, that is, for atoms of H, He, Li, .... In a neutral atom, a nucleus with charge +Ze holds Z electrons with a total charge - Ze. An atom can lose or gain electrons and become a positive or negative ion (k = 1, 2, 3, ... - the multiplicity of its ionization). An atom of a certain element often includes its ions. When writing, ions are distinguished from a neutral atom by the index k + and k -; for example, O is a neutral oxygen atom, O +, O 2+, O 3+, ..., O 8+, O -, O 2- are its positive and negative ions. The combination of a neutral atom and ions of other elements with the same number of electrons forms an isoelectronic series, for example, a series of hydrogen-like atoms H, He +, Li 2+, Be 3+,....

The multiplicity of the charge of the nucleus of an atom to the elementary charge e was explained on the basis of ideas about the structure of the nucleus: Z is equal to the number of protons in the nucleus, the charge of a proton is +e. The mass of an atom increases with increasing Z. The mass of the nucleus of an atom is approximately proportional to the mass number A - the total number of protons and neutrons in the nucleus. The mass of an electron (0.91 x 10 -27 g) is significantly less (about 1840 times) than the mass of a proton or neutron (1.67 x 10 -24 g), so the mass of an atom is mainly determined by the mass of its nucleus.

Atoms of a given element may differ in nuclear mass (the number of protons Z is constant, the number of neutrons A-Z can vary); Such varieties of atoms of the same element are called isotopes. The difference in the mass of the nucleus has almost no effect on the structure of the electronic shells of a given atom, which depends on Z, and the properties of the atom. The greatest differences in properties (isotope effects) are obtained for hydrogen isotopes (Z = 1) due to the large difference in the masses of the ordinary light atom of hydrogen (A = 1), deuterium (A = 2) and tritium (A = 3).

The mass of an atom varies from 1.67 × 10 -24 g (for the main isotope, hydrogen atom, Z = 1, A = 1) to approximately 4 × 10 -22 g (for atoms of transuranium elements). The most accurate values ​​of atomic masses can be determined by mass spectroscopy methods. The mass of an atom is not exactly equal to the sum of the mass of the nucleus and the masses of the electrons, but somewhat less - by the mass defect ΔM = W/c 2, where W is the energy of formation of an atom from the nucleus and electrons (binding energy), c is the speed of light. This correction is of the order of the electron mass m e for heavy atoms, and for light atoms it is negligible (about 10 -4 m e).

Atomic energy and its quantization. Due to its small size and large mass, the atomic nucleus can be approximately considered pointlike and at rest at the center of mass of the atom (the common center of mass of the nucleus and electrons is located near the nucleus, and the speed of movement of the nucleus relative to the center of mass of the atom is small compared to the speed of movement of electrons). Accordingly, an atom can be considered as a system in which N electrons with charges e move around a stationary attracting center. The movement of electrons in an atom occurs in a limited volume, that is, it is bound. The total internal energy of an atom E is equal to the sum of the kinetic energies T of all electrons and the potential energy U - the energy of their attraction by the nucleus and repulsion from each other.

According to the theory of the atom, proposed in 1913 by Niels Bohr, in a hydrogen atom one electron with charge -e moves around a stationary center with charge +e. In accordance with classical mechanics, the kinetic energy of such an electron is equal to

where v is the speed, p = m e v is the momentum (momentum) of the electron. The potential energy (reduced to the energy of the Coulomb attraction of an electron by the nucleus) is equal to

and depends only on the distance r of the electron from the nucleus. Graphically, the function U(r) is represented by a curve that decreases without limit as r decreases, i.e., as the electron approaches the nucleus. The value of U(r) at r→∞ is taken to be zero. At negative values ​​of total energy E = T + U< 0 движение электрона является связанным: оно ограничено в пространстве значениями r=r мaкc . При положительных значениях полной энергии Е = Т + U >0 the movement of the electron is free - it can go to infinity with energy E = T = (1/2)m e v 2, which corresponds to the ionized hydrogen atom H +. Thus, a neutral hydrogen atom is a system of electrostatically bound nucleus and electron with energy E< 0.

The total internal energy of an atom E is its main characteristic as a quantum system (see Quantum mechanics). An atom can remain for a long time only in states with a certain energy - stationary (unchangeable in time) states. The internal energy of a quantum system consisting of bound microparticles (including an atom) can take one of a discrete (discontinuous) series of values

Each of these “allowed” energy values ​​corresponds to one or more stationary quantum states. The system cannot have intermediate energy values ​​(for example, those lying between E 1 and E 2, E 2 and E 3, etc.); such a system is said to have quantized energy. Any change in E is associated with a quantum (jump) transition of the system from one stationary quantum state to another (see below).

Possible discrete values ​​(3) of the energy of an atom can be graphically depicted by analogy with the potential energy of a body raised to different heights (to different levels), in the form of a diagram of energy levels, where each energy value corresponds to a straight line drawn at a height E i, i= 1 , 2, 3, ... (Fig. 1). The lowest level E 1, corresponding to the lowest possible energy of the atom, is called the ground level, and all the others (E i >E 1), i = 2, 3, 4, ...) are called excited, because for the transition to them ( transition to the corresponding stationary excited states from the ground) it is necessary to excite the system - to impart energy E i -E 1 to it from the outside.

Quantization of atomic energy is a consequence of the wave properties of electrons. According to the principle of wave-particle duality, the movement of a microparticle of mass m with speed v corresponds to a wavelength λ = h/mv, where h is Planck’s constant. For an electron in an atom, λ is of the order of 10 -8 cm, that is, of the order of the linear dimensions of the atom, and taking into account the wave properties of the electron in the atom is necessary. The associated motion of an electron in an atom is similar to a standing wave, and it should be considered not as the movement of a material point along a trajectory, but as a complex wave process. For a standing wave in a limited volume, only certain values ​​of the wavelength λ (and, consequently, the oscillation frequency v) are possible. According to quantum mechanics, the energy of an atom E is related to v by the relation E = hν and therefore can only take on certain values. The free translational motion of a microparticle, not limited in space, for example, the motion of an electron separated from an atom (with energy E> 0), is similar to the propagation of a traveling wave in an unlimited volume, for which any values ​​of λ (and v) are possible. The energy of such a free microparticle can take on any value (it is not quantized, it has a continuous energy spectrum). This continuous sequence corresponds to an ionized atom. The value E ∞ = 0 corresponds to the ionization boundary; the difference E ∞ -E 1 = E ion is called ionization energy (see the article Ionization potential); for a hydrogen atom it is 13.6 eV.

Electron density distribution. The exact position of an electron in an atom at a given time cannot be determined due to uncertainties in the relationship. The state of an electron in an atom is determined by its wave function, which in a certain way depends on its coordinates; The square of the modulus of the wave function characterizes the probability density of finding an electron at a given point in space. The wave function is explicitly the solution of the Schrödinger equation.

Thus, the state of an electron in an atom can be characterized by the distribution of its electric charge in space with a certain density - the distribution of electron density. Electrons are, as it were, “smeared” in space and form an “electron cloud”. This model characterizes electrons in an atom more correctly than the model of a point electron moving along strictly defined orbits (in Bohr’s atomic theory). At the same time, each such Bohr orbit can be associated with a specific electron density distribution. For the ground energy level E 1, the electron density is concentrated near the nucleus; for excited energy levels E 2, E 3, E 4 ... it is distributed over increasingly large average distances from the nucleus. In a multielectron atom, electrons are grouped into shells that surround the nucleus at various distances and are characterized by certain electron density distributions. The strength of the bond between electrons and the nucleus in the outer shells is less than in the inner shells, and the weakest electrons are bound in the outermost shell, which has the largest dimensions.

Accounting for electron spin and nuclear spin. In the theory of the atom, it is very important to take into account the spin of the electron - its own (spin) angular momentum, which, from a visual point of view, corresponds to the rotation of the electron around its own axis (if the electron is considered as a small-sized particle). The spin of the electron is associated with a hundred intrinsic (spin) magnetic moment. Therefore, in an atom it is necessary to take into account, along with electrostatic interactions, magnetic interactions determined by the spin magnetic moment and the orbital magnetic moment associated with the movement of the electron around the nucleus; magnetic interactions are small compared to electrostatic ones. The most significant influence of spin is in multielectron atoms: the filling of the electron shells of an atom with a certain number of electrons depends on the spin of the electrons.

The nucleus in an atom can also have its own mechanical moment - nuclear spin, which is associated with a nuclear magnetic moment that is hundreds and thousands of times smaller than the electron one. The existence of spins leads to additional, very small interactions between the nucleus and electrons (see below).

Quantum states of the hydrogen atom. The most important role in the quantum theory of the atom is played by the theory of the simplest one-electron atom, consisting of a nucleus with charge +Ze and an electron with charge -e, that is, the theory of the hydrogen atom H and hydrogen-like ions He +, Li 2+, Be 3+,..., commonly called the theory of the hydrogen atom. Using the methods of quantum mechanics, it is possible to obtain an accurate and complete characterization of the states of an electron in a one-electron atom. The problem of a many-electron atom can only be solved approximately; in this case, they proceed from the results of solving the problem of a one-electron atom.

The energy of a one-electron atom in the non-relativistic approximation (without taking into account the electron spin) is equal to

the integer n = 1, 2, 3, ... defines the possible discrete energy values ​​- energy levels - and is called the principal quantum number, R is the Rydberg constant equal to 13.6 eV. The energy levels of the atom converge (condense) to the ionization boundary E ∞ = 0, corresponding to n = ∞. For hydrogen-like ions, only the scale of energy values ​​changes (Z 2 times). The ionization energy of a hydrogen-like atom (electron binding energy) is (in eV)

which gives for H, He +, Li 2+, ... values ​​13.6 eV, 54.4 eV, 122.4 eV, ....

Basic formula (4) corresponds to the expression U(r) = -Ze 2 /r for the potential energy of an electron in the electric field of a nucleus with charge +Ze. This formula was first derived by N. Bohr by considering the motion of an electron around a nucleus in a circular orbit of radius r and is an exact solution to the Schrödinger equation for such a system. Energy levels (4) correspond to orbits of radius

where the constant a 0 = 0.529·10 -8 cm = = 0.529 A is the radius of the first circular orbit of the hydrogen atom corresponding to its ground level (this Bohr radius is often used as a convenient unit for measuring lengths in atomic physics). The radius of the orbits is proportional to the square of the principal quantum number n 2 and inversely proportional to Z; for hydrogen-like ions, the linear size scale decreases by a factor of Z compared to the hydrogen atom. A relativistic description of the hydrogen atom, taking into account the spin of the electron, is given by the Dirac equation.

According to quantum mechanics, the state of the hydrogen atom is completely determined by the discrete values ​​of four physical quantities: energy E; orbital momentum M l (momentum of the electron relative to the nucleus); projections M lz of the orbital momentum onto an arbitrarily chosen direction z; projections M sz of the spin moment (intrinsic angular momentum of the electron M s). The possible values ​​of these physical quantities, in turn, are determined by the quantum numbers n, l, m l, m s, respectively. In the approximation, when the energy of a hydrogen atom is described by formula (4), it is determined only by the principal quantum number n, which takes on the integer values ​​1, 2, 3, .... An energy level with a given n corresponds to several states that differ in the values ​​of the orbital (azimuthal) quantum number l = 0, 1, ..., n-1. States with given values ​​of n and l are usually denoted as 1s, 2s, 2р, 3s, ..., where the numbers indicate the value of n, and the letters s, р, d, f (hereinafter in the Latin alphabet) - respectively, the values ​​l = 0, 1, 2, 3. For given n and l, the number of different states is equal to 2(2l + 1) - the number of combinations of values ​​of the magnetic orbital quantum number m l magnetic spin number m s (the first takes 2l + 1 values, the second - 2 values). The total number of different states with given n and l is equal to 2n 2. Thus, each energy level of the hydrogen atom corresponds to 2.8, 18,...2n 2 (with n = 1, 2, 3, ...) different stationary quantum states. If only one quantum state corresponds to an energy level, then it is called non-degenerate, if two or more - degenerate (see Degeneracy in quantum theory), and the number of such states g is called the degree or multiplicity of degeneracy (for non-degenerate energy levels g = 1). The energy levels of the hydrogen atom are degenerate, and their degree of degeneracy g n = 2n 2 .

For different states of the hydrogen atom, different electron density distributions are obtained. It depends on the quantum numbers n, l and In this case, the electron density for s-states (l=0) is different from zero in the center, i.e. at the location of the nucleus, and does not depend on the direction (spherically symmetric), and for the rest states (l>0) it is equal to zero at the center and depends on the direction. The electron density distribution for states of the hydrogen atom with n = 1, 2, 3 is shown in Figure 2; the dimensions of the “electron cloud” grow in accordance with formula (6) in proportion to n2 (the scale in Figure 2 decreases when moving from n = 1 to n = 2 and from n = 2 to n = 3). The quantum states of an electron in hydrogen-like ions are characterized by the same four quantum numbers n, l, m l and m s as in the hydrogen atom. The distribution of electron density is also preserved, only it increases by Z times.

Action of external fields on an atom. An atom as an electrical system in external electric and magnetic fields acquires additional energy. The electric field polarizes the atom - it displaces electron clouds relative to the nucleus (see Polarizability of atoms, ions and molecules), and the magnetic field orients in a certain way the magnetic moment of the atom, associated with the movement of the electron around the nucleus (with orbital momentum M l) and its spin. Different states of a hydrogen atom with the same energy E n in an external field correspond to different values ​​of additional energy ΔE, and the degenerate energy level E n is split into a number of sublevels. Both the splitting of energy levels in an electric field - the Stark effect - and their splitting in a magnetic field - the Zeeman effect - are proportional to the strengths of the corresponding fields.

Small magnetic interactions inside an atom also lead to splitting of energy levels. For the hydrogen atom and hydrogen-like ions, there is a spin-orbit interaction - the interaction of the spin and orbital moments of the electron; it determines the so-called fine structure of energy levels - the splitting of excited levels E n (for n>1) into sublevels. For all energy levels of the hydrogen atom, a hyperfine structure is also observed, due to very small magnetic interactions of the nuclear spin with electronic moments.

Electron shells of multielectron atoms. The theory of an atom containing 2 or more electrons is fundamentally different from the theory of a hydrogen atom, since in such an atom there are identical particles interacting with each other - electrons. The mutual repulsion of electrons in a multielectron atom significantly reduces the strength of their bond with the nucleus. For example, the energy of removal of a single electron in a helium ion (He +) is 54.4 eV, while in a neutral helium atom, as a result of the repulsion of electrons, the energy of removal of one of them decreases to 24.6 eV. For the outer electrons of heavier atoms, the decrease in their bond strength due to repulsion by the inner electrons is even more significant. An important role in multielectron atoms is played by the properties of electrons as identical microparticles (see Identity principle) with spin s = 1/2, for which the Pauli principle is valid. According to this principle, in a system of electrons there cannot be more than one electron in each quantum state, which leads to the formation of electron shells of the atom, filled with strictly defined numbers of electrons.

Considering the indistinguishability of electrons interacting with each other, it makes sense to talk only about the quantum states of the atom as a whole. However, it is possible to approximately consider the quantum states of individual electrons and characterize each of them by a set of quantum numbers n, l, m l and m s, similarly to an electron in a hydrogen atom. In this case, the electron energy turns out to depend not only on n, as in the hydrogen atom, but also on l; it still does not depend on m l and m s. Electrons with given n and l in a multielectron atom have the same energy and form a specific electron shell. Such equivalent electrons and the shells formed by them are denoted, like quantum states and energy levels with given n and l, by the symbols ns, nр, nd, nf, ... (for 1 = 0, 1, 2,3,...) and they talk about 2p electrons, 3s-o6 shells, etc.

According to the Pauli principle, any 2 electrons in an atom must be in different quantum states and, therefore, differ in at least one of the four quantum numbers n, l, m l and m s, and for equivalent electrons (n ​​and l are the same) - in the values ​​of m l and m s . The number of pairs m l, m s, i.e. the number of different quantum states of an electron with given n and l, is the degree of degeneracy of its energy level g l = 2 (2l+1) = 2, 6, 10, 14, .... It determines the number of electrons in completely filled electron shells. Thus, the s-, p-, d-, f-, ... shells are filled with 2, 6, 10, 14, ... electrons, regardless of the value of n. Electrons with a given n form a layer consisting of shells with l = 0, 1, 2, ..., n - 1 and filled with 2n 2 electrons, the so-called K-, L-, M, N-layer. When completely filled we have:

In each layer, shells with smaller l are characterized by higher electron density. The strength of the bond between the electron and the nucleus decreases with increasing n, and for a given n, with increasing l. The weaker the electron is bound in the corresponding shell, the higher its energy level lies. A nucleus with a given Z attaches electrons in order of decreasing strength of their bond: first two 1s electrons, then two 2s electrons, six 2p electrons, etc. The atom of each chemical element has a certain distribution of electrons across shells - its electronic configuration, for example:

(the number of electrons in a given shell is indicated by the index at the top right). Periodicity in the properties of elements is determined by the similarity of the outer electron shells of the atom. For example, neutral atoms P, As, Sb, Bi (Z = 15, 33, 51, 83) have three p-electrons in the outer electron shell, like the N atom, and are similar to it in chemical and many physical properties.

Each atom is characterized by a normal electron configuration, which occurs when all the electrons in the atom are most tightly bound, and excited electronic configurations, when one or more electrons are more loosely bound - found at higher energy levels. For example, for a helium atom, along with the normal 1s2, excited electronic configurations are possible: 1s2s, 1s2p, ... (one electron is excited), 2s 2, 2s2p, ... (both electrons are excited). A certain electronic configuration corresponds to one energy level of the atom as a whole, if the electron shells are completely filled (for example, the normal configuration of the Ne atom 1s 2 2s 2 2р 6), and a number of energy levels if there are partially filled shells (for example, the normal configuration of the nitrogen atom 1s 2 2s 2 2р 3 for which the shell 2р is half filled). In the presence of partially filled d- and f-shells, the number of energy levels corresponding to each configuration can reach many hundreds, so the scheme of energy levels of an atom with partially filled shells turns out to be very complex. The ground energy level of an atom is the lowest level of normal electron configuration.

Quantum transitions in the atom. During quantum transitions, an atom moves from one stationary state to another - from one energy level to another. When transitioning from a higher energy level E i to a lower energy level E k, the atom gives up energy E i - E k, and during the reverse transition it receives it. As for any quantum system, for an atom quantum transitions can be of two types: with radiation (optical transitions) and without radiation (non-radiative, or non-optical, transitions). The most important characteristic of a quantum transition is its probability, which determines how often this transition can occur.

In quantum transitions with radiation, the atom absorbs (transition E k → E i) or emits (transition E i → E k) electromagnetic radiation. Electromagnetic energy is absorbed and emitted by an atom in the form of a light quantum - a photon - characterized by a certain oscillation frequency v, according to the relationship:

where hv is the photon energy. Relationship (7) represents the law of conservation of energy for microscopic processes associated with radiation.

An atom in the ground state can only absorb photons, but in excited states it can both absorb and emit them. A free atom in the ground state can exist indefinitely. The duration of an atom's stay in an excited state (the lifetime of this state) is limited; the atom spontaneously (spontaneously), partially or completely loses its excitation energy, emitting a photon and moving to a lower energy level; Along with such spontaneous emission, stimulated emission is also possible, which occurs, like absorption, under the influence of photons of the same frequency. The higher the probability of a spontaneous transition, the shorter the lifetime of an excited atom; for a hydrogen atom it is about 10 -8 s.

The set of frequencies v of possible transitions with radiation determines the atomic spectrum of the corresponding atom: the set of frequencies of transitions from lower to upper levels is its absorption spectrum, the set of frequencies of transitions from upper to lower levels is the emission spectrum. Each such transition in the atomic spectrum corresponds to a certain spectral line of frequency v.

In non-radiative quantum transitions, an atom gains or loses energy when interacting with other particles that it collides with in a gas or is bound for a long time in a molecule, liquid or solid. In a gas, the atom can be considered free during the time intervals between collisions; During a collision (impact), an atom can move to a lower or higher energy level. Such a collision is called inelastic (as opposed to an elastic collision, in which only the kinetic energy of the translational motion of the atom changes, and its internal energy remains unchanged). An important special case is the collision of a free atom with an electron; Usually the electron moves faster than the atom, the collision time is very short and we can talk about an electron impact. Exciting an atom by electron impact is one method of determining its energy levels.

Chemical and physical properties of the atom. Most of the properties of an atom are determined by the structure and characteristics of its outer electron shells, in which electrons are bound to the nucleus relatively weakly (binding energies from several eV to several tens of eV). The structure of the inner shells of an atom, the electrons of which are bound much more tightly (binding energies of hundreds, thousands and tens of thousands of eV), appears only when the atom interacts with fast particles and high-energy photons (more than hundreds of eV). Such interactions determine the X-ray spectra of the atom and the scattering of fast particles (see Particle diffraction). The mass of an atom determines its mechanical properties during the movement of the atom as a whole - momentum, kinetic energy. Various resonant and other physical properties of the atom depend on the mechanical and associated magnetic and electrical moments of the atom (see Electron paramagnetic resonance, Nuclear magnetic resonance, Nuclear quadrupole resonance).

The electrons in the outer shells of an atom are easily affected by external influences. When atoms come together, strong electrostatic interactions occur, which can lead to the formation of a chemical bond. Weaker electrostatic interactions of two atoms are manifested in their mutual polarization - the displacement of electrons relative to the nuclei, which is strongest for weakly bound outer electrons. Polarization forces of attraction between atoms arise, which must be taken into account even at large distances between them. Atom polarization also occurs in external electric fields; As a result, the energy levels of the atom are shifted and, most importantly, the degenerate energy levels are split (Stark effect). Polarization of an atom can occur under the influence of the electric field of a light (electromagnetic) wave; it depends on the frequency of light, which determines the dependence on it and the refractive index (see Dispersion of light), associated with the polarizability of the atom. The close connection between the optical characteristics of an atom and its electrical properties is especially clearly manifested in its optical spectra.

The magnetic properties of atoms are determined mainly by the structure of their electronic shells. The magnetic moment of an atom depends on its mechanical moment (see Magneto-mechanical ratio); in an atom with completely filled electron shells it is zero, just like the mechanical moment. Atoms with partially filled outer electron shells usually have non-zero magnetic moments and are paramagnetic. In an external magnetic field, all levels of atoms whose magnetic moment is not equal to zero are split - the Zeeman effect takes place. All atoms have diamagnetism, which is caused by the appearance of a magnetic moment in them under the influence of an external magnetic field (the so-called induced magnetic moment, similar to the electric dipole moment of an atom).

With the sequential ionization of an atom, that is, with the removal of its electrons, starting with the outermost ones in order of increasing strength of their bond, all the properties of the atom, determined by its outer shell, change accordingly. More and more tightly bound electrons become external; as a result, the ability of an atom to polarize in an electric field greatly decreases, the distances between energy levels and the frequencies of optical transitions between these levels increase (which leads to a shift of the spectra towards increasingly shorter wavelengths). A number of properties exhibit periodicity: the properties of ions with similar external electrons are similar; for example, N 3+ (two 2s electrons) shows similarity to N 5+ (two 1s electrons). This applies to the characteristics and relative positions of energy levels and to optical spectra, to the magnetic moments of an atom, and so on. The most dramatic change in properties occurs when the last electron is removed from the outer shell, when only completely filled shells remain, for example, when going from N 4+ to N 5+ (electronic configurations 1s 2 2s and 1s 2). In this case, the ion is most stable and its total mechanical and total magnetic moments are equal to zero.

The properties of an atom in a bound state (for example, part of a molecule) differ from the properties of a free atom. The properties of an atom undergo the greatest changes, determined by the outermost electrons that take part in the attachment of a given atom to another. At the same time, the properties determined by the electrons of the inner shells may remain virtually unchanged, as is the case for X-ray spectra. Some properties of an atom may experience relatively small changes, from which information can be obtained about the nature of the interactions of bonded atoms. An important example is the splitting of atomic energy levels in crystals and complex compounds, which occurs under the influence of electric fields created by surrounding ions.

Experimental methods for studying the structure of an atom, its energy levels, its interactions with other atoms, elementary particles, molecules, external fields, and so on are varied, but the main information is contained in its spectra. Atomic spectroscopy methods in all wavelength ranges, and in particular modern laser spectroscopy methods, make it possible to study increasingly subtle effects associated with the atom. Since the beginning of the 19th century, the existence of the atom was obvious to scientists, but an experiment to prove the reality of its existence was carried out by J. Perrin at the beginning of the 20th century. With the development of microscopy, it became possible to obtain images of atoms on the surface of solids. The atom was first seen by E. Muller (USA, 1955) using the field ion microscope he invented. Modern atomic force and tunneling microscopes make it possible to obtain images of solid surfaces with good resolution at the atomic level (see Figure 3).

Rice. 3. Image of the atomic structure of the silicon surface obtained by Oxford University professor M. Capstell using a scanning tunneling microscope.

So-called exotic atoms exist and are widely used in various studies, for example muonic atoms, i.e. atoms in which all or part of the electrons are replaced by negative muons, muonium, positronium, as well as hadronic atoms consisting of charged pions, kaons, protons, deuterons, etc. The first observations of the antihydrogen atom (2002) - an atom consisting of a positron and an antiproton - were also made.

Lit.: Born M. Atomic physics. 3rd ed. M., 1970; Fano U., Fano L. Physics of atoms and molecules. M., 1980; Shpolsky E.V. Atomic physics. 7th ed. M., 1984. T. 1-2; Elyashevich M. A. Atomic and molecular spectroscopy. 2nd ed. M., 2000.