The size and mass of an atom. Big encyclopedia of oil and gas

Atom size determined by the radius of its outer electron shell. The dimensions of all atoms are ~ 10 -10 m. And the size of the nucleus is 5 orders of magnitude smaller, in total - 10 -15 m. Visually, this can be represented as follows: if the atom is increased to the size of a 20-story building, then the nucleus of the atom will look like a millimeter speck of dust in the central room in this house. However, it is difficult to imagine a house, the mass of which is almost completely concentrated in this speck of dust. And the atom is just that.

Atoms are very small and very light. An atom is as many times lighter than an apple as an apple is lighter than a globe. If the world "gets heavier" so that an atom begins to weigh like a drop of water, then people in such a world will become heavy, like planets: children - like Mercury and Mars, and adults - like Venus and Earth.

You can't see an atom even with a microscope. The best optical microscopes make it possible to distinguish the details of an object if the distance between them is ~0.2 µm. In an electron microscope, this distance was reduced to ~2-3 Å. For the first time, it was possible to distinguish and photograph individual atoms using an ion projector. But no one saw how the atom is arranged inside. All data on the structure of atoms are obtained from experiments on particle scattering.

Mass of the atomic nucleus several thousand times the mass of its electron shell. This is due to the fact that the nuclei of atoms consist of very heavy, in comparison with the electron, particles - protons. p and neutrons n. Their masses are almost the same and about 2000 times the mass of an electron. Wherein proton- positively charged particles, and neutron- neutral. The charge of a proton is equal in magnitude to the charge of an electron. The number of protons in the nucleus is equal to the number of electrons in the shell, and this ensures the electrical neutrality of the atom. The number of neutrons can be different, in the nucleus of a light hydrogen atom there are no neutrons at all, and in the nucleus of a carbon atom there can be 6, and 7, and 8.

Mass of an electronme ≈ 0.91. 10 -30 kg, proton massm p≈ 1.673. 10 -27 kg = 1836m e , neutron massm n \u003d 1.675. 10 -27 kg≈ 1840 me.

mass of an atom less than the sum of the masses of the nucleus and electrons per size ∆m, called mass defect, which arises due to the Coulomb interaction of the nucleus and electrons. The mass defect of atoms (unlike nuclei) is very small, and although it increases with increasing Z, not a single atom exceeds the mass of an electron. material from the site

Of course, an atom cannot be put on a scale and weighed, it is too small. The masses of atoms were first determined by chemists. Moreover, they measured them in relative units, taking the mass of a hydrogen atom as a unit and using Dalton's law, according to which chemical substances are formed by combining atoms of chemical elements in a strictly defined proportion. And now the masses of atoms are most often measured in relative units, but 1/12 of the mass of the carbon atom C 12.1 a.m. is used as the atomic mass unit (a.m.u.). e.m. = 1.66057 . 10 -27 kg.

An atom is the smallest particle of a chemical substance that is capable of retaining its properties. The word "atom" comes from the ancient Greek "atomos", which means "indivisible". Depending on how many and what particles are in the atom, you can determine the chemical element.

Briefly about the structure of the atom

As you can 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. Every atom in its normal state is neutral. The size of this particle can be completely 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 carry no charge. However, protons, along with 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..

The first information about the atom

The fact that matter can consist of small particles was suspected even by the ancient Greeks. They believed that everything that exists is made up of atoms. However, this view was purely philosophical in nature and cannot be interpreted scientifically.

An English scientist was the first to obtain basic information about the structure of the atom. 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 of oxygen is carbon monoxide.

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

Rutherford's research

Almost a century later, the 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 critical steps that chemistry could make. Basic information about the structure of the atom testified that it is similar to the solar system: particles-electrons rotate around the nucleus in strictly defined orbits, just as the planets do.

Electronic shell of atoms and formulas of atoms of chemical elements

The electron shell of each of the atoms contains exactly as many electrons as there are protons in its nucleus. That is why the atom is neutral. In 1913, another scientist received basic information about the structure of the atom. Niels Bohr's formula was similar to Rutherford's. According to his concept, electrons also revolve around the nucleus located in the center. Bohr finalized Rutherford's theory, introduced harmony into its facts.

Even then, the formulas of some chemicals were drawn up. 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.

Schrödinger model

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

He proposed a new model of its structure - a wave one. By this time, scientists had already proved that the electron was endowed not only with the nature of a particle, but had 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. The level number can be used to characterize the energy of an electron. 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 can have one sublevel, the second - two, the third - three, and so on (see the above electronic formulas for nitrogen and sodium).

Even smaller particles

On the this moment, of course, even smaller particles than the electron, proton and neutron have been discovered. It is known that the proton consists of quarks. There are even smaller particles of the universe - for example, a neutrino, which is a hundred times smaller 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 than the entire observable universe.

Basic information about the structure of the atom: radioactivity

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

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

During the decay of isotopes, radioactive radiation is emitted. This phenomenon was first discovered by Becquerel. The main type of radioactive radiation is alpha decay. It releases an alpha particle. There is also beta decay, in which a beta particle is ejected from the nucleus of an atom, respectively.

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 obtained in nuclear reactors. Many of these isotopes are used in medicine, especially in diagnostics..

Proportions within an atom

If we imagine an atom, the dimensions of which will be comparable to the size 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 a pinhead. Then the nucleus 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 really looks like. In fact, it literally does not look like anything - not for the reason that not enough good microscopes are used in science. The dimensions of an atom are in those areas where the concept of "visibility" simply does not exist.

Atoms are very small. But how small are these dimensions 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 human hand, then next to it would be viruses 300 meters long. Bacteria would be 3 km long and a human hair would be 150 km thick. In the supine position, he could go beyond the boundaries of the earth's atmosphere. And if such proportions were real, then a human hair in length could reach the moon. This is such a complex and interesting atom, the study of which scientists continue to study to this day.

Forming bonds with each other, atoms are combined into molecules and large solids.

Humanity has known about the existence of the smallest particles of matter since ancient times, but confirmation of the existence of atoms was received only at the end of the 19th century. But almost immediately it became clear that atoms, in turn, have a complex structure, which determines their properties.

The concept of an atom as the smallest indivisible particle of matter was first proposed ancient Greek philosophers. In the 17th and 18th centuries, chemists established that chemicals react in certain proportions, which are expressed in terms of small numbers. In addition, they identified certain simple substances, which they called chemical elements. These discoveries led to a revival of the idea of ​​indivisible particles. The development of thermodynamics and statistical physics showed that the thermal properties of bodies can be explained by the motion of such particles. In the end, the sizes of atoms were experimentally determined.

In the late 19th and early 20th centuries, physicists discovered the first of the subatomic particles, the electron, and somewhat later the atomic nucleus, thus showing that the atom is not indivisible. The development of quantum mechanics made it possible to explain not only the structure of atoms, but also their properties: optical spectra, the ability to enter into reactions and form molecules, i.e.

General characteristics of the structure of the atom

Modern ideas about the structure of the atom are based on quantum mechanics.

At the popular level, the structure of the atom can be described in terms of the wave model, which is based on the Bohr model, but also takes into account additional information on quantum mechanics.

For this model:

Atoms consist of elementary particles (protons, electrons and neutrons). The mass of an atom is mostly concentrated in the nucleus, so most of the volume is relatively empty. The nucleus is surrounded by electrons. The number of electrons is equal to the number of protons in the nucleus, the number of protons determines the ordinal number of the element in the periodic system. In a neutral atom, the total negative charge of the electrons is equal to the positive charge of the protons. Atoms of the same element with different numbers of neutrons are called isotopes.
At the center of an atom is a tiny, positively charged nucleus made up of protons and neutrons.
The nucleus of an atom is about 10,000 times smaller than the atom itself. Thus, if an atom is enlarged to the size of the Borispol airport, the size of the nucleus will be smaller than the size of a table tennis ball.
The nucleus is surrounded by an electron cloud that occupies most its volume. In an electron cloud, shells can be distinguished, for each of which there are several possible orbitals. The filled orbitals make up the electronic configuration characteristic of each chemical element.
Each orbital can contain up to two electrons, characterized by three quantum numbers: basic, orbital and magnetic.
Each electron in an orbital has a unique value for the fourth quantum number: spin.
Orbitals are defined by a specific probability distribution of where exactly an electron can be found. Examples of orbitals and their designations are shown in the figure on the right. The "boundary" of an orbital is the distance at which the probability that an electron can be outside of it is less than 90%.
Each shell can contain no more than a strictly defined number of electrons. For example, the shell closest to the nucleus can have a maximum of two electrons, the next - 8, the third from the nucleus - 18, and so on.
When electrons join an atom, they drop into a low-energy orbital. Only outer shell electrons can participate in the formation of interatomic bonds. Atoms can donate and gain electrons, becoming positively or negatively charged ions. The chemical properties of an element are determined by the ease with which the nucleus can donate or acquire electrons. It depends both on the number of electrons and on the degree of filling of the outer shell.
Atom size

The size of an atom is a quantity difficult to measure, because the central nucleus is surrounded by a blurry electron cloud. For atoms that form solid crystals, the distance between adjacent sites of the crystal lattice can serve as an approximate value of their size. For atoms, crystals are not formed, other evaluation techniques are used, including theoretical calculations. For example, the size of a hydrogen atom is estimated as 1.2 × 10-10 m. This value can be compared with the size of a proton (which is the nucleus of a hydrogen atom): 0.87 × 10-15 m and make sure that the nucleus of a hydrogen atom is 100 000 times smaller than the atom itself. Atoms of other elements retain approximately the same ratio. The reason for this is that elements with a large positively charged nucleus attract electrons more strongly.

Another characteristic of the size of an atom is the van der Waals radius - the distance that another atom can approach a given atom. Interatomic distances in molecules are characterized by the length of chemical bonds or covalent radius.

Core

The main mass of an atom is concentrated in the nucleus, which consists of nucleons: protons and neutrons, interconnected by the forces of nuclear interaction.

The number of protons in an atom's nucleus determines its atomic number and the element to which the atom belongs. For example, carbon atoms contain 6 protons. All atoms with a particular atomic number have the same physical characteristics and exhibit the same chemical properties. The elements are listed in the periodic table in ascending order of atomic number.

The total number of protons and neutrons in an element's atom determines its atomic mass, since a proton and a neutron have a mass of approximately 1 amu. Neutrons in a nucleus do not affect which element an atom belongs to, but a chemical element can have atoms with the same number of protons and a different number of neutrons. Such atoms have the same atomic number but different atomic mass and are called isotopes of the element. When writing the name of an isotope, the atomic mass is written after it. For example, the isotope carbon-14 contains 6 protons and 8 neutrons, for a total atomic mass of 14. Another popular notation method is to superscript the atomic mass before the element symbol. For example, carbon-14 is referred to as 14C.

The atomic mass of an element given in the periodic table is an average of the masses of naturally occurring isotopes. Averaging is carried out according to the abundance of the isotope in nature.

With an increase in the atomic number, the positive charge of the nucleus increases, and, consequently, the Coulomb repulsion between protons. More and more neutrons are needed to hold protons together. However, a large number of neutrons is unstable, and this circumstance imposes a limitation on the possible charge of the nucleus and the number of chemical elements that exist in nature. Chemical elements with high atomic numbers have a very short lifetime, can only be created by bombarding the nuclei of light elements with ions, and are observed only during experiments using accelerators. As of February 2008, ununoctium is the heaviest synthesized chemical element.

Many isotopes of chemical elements are unstable and decay over time. This phenomenon is used by the radioelement test to determine the age of objects. great importance for archeology and paleontology.

Bohr model

The Bohr model is the first physical model that was able to correctly describe the optical spectra of the hydrogen atom. After the development of the exact methods of quantum mechanics, the Bohr model has only historical significance, but due to its simplicity, it is still widely taught and used for a qualitative understanding of the structure of the atom.

Bohr's model is based on Rutherford's planetary model, which describes the atom as a small positively charged nucleus with negatively charged electrons in orbits at different levels, which resembles the structure solar system. Rutherford proposed a planetary model to explain the results of his experiments on the scattering of alpha particles by metal foil. According to the planetary model, an atom consists of a heavy nucleus around which electrons revolve. But the fact that the electrons rotating around the nucleus do not fall in a spiral onto it was incomprehensible to the physicists of that time. Indeed, according to classical theory electromagnetism, an electron that revolves around the nucleus must radiate electromagnetic waves(light), which would lead to a gradual loss of energy and fall to the core. So how can an atom exist at all? Moreover, the study of the electromagnetic spectrum of atoms showed that the electrons in an atom can only emit light of a certain frequency.

These difficulties were overcome in the model proposed by Niels Bohr in 1913, which postulates that:

Electrons can only be in orbits that have discrete quantized energies. That is, not all orbits are possible, but only some specific ones. The exact values ​​of the energies of admissible orbits depend on the atom.
The laws of classical mechanics do not apply when electrons move from one allowable orbit to another.
When an electron moves from one orbit to another, the difference in energy is emitted (or absorbed) by a single quantum of light (photon), whose frequency is directly related to the energy difference between the two orbits.

where ν is the frequency of the photon, E is the energy difference, and h is a constant of proportionality, also known as Planck's constant.
Determine what can be written

where ω is the angular frequency of the photon.
Permissible orbits depend on the quantized values ​​of the orbital angular momentum L, described by the equation

where n = 1,2,3,...
and is called the quantum number of angular momentum.
These assumptions made it possible to explain the results of the then observations, for example, why the spectrum consists of discrete lines. Assumption (4) states that the smallest value of n is 1. Accordingly, the smallest allowable atomic radius is 0.526 Å (0.0529 nm = 5.28 10-11 m). This value is known as the Bohr radius.

Bohr's model is sometimes referred to as Semiclassical because although it includes some ideas from quantum mechanics, it is not a complete quantum mechanical description of the hydrogen atom. However, Bohr's model was a significant step towards such a description.

With a strict quantum mechanical description of the hydrogen atom, the energy levels are found from the solution of the stationary Schrödinger equation. These levels are characterized by the above three quantum numbers, the formula for quantizing the angular momentum is different, the quantum number of the angular momentum is zero for spherical s-orbitals, one for prolate dumbbell-shaped p-orbitals, etc. (see picture above).

The energy of the atom and its quantization

The energy values ​​that an atom can have are calculated and interpreted based on the provisions of quantum mechanics. This takes into account such factors as the electrostatic interaction of electrons with the nucleus and electrons among themselves, the spins of electrons, the principle of identical particles. In quantum mechanics, the state in which an atom is located is described by a wave function, which can be found from the solution of the Schrödinger equation. There is a certain set of states, each of which has a certain energy value. The state with the lowest energy is called the ground state. Other states are called excited. An atom is in an excited state for a finite time, emitting sooner or later a quantum of an electromagnetic field (photon) and passing into the ground state. An atom can stay in the ground state for a long time. In order to be aroused, he needs external energy, which can only come to him from external environment. An atom emits or absorbs light only at certain frequencies, corresponding to the difference in the energies of its states.

The possible states of an atom are indexed by quantum numbers such as spin, quantum number of orbital momentum, quantum number of total momentum. You can read more about their classification in the article electronic term

Electronic shells of complex atoms

Complex atoms have dozens, and for very heavy elements, even hundreds of electrons. According to the principle of identical particles, the electronic states of atoms are formed by all electrons, and it is impossible to determine where each of them is located. However, in the so-called one-electron approximation, one can speak of certain energy states of individual electrons.

According to these ideas, there is a certain set of orbitals that are filled with the electrons of the atom. These orbitals form a certain electronic configuration. Each orbital can contain no more than two electrons (Pauli exclusion principle). Orbitals are grouped into shells, each of which can only have a certain fixed number of orbitals (1, 4, 10, etc.). Orbitals are divided into internal and external. In the ground state of an atom, the inner shells are completely filled with electrons.

In inner orbitals, electrons are very close to the nucleus and are strongly attached to it. To pull an electron out of the inner orbit, you need to provide it with a lot of energy, up to several thousand electron volts. An electron on the inner shell can obtain such energy only by absorbing an X-ray quantum. The energies of the inner shells of atoms are individual for each chemical element, and therefore an atom can be identified by the X-ray absorption spectrum. This circumstance is used in x-ray analysis.

In the outer shell, the electrons are far from the nucleus. It is these electrons that are involved in the formation of chemical bonds, therefore the outer shell is called valence, and the outer shell electrons are called valence electrons.

Quantum transitions in the atom

Transitions between different states of atoms are possible, caused by an external perturbation, more often electromagnetic field. Due to the quantization of the states of an atom, the optical spectra of atoms consist of individual lines if the energy of a light quantum does not exceed the ionization energy. At higher frequencies, the optical spectra of atoms become continuous. The probability of excitation of an atom by light decreases with a further increase in frequency, but increases sharply at certain frequencies characteristic of each chemical element in the X-ray range.

Excited atoms emit light quanta with the same frequencies at which absorption occurs.

Transitions between different states of atoms can also be caused by interactions with fast charged particles.

Chemical and physical properties atom

The chemical properties of an atom are determined mainly by valence electrons - electrons in the outer shell. The number of electrons in the outer shell determines the valency of the atom.

The atoms of the last column of the periodic table of elements have a completely filled outer shell, and for the transition of an electron to the next shell, a very large amount of energy must be provided to the atom. Therefore, these atoms are inert, not inclined to enter into chemical reactions. Inert gases thin out and crystallize only at very low temperatures.

The atoms of the first column of the periodic table of elements have one electron on the outer shell, and are chemically active. Their valency is 1. characteristic type chemical bond for these atoms in the crystallized state is a metallic bond.

The atoms of the second column of the periodic table in the ground state have 2 s-electrons on the outer shell. Their outer shell is filled, so they must be inert. But the transition from the ground state with the s2 electron shell configuration to the state with the s1p1 configuration requires very little energy, so these atoms have a valence of 2, but they show less activity.

The atoms of the third column of the periodic table of elements have the electronic configuration s2p1 in the ground state. They can show different valencies: 1, 3, 5. The last possibility arises when the electron shell of the atom is completed to 8 electrons and becomes closed.

Atoms in the fourth column of the periodic table of elements have a valence of 4 (for example, carbon dioxide CO2), although a valence of 2 is also possible (for example, carbon monoxide CO). Before this column belongs carbon - an element that forms a wide variety of chemical compounds. A special branch of chemistry is devoted to carbon compounds - organic chemistry. Other elements of this column - silicon, germanium under normal conditions are solid-state semiconductors.

The elements of the fifth column have a valence of 3 or 5.

The elements of the sixth column of the periodic table in the ground state have an s2p4 configuration and a common spin of 1. Therefore, they are divalent. There is also the possibility of an atom transitioning to an excited state s2p3s" with spin 2, in which the valency is 4 or 6.

The elements of the seventh column of the periodic table lack one electron in the outer shell in order to fill it. They are mostly monovalent. However, they can enter into chemical compounds in excited states, showing valences of 3,5,7.

Transition elements are characterized by the filling of the outer s-shell before the d-shell is completely filled. Therefore, they mostly have a valence of 1 or 2, but in some cases one of the d-electrons is involved in the formation of chemical bonds, and the valence becomes equal to three.

When chemical compounds are formed, atomic orbitals are modified, deformed and become molecular orbitals. In this case, the process of hybridization of orbitals takes place - the formation of new orbitals, as a specific sum of the base ones.

History of the concept of atom

Read more in the article atomistics
The concept of atom, like the word itself, is of ancient Greek origin, although the truth of the hypothesis of the existence of atoms was confirmed only in the 20th century. The main idea behind this concept for all centuries was the idea of ​​the world as a set of a huge number of indivisible elements that are very simple in structure and have existed since the beginning of time.

The first preachers of the atomistic doctrine

The philosopher Leucippus was the first to preach atomistic teachings in the 5th century BC. Then the baton was picked up by his student Democritus. Only fragments of their works have survived, from which it becomes clear that they proceeded from a small number of rather abstract physical hypotheses:

"Sweetness and bitterness, heat and cold are the meaning of the definition, in fact [only] atoms and emptiness."

According to Democritus, all nature consists of atoms, the smallest particles of matter that rest or move in a completely empty space. All atoms have a simple form, and atoms of the same kind are identical; the variety of nature reflects the variety of forms of atoms and the variety of ways in which atoms can interlock with each other. Both Democritus and Leucippus believed that atoms, having begun to move, continue to move according to the laws of nature.

The most difficult for the ancient Greeks was the question of the physical reality of the basic concepts of atomism. In what sense could one speak of the reality of emptiness if, having no matter, it cannot have any physical properties? The ideas of Leucippus and Democritus could not serve as a satisfactory basis for the theory of matter on the physical plane, since they did not explain what atoms are not made of, nor why atoms are indivisible.

A generation after Democritus, Plato proposed his solution to this problem: “the smallest particles do not belong to the realm of matter, but to the realm of geometry; they are different bodily geometric figures bounded by flat triangles.

The concept of the atom in Indian philosophy

A thousand years later, the abstract reasoning of the ancient Greeks penetrated into India and was adopted by some schools of Indian philosophy. But if Western philosophy believed that the atomistic theory should become a concrete and objective basis for the theory of the material world, Indian philosophy always perceived the material world as an illusion. When atomism appeared in India, it took the form of a theory according to which reality in the world has a process, not a substance, and that we are present in the world as links in a process, and not as clots of matter.

That is, both Plato and Indian philosophers thought something like this: if nature consists of small, but finite in size, shares, then why can’t they be divided, at least in the imagination, into even smaller particles, which became the subject of further consideration?

Atomistic theory in Roman science

The Roman poet Lucretius (96 - 55 BC) was one of the few Romans who showed an interest in pure science. In his poem On the Nature of Things (De rerum natura), he built up in detail the facts that testify in favor of the atomistic theory. For example, a wind that blows with great force, although no one can see it, is probably composed of particles, leaking to see them. We can feel things from a distance by smell, sound and heat that spread without being seen.

Lucretius connects the properties of things with the properties of their constituents, i.e. atoms: liquid atoms are small and rounded, which is why liquid flows so easily and seeps through porous matter, while solid atoms have hooks that hold them together. In the same way, various taste sensations and sounds of different loudness are composed of atoms of appropriate shapes - from simple and harmonious to sinuous and irregular.

But the teachings of Lucretius were condemned by the church, since he gave a rather materialistic interpretation of them: for example, the idea that God, having launched the atomic mechanism once, no longer interferes with its work, or that the soul dies with the body.

The first theories about the structure of the atom

One of the first theories about the structure of the atom, which already has modern outlines, was described by Galileo (1564-1642). According to his theory, matter consists of particles that are not at rest, but move in all directions under the influence of heat; heat is nothing but the movement of particles. The structure of the particles is complex, and if you deprive any part of its material shell, then light will spurt from within. Galileo was the first to present, albeit in fantastic form, the structure of the atom.

Scientific Foundations

In the 19th century, John Dalton obtained evidence for the existence of atoms, but assumed that they were indivisible. Ernest Rutherford showed experimentally that an atom consists of a nucleus surrounded by negatively charged particles - electrons.

Let us consider the dependence of some properties of atoms on the structure of their electron shells. Let us dwell, first of all, on the patterns of change in atomic and ionic radii.

Electron clouds do not have sharply defined boundaries. Therefore, the concept of the size of an atom is not strict. But if we imagine the atoms in the crystals of a simple substance in the form of balls in contact with each other, then the distance between the centers of neighboring balls (i.e., between the nuclei of neighboring atoms) can be taken equal to twice the radius of the atom. So, the smallest internuclear distance in copper crystals is ; this allows us to consider that the radius of the copper atom is equal to half this value, i.e. .

The dependence of atomic radii on the charge of the atomic nucleus Z has a periodic character. Within one period, with increasing Z, there is a tendency to a decrease in the size of the atom, which is especially clearly observed in short periods (atomic radii are given in nm):

This is explained by the increasing attraction of the electrons of the outer layer to the nucleus as its charge increases.

With the beginning of the construction of a new electron layer, more distant from the nucleus, i.e., during the transition to the next period, the atomic radii increase (compare, for example, the radii of fluorine and sodium atoms). As a result, within the subgroup, as the charge of the nucleus increases, the sizes of atoms increase. Let us give as an example the values ​​of atomic radii (in nm) of elements of some main subgroups:

The electrons of the outer layer, which are the least strongly bound to the nucleus, can break away from the atom and join other atoms, becoming part of the outer layer of the latter.

Atoms that have lost one or more electrons become positively charged, since the charge of the atom's nucleus exceeds the sum of the charges of the remaining electrons. Conversely, atoms that have attached extra electrons to themselves become negatively charged. The resulting charged particles are called ions.

Ions are denoted by the same symbols as atoms, indicating their charge at the top right: for example, a positive three-charged aluminum ion is denoted, a negative singly charged chlorine ion is.

The loss of electron atoms leads to a decrease in its effective size, and the addition of excess electrons leads to an increase. Therefore, the radius of a positively charged ion (cation) is always less, and the radius of a negatively charged non (anion) is always greater than the radius of the corresponding electrically neutral atom. So, the radius of the potassium atom is , and the radius of the ion, the radii of the chlorine atom and ion, respectively, are 0.099 and . In this case, the radius of the ion differs the more from the radius of the atom, the greater the charge of the ion. For example, the radii of the chromium atom and ions and are 0.127, 0.083 and , respectively.

Within one subgroup, the radii of ions of the same charge increase with increasing nuclear charge. This is illustrated by the following examples (ion radii are given in nm):

This regularity is explained by the increase in the number of electron layers and the growing distance of the outer electrons from the nucleus.

DEFINITION

Atom is the smallest chemical particle.

The variety of chemical compounds is due to the different combination 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 electron shell is of paramount importance.

Models of the structure of the atom

At the beginning of the 19th century, D. Dalton revived the atomistic 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 (the atoms of the same element have the same properties, and the atoms of other elements have different properties, the concept of atomic mass was introduced), the atom was considered indivisible.

After receiving experimental evidence (late XIX - early XX 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 the structure of the atom. One of the first models was proposed J. Thomson(1904) (Fig. 1): the atom was presented 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 structure of the atom (Fig. 1), similar to the structure of the solar system. According to the planetary model, in the center of the atom there is a very small nucleus with a charge Z e, the size of which is approximately 1,000,000 times smaller than the size of the atom itself. The nucleus contains almost the entire mass of the atom and has a positive charge. Electrons move in orbits around the nucleus, the number of which is determined by the charge of the nucleus. The outer trajectory of the electrons determines the outer 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 the structure of the atom according to Thomson and Rutherford

Experiments on the study of atomic spectra showed the imperfection of the planetary model of the structure of the atom, since this model contradicts the line structure of atomic spectra. Based on the Rutherford model, Einstein's theory of light quanta and quantum theory plank radiation Niels Bohr (1913) formulated postulates, which contains atomic theory(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 the 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 showed 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 creating modern model of the structure of the atom.

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 - it exhibits the properties of both particles and waves (particle-wave dualism);

— quantum mechanics denies the existence of certain orbits for microparticles (for moving electrons it is impossible to determine the exact position, because they move in space near the nucleus, one 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 sufficiently high (90%), is called orbital.

quantum numbers. Pauli principle. Rules of Klechkovsky

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

n is the principal quantum number. Characterizes the total energy 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 at 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 the ground state. States with more high 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, in reality, n exists from 1 to 7. The main quantum number determines the size of the electron cloud and determines the average radius of the electron in the atom.

l is the orbital quantum number. It 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 at the 1st level there is a 1st sublevel.

me is the 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 the p-orbital space

s is the spin quantum number. Characterizes the electron's own rotation around the axis. It takes the 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 orbitals with electrons is determined by Klechkovsky's rules: orbitals are filled with electrons in ascending order of the sum (n + l) for these orbitals, if the sum (n + l) is the same, then the orbital with the lower value of n is filled first.

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

The main 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 over 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 on 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, the electrons fill the s-sublevel, then the p-sublevel. For example:

5 B 1s 2 2s 2 2p 1

The relationship of the electronic structure of the atom with the position of the element in the Periodic system

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

5 B 1s 2 2s 2 2p 1

For atoms of some elements, the phenomenon of "leakage" of an electron from an external energy level to the penultimate one is observed. Electron slip 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

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

5 B 1s 2 2s 2 2p 1

The group number for the elements of the main subgroups is equal to the number of electrons in the external energy level, such electrons are called valence electrons (they participate in the formation of a chemical bond). The valence electrons of the elements of the side subgroups can be electrons of the outer energy level and the d-sublevel of the penultimate level. The number of the group of elements of the side subgroups of 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 phosphorus, rubidium and zirconium atoms. List 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