The formula for the structure of an atom. The structure of the electron shells of atoms

Atom- the smallest particle of a substance that is chemically indivisible. In the 20th century, the complex structure of the atom was elucidated. Atoms are made up of positively charged nuclei and a shell formed by negatively charged electrons. The total charge of a free atom is zero, since the charges of the nucleus and electron shell balance each other. In this case, the charge of the nucleus is equal to the number of the element in the periodic table ( atomic number) and is equal to the total number of electrons (electron charge is −1).

The atomic nucleus is made up of positively charged protons and neutral particles - neutrons that have no charge. The generalized characteristics of elementary particles in the composition of an atom can be presented in the form of a table:

The number of protons is equal to the charge of the nucleus, therefore, equal to the atomic number. To find the number of neutrons in an atom, it is necessary to subtract the nuclear charge (the number of protons) from the atomic mass (the sum of the masses of protons and neutrons).

For example, in the sodium atom 23 Na, the number of protons is p = 11, and the number of neutrons is n = 23 − 11 = 12

The number of neutrons in atoms of the same element can be different. Such atoms are called isotopes .

The electron shell of the atom also has a complex structure. Electrons are located on energy levels (electronic layers).

The level number characterizes the electron energy. This is due to the fact that elementary particles can transmit and receive energy not in arbitrarily small quantities, but in certain portions - quanta. The higher the level, the more energy the electron has. Since the lower the energy of the system, the more stable it is (compare the low stability of a stone on top of a mountain, which has a large potential energy, and the stable position of the same stone on the plain below, when its energy is much lower), the levels with low electron energy are filled first and only then - high.

The maximum number of electrons that a level can hold can be calculated using the formula:
N \u003d 2n 2, where N is the maximum number of electrons in the level,
n - level number.

Then for the first level N = 2 1 2 = 2,

for the second N = 2 2 2 = 8, etc.

The number of electrons at the outer level for the elements of the main (A) subgroups is equal to the group number.

In most modern periodic tables, the arrangement of electrons by levels is indicated in the cell with the element. Very important understand that the levels are read down up, which corresponds to their energy. Therefore, a column of numbers in a cell with sodium:
1
8
2

at the 1st level - 2 electrons,

at the 2nd level - 8 electrons,

at the 3rd level - 1 electron
Be careful, a very common mistake!

The distribution of electrons over levels can be represented as a diagram:
11 Na)))
2 8 1

If the periodic table does not indicate the distribution of electrons by levels, you can be guided by:

• the maximum number of electrons: at the 1st level, no more than 2 e - ,
  on the 2nd - 8 e - ,
  at the external level - 8 e − ;
• the number of electrons in the outer level (for the first 20 elements, it is the same as the group number)

Then for sodium the course of reasoning will be as follows:

1. The total number of electrons is 11, therefore, the first level is filled and contains 2 e − ;
2. The third, outer level contains 1 e − (I group)
3. The second level contains the remaining electrons: 11 − (2 + 1) = 8 (completely filled)

* For a clearer distinction between a free atom and an atom in a compound, a number of authors propose using the term "atom" only to refer to a free (neutral) atom, and to refer to all atoms, including those in compounds, they propose the term "atomic particles". Time will tell how the fate of these terms will turn out. From our point of view, an atom, by definition, is a particle, therefore, the expression "atomic particles" can be considered as a tautology ("butter oil").

2. Task. Calculation of the amount of substance of one of the reaction products, if the mass of the starting substance is known.
Example:

What amount of hydrogen substance will be released during the interaction of zinc with hydrochloric acid weighing 146 g?

Decision:

1. We write the reaction equation: Zn + 2HCl \u003d ZnCl 2 + H 2
2. Find the molar mass of hydrochloric acid: M (HCl) \u003d 1 + 35.5 \u003d 36.5 (g / mol)
   (we look at the molar mass of each element, numerically equal to the relative atomic mass, in the periodic table under the sign of the element and round it up to integers, except for chlorine, which is taken as 35.5)
3. Find the amount of hydrochloric acid substance: n (HCl) \u003d m / M \u003d 146 g / 36.5 g / mol \u003d 4 mol
4. We write the available data above the reaction equation, and under the equation - the number of moles according to the equation (equal to the coefficient in front of the substance):
   4 mol x mol
   Zn + 2HCl \u003d ZnCl 2 + H 2
   2 mol 1 mol
5. We make a proportion:
   4 mol - x mole
   2 mol - 1 mol
   (or with explanation:
   from 4 moles of hydrochloric acid you get x mole of hydrogen
   and out of 2 mol - 1 mol)
6. We find x:
   x= 4 mol 1 mol / 2 mol = 2 mol

Answer: 2 mol.

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 the quantum theory of radiation, Planck 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 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, 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 a smaller 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

Electrons

The concept of an atom originated in the ancient world to denote the particles of matter. In Greek, atom means "indivisible".

The Irish physicist Stoney, on the basis of experiments, came to the conclusion that electricity is carried by the smallest particles that exist in the atoms of all chemical elements. In 1891, Stoney proposed to call these particles electrons, which in Greek means "amber". A few years after the electron got its name, English physicist Joseph Thomson and French physicist Jean Perrin proved that electrons carry a negative charge. This is the smallest negative charge, which in chemistry is taken as a unit (-1). Thomson even managed to determine the speed of the electron (the speed of an electron in orbit is inversely proportional to the orbit number n. The radii of the orbits grow in proportion to the square of the orbit number. In the first orbit of the hydrogen atom (n=1; Z=1), the speed is ≈ 2.2 106 m / c, that is, about a hundred times less than the speed of light c=3 108 m/s.) and the mass of an electron (it is almost 2000 times less than the mass of a hydrogen atom).

The state of electrons in an atom

The state of an electron in an atom is a set of information about the energy of a particular electron and the space in which it is located. An electron in an atom does not have a trajectory of motion, i.e., one can only speak of the probability of finding it in the space around the nucleus.

It can be located in any part of this space surrounding the nucleus, and the totality of its various positions is considered as an electron cloud with a certain negative charge density. Figuratively, this can be imagined as follows: if it were possible to photograph the position of an electron in an atom in hundredths or millionths of a second, as in a photo finish, then the electron in such photographs would be represented as points. Overlaying countless such photographs would result in a picture of an electron cloud with the highest density where there will be most of these points.

The space around the atomic nucleus, in which the electron is most likely to be found, is called the orbital. It contains approximately 90% e-cloud, and this means that about 90% of the time the electron is in this part of space. Distinguished by shape 4 currently known types of orbitals, which are denoted by Latin letters s, p, d and f. A graphic representation of some forms of electronic orbitals is shown in the figure.

The most important characteristic of the motion of an electron in a certain orbit is the energy of its connection with the nucleus. Electrons with similar energy values ​​form a single electron layer, or energy level. Energy levels are numbered starting from the nucleus - 1, 2, 3, 4, 5, 6 and 7.

An integer n, denoting the number of the energy level, is called the main quantum number. It characterizes the energy of electrons occupying a given energy level. The electrons of the first energy level, closest to the nucleus, have the lowest energy. Compared with the electrons of the first level, the electrons of the next levels will be characterized by a large amount of energy. Consequently, the electrons of the outer level are the least strongly bound to the nucleus of the atom.

The largest number of electrons in the energy level is determined by the formula:

N = 2n2,

where N is the maximum number of electrons; n is the level number, or the main quantum number. Consequently, the first energy level closest to the nucleus can contain no more than two electrons; on the second - no more than 8; on the third - no more than 18; on the fourth - no more than 32.

Starting from the second energy level (n = 2), each of the levels is subdivided into sublevels (sublayers), which differ somewhat from each other in the binding energy with the nucleus. The number of sublevels is equal to the value of the main quantum number: the first energy level has one sublevel; the second - two; third - three; fourth - four sublevels. Sublevels, in turn, are formed by orbitals. Each valuen corresponds to the number of orbitals equal to n.

It is customary to designate sublevels in Latin letters, as well as the shape of the orbitals of which they consist: s, p, d, f.

Protons and neutrons

An atom of any chemical element is comparable to a tiny solar system. Therefore, such a model of the atom, proposed by E. Rutherford, is called planetary.

The atomic nucleus, in which the entire mass of the atom is concentrated, consists of particles of two types - protons and neutrons.

Protons have a charge equal to the charge of electrons, but opposite in sign (+1), and a mass equal to the mass of a hydrogen atom (it is accepted in chemistry as a unit). Neutrons carry no charge, they are neutral and have a mass equal to that of a proton.

Protons and neutrons are collectively called nucleons (from the Latin nucleus - nucleus). The sum of the number of protons and neutrons in an atom is called the mass number. For example, the mass number of an aluminum atom:

13 + 14 = 27

number of protons 13, number of neutrons 14, mass number 27

Since the mass of the electron, which is negligible, can be neglected, it is obvious that the entire mass of the atom is concentrated in the nucleus. Electrons represent e - .

Because the atom electrically neutral, it is also obvious that the number of protons and electrons in an atom is the same. It is equal to the serial number of the chemical element assigned to it in the Periodic system. The mass of an atom is made up of the mass of protons and neutrons. Knowing the serial number of the element (Z), i.e., the number of protons, and the mass number (A), equal to the sum of the numbers of protons and neutrons, you can find the number of neutrons (N) using the formula:

N=A-Z

For example, the number of neutrons in an iron atom is:

56 — 26 = 30

isotopes

Varieties of atoms of the same element that have the same nuclear charge but different mass numbers are called isotopes. Chemical elements found in nature are a mixture of isotopes. So, carbon has three isotopes with a mass of 12, 13, 14; oxygen - three isotopes with a mass of 16, 17, 18, etc. Usually given in the Periodic system, the relative atomic mass of a chemical element is the average value of the atomic masses of a natural mixture of isotopes of a given element, taking into account their relative abundance in nature. The chemical properties of the isotopes of most chemical elements are exactly the same. However, hydrogen isotopes differ greatly in properties due to the dramatic fold increase in their relative atomic mass; they have even been given individual names and chemical symbols.

Elements of the first period

Scheme of the electronic structure of the hydrogen atom:

Schemes of the electronic structure of atoms show the distribution of electrons over electronic layers (energy levels).

The graphical electronic formula of the hydrogen atom (shows the distribution of electrons over energy levels and sublevels):

Graphic electronic formulas of atoms show the distribution of electrons not only in levels and sublevels, but also in orbits.

In a helium atom, the first electron layer is completed - it has 2 electrons. Hydrogen and helium are s-elements; for these atoms, the s-orbital is filled with electrons.

All elements of the second period the first electron layer is filled, and the electrons fill the s- and p-orbitals of the second electron layer in accordance with the principle of least energy (first s, and then p) and the rules of Pauli and Hund.

In the neon atom, the second electron layer is completed - it has 8 electrons.

For atoms of elements of the third period, the first and second electron layers are completed, so the third electron layer is filled, in which electrons can occupy 3s-, 3p- and 3d-sublevels.

A 3s ​​electron orbital is completed at the magnesium atom. Na and Mg are s-elements.

For aluminum and subsequent elements, the 3p sublevel is filled with electrons.

The elements of the third period have unfilled 3d orbitals.

All elements from Al to Ar are p-elements. s- and p-elements form the main subgroups in the Periodic system.

Elements of the fourth - seventh periods

A fourth electron layer appears at the potassium and calcium atoms, the 4s sublevel is filled, since it has less energy than the 3d sublevel.

K, Ca - s-elements included in the main subgroups. For atoms from Sc to Zn, the 3d sublevel is filled with electrons. These are 3d elements. They are included in the secondary subgroups, they have a pre-external electron layer filled, they are referred to as transition elements.

Pay attention to the structure of the electron shells of chromium and copper atoms. In them, a “failure” of one electron from the 4s- to the 3d-sublevel occurs, which is explained by the greater energy stability of the resulting electronic configurations 3d 5 and 3d 10:

In the zinc atom, the third electron layer is completed - all the 3s, 3p and 3d sublevels are filled in it, in total there are 18 electrons on them. In the elements following zinc, the fourth electron layer continues to be filled, the 4p sublevel.

Elements from Ga to Kr are p-elements.

The outer layer (fourth) of the krypton atom is complete and has 8 electrons. But there can only be 32 electrons in the fourth electron layer; the 4d- and 4f-sublevels of the krypton atom still remain unfilled. The elements of the fifth period are filling the sub-levels in the following order: 5s - 4d - 5p. And there are also exceptions related to " failure» electrons, y 41 Nb, 42 Mo, 44 ​​Ru, 45 Rh, 46 Pd, 47 Ag.

In the sixth and seventh periods, f-elements appear, i.e., elements in which the 4f- and 5f-sublevels of the third outer electronic layer are filled, respectively.

4f elements are called lanthanides.

5f elements are called actinides.

The order of filling of electronic sublevels in the atoms of elements of the sixth period: 55 Cs and 56 Ba - 6s-elements; 57 La … 6s 2 5d x - 5d element; 58 Ce - 71 Lu - 4f elements; 72 Hf - 80 Hg - 5d elements; 81 T1 - 86 Rn - 6d elements. But even here there are elements in which the order of filling of electronic orbitals is “violated”, which, for example, is associated with greater energy stability of half and completely filled f-sublevels, i.e., nf 7 and nf 14. Depending on which sublevel of the atom is filled with electrons last, all elements are divided into four electronic families, or blocks:

• s-elements. The s-sublevel of the outer level of the atom is filled with electrons; s-elements include hydrogen, helium and elements of the main subgroups of groups I and II.

• p-elements. The p-sublevel of the outer level of the atom is filled with electrons; p-elements include elements of the main subgroups of III-VIII groups.

• d-elements. The d-sublevel of the preexternal level of the atom is filled with electrons; d-elements include elements of secondary subgroups of groups I-VIII, i.e., elements of intercalary decades of large periods located between s- and p-elements. They are also called transition elements.

• f-elements. The f-sublevel of the third outside level of the atom is filled with electrons; these include the lanthanides and antinoids.

The Swiss physicist W. Pauli in 1925 established that in an atom in one orbital there can be no more than two electrons having opposite (antiparallel) spins (translated from English - “spindle”), i.e. having such properties that can be conditionally imagined as the rotation of an electron around its imaginary axis: clockwise or counterclockwise.

This principle is called Pauli principle. If there is one electron in the orbital, then it is called unpaired, if there are two, then these are paired electrons, that is, electrons with opposite spins. The figure shows a diagram of the division of energy levels into sublevels and the order in which they are filled.

Very often, the structure of the electron shells of atoms is depicted using energy or quantum cells - they write down the so-called graphic electronic formulas. For this record, the following notation is used: each quantum cell is denoted by a cell that corresponds to one orbital; each electron is indicated by an arrow corresponding to the direction of the spin. When writing a graphical electronic formula, two rules should be remembered: Pauli principle and F. Hund's rule, according to which electrons occupy free cells first one at a time and at the same time have the same spin value, and only then they pair, but the spins, according to the Pauli principle, will already be oppositely directed.

Hund's rule and Pauli's principle

Hund's rule- the rule of quantum chemistry, which determines the order of filling the orbitals of a certain sublayer and is formulated as follows: the total value of the spin quantum number of electrons of this sublayer should be maximum. Formulated by Friedrich Hund in 1925.

This means that in each of the orbitals of the sublayer, one electron is first filled, and only after the unfilled orbitals are exhausted, a second electron is added to this orbital. In this case, there are two electrons with half-integer spins of the opposite sign in one orbital, which pair (form a two-electron cloud) and, as a result, the total spin of the orbital becomes equal to zero.

Other wording: Below in energy lies the atomic term for which two conditions are satisfied.

1. Multiplicity is maximum
2. When the multiplicities coincide, the total orbital momentum L is maximum.

Let's analyze this rule using the example of filling the orbitals of the p-sublevel p- elements of the second period (that is, from boron to neon (in the diagram below, horizontal lines indicate orbitals, vertical arrows indicate electrons, and the direction of the arrow indicates the orientation of the spin).

Klechkovsky's rule

Klechkovsky's rule - as the total number of electrons in atoms increases (with an increase in the charges of their nuclei, or the ordinal numbers of chemical elements), atomic orbitals are populated in such a way that the appearance of electrons in higher-energy orbitals depends only on the principal quantum number n and does not depend on all other quantum numbers. numbers, including those from l. Physically, this means that in a hydrogen-like atom (in the absence of interelectron repulsion) the orbital energy of an electron is determined only by the spatial remoteness of the electron charge density from the nucleus and does not depend on the features of its motion in the field of the nucleus.

Klechkovsky's empirical rule and the sequence of sequences of a somewhat contradictory real energy sequence of atomic orbitals arising from it only in two cases of the same type: for atoms Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, Pt, Au, there is a “failure” of an electron with s - sublevel of the outer layer to the d-sublevel of the previous layer, which leads to an energetically more stable state of the atom, namely: after filling the orbital 6 with two electrons s

Chemicals are the things that make up the world around us.

The properties of each chemical substance are divided into two types: these are chemical, which characterize its ability to form other substances, and physical, which are objectively observed and can be considered in isolation from chemical transformations. So, for example, the physical properties of a substance are its state of aggregation (solid, liquid or gaseous), thermal conductivity, heat capacity, solubility in various media (water, alcohol, etc.), density, color, taste, etc.

The transformation of some chemical substances into other substances is called chemical phenomena or chemical reactions. It should be noted that there are also physical phenomena, which, obviously, are accompanied by a change in any physical properties of a substance without its transformation into other substances. Physical phenomena, for example, include the melting of ice, the freezing or evaporation of water, etc.

The fact that during any process a chemical phenomenon takes place can be concluded by observing the characteristic signs of chemical reactions, such as a change in color, the formation of a precipitate, the evolution of gas, the evolution of heat and / or light.

So, for example, a conclusion about the course of chemical reactions can be made by observing:

The formation of sediment when boiling water, called scale in everyday life;

The release of heat and light during the burning of a fire;

Changing the color of a slice of a fresh apple in the air;

The formation of gas bubbles during the fermentation of dough, etc.

The smallest particles of matter, which in the process of chemical reactions practically do not undergo changes, but only in a new way are connected to each other, are called atoms.

The very idea of ​​the existence of such units of matter arose back in ancient Greece in the minds of ancient philosophers, which actually explains the origin of the term "atom", since "atomos" literally translated from Greek means "indivisible".

However, contrary to the idea of ​​the ancient Greek philosophers, atoms are not the absolute minimum of matter, i.e. themselves have a complex structure.

Each atom consists of the so-called subatomic particles - protons, neutrons and electrons, denoted respectively by the symbols p + , n o and e - . The superscript in the notation used indicates that the proton has a unit positive charge, the electron has a unit negative charge, and the neutron has no charge.

As for the qualitative structure of the atom, each atom has all the protons and neutrons concentrated in the so-called nucleus, around which the electrons form an electron shell.

The proton and neutron have practically the same masses, i.e. m p ≈ m n , and the electron mass is almost 2000 times less than the mass of each of them, i.e. m p / m e ≈ m n / m e ≈ 2000.

Since the fundamental property of an atom is its electrical neutrality, and the charge of one electron is equal to the charge of one proton, it can be concluded from this that the number of electrons in any atom is equal to the number of protons.

So, for example, the table below shows the possible composition of atoms:

The type of atoms with the same nuclear charge, i.e. with the same number of protons in their nuclei is called a chemical element. Thus, from the table above, we can conclude that atom1 and atom2 belong to one chemical element, and atom3 and atom4 belong to another chemical element.

Each chemical element has its own name and individual symbol, which is read in a certain way. So, for example, the simplest chemical element, the atoms of which contain only one proton in the nucleus, has the name "hydrogen" and is denoted by the symbol "H", which is read as "ash", and the chemical element with a nuclear charge of +7 (i.e. containing 7 protons) - "nitrogen", has the symbol "N", which is read as "en".

As you can see from the table above, the atoms of one chemical element can differ in the number of neutrons in the nuclei.

Atoms belonging to the same chemical element, but having a different number of neutrons and, as a result, mass, are called isotopes.

So, for example, the chemical element hydrogen has three isotopes - 1 H, 2 H and 3 H. The indices 1, 2 and 3 above the H symbol mean the total number of neutrons and protons. Those. knowing that hydrogen is a chemical element, which is characterized by the fact that there is one proton in the nuclei of its atoms, we can conclude that there are no neutrons at all in the 1 H isotope (1-1 = 0), in the 2 H isotope - 1 neutron (2-1=1) and in the isotope 3 H - two neutrons (3-1=2). Since, as already mentioned, a neutron and a proton have the same masses, and the mass of an electron is negligible compared to them, this means that the 2 H isotope is almost twice as heavy as the 1 H isotope, and the 3 H isotope is even three times as heavy. . In connection with such a large spread in the masses of hydrogen isotopes, the 2 H and 3 H isotopes were even given separate individual names and symbols, which is not typical of any other chemical element. The 2 H isotope was named deuterium and given the symbol D, and the 3 H isotope was given the name tritium and given the symbol T.

If we take the mass of the proton and neutron as unity, and neglect the mass of the electron, in fact, the upper left index, in addition to the total number of protons and neutrons in the atom, can be considered its mass, and therefore this index is called the mass number and is denoted by the symbol A. Since the charge of the nucleus of any protons correspond to the atom, and the charge of each proton is conditionally considered equal to +1, the number of protons in the nucleus is called the charge number (Z). Denoting the number of neutrons in an atom with the letter N, mathematically the relationship between mass number, charge number and number of neutrons can be expressed as:

According to modern concepts, the electron has a dual (particle-wave) nature. It has the properties of both a particle and a wave. Like a particle, an electron has a mass and a charge, but at the same time, the flow of electrons, like a wave, is characterized by the ability to diffraction.

To describe the state of an electron in an atom, the concepts of quantum mechanics are used, according to which the electron does not have a specific trajectory of motion and can be located at any point in space, but with different probabilities.

The region of space around the nucleus where an electron is most likely to be found is called the atomic orbital.

An atomic orbital can have a different shape, size and orientation. An atomic orbital is also called an electron cloud.

Graphically, one atomic orbital is usually denoted as a square cell:

Quantum mechanics has an extremely complex mathematical apparatus, therefore, within the framework of a school chemistry course, only the consequences of quantum mechanical theory are considered.

According to these consequences, any atomic orbital and an electron located on it are completely characterized by 4 quantum numbers.

• The main quantum number - n - determines the total energy of an electron in a given orbital. The range of values ​​of the main quantum number is all natural numbers, i.e. n = 1,2,3,4, 5 etc.
• The orbital quantum number - l - characterizes the shape of the atomic orbital and can take any integer values ​​from 0 to n-1, where n, recall, is the main quantum number.

Orbitals with l = 0 are called s-orbitals. s-orbitals are spherical and do not have a direction in space:

Orbitals with l = 1 are called p-orbitals. These orbitals have the shape of a three-dimensional figure eight, i.e. the shape obtained by rotating the figure eight around the axis of symmetry, and outwardly resemble a dumbbell:

Orbitals with l = 2 are called d-orbitals, and with l = 3 – f-orbitals. Their structure is much more complex.

3) Magnetic quantum number - m l - determines the spatial orientation of a particular atomic orbital and expresses the projection of the orbital angular momentum on the direction of the magnetic field. The magnetic quantum number m l corresponds to the orientation of the orbital relative to the direction of the external magnetic field strength vector and can take any integer values ​​from –l to +l, including 0, i.e. the total number of possible values ​​is (2l+1). So, for example, with l = 0 m l = 0 (one value), with l = 1 m l = -1, 0, +1 (three values), with l = 2 m l = -2, -1, 0, +1 , +2 (five values ​​of the magnetic quantum number), etc.

So, for example, p-orbitals, i.e. orbitals with an orbital quantum number l = 1, having the shape of a “three-dimensional figure eight”, correspond to three values ​​of the magnetic quantum number (-1, 0, +1), which, in turn, corresponds to three directions in space perpendicular to each other.

4) The spin quantum number (or simply spin) - m s - can be conditionally considered responsible for the direction of rotation of an electron in an atom, it can take on values. Electrons with different spins are indicated by vertical arrows pointing in different directions: ↓ and .

The set of all orbitals in an atom that have the same value of the principal quantum number is called the energy level or electron shell. Any arbitrary energy level with some number n consists of n 2 orbitals.

The set of orbitals with the same values ​​of the principal quantum number and the orbital quantum number is an energy sublevel.

Each energy level, which corresponds to the main quantum number n, contains n sublevels. In turn, each energy sublevel with an orbital quantum number l consists of (2l+1) orbitals. Thus, the s-sublayer consists of one s-orbital, the p-sublayer - three p-orbitals, the d-sublayer - five d-orbitals, and the f-sublayer - seven f-orbitals. Since, as already mentioned, one atomic orbital is often denoted by one square cell, the s-, p-, d- and f-sublevels can be graphically depicted as follows:

Each orbital corresponds to an individual strictly defined set of three quantum numbers n, l and m l .

The distribution of electrons in orbitals is called the electronic configuration.

The filling of atomic orbitals with electrons occurs in accordance with three conditions:

• The principle of minimum energy: Electrons fill orbitals starting from the lowest energy sublevel. The sequence of sublevels in order of increasing energy is as follows: 1s<2s<2p<3s<3p<4s≤3d<4p<5s≤4d<5p<6s…;

In order to make it easier to remember this sequence of filling electronic sublevels, the following graphic illustration is very convenient:

• Pauli principle: Each orbital can hold at most two electrons.

If there is one electron in the orbital, then it is called unpaired, and if there are two, then they are called an electron pair.

• Hund's rule: the most stable state of an atom is one in which, within one sublevel, the atom has the maximum possible number of unpaired electrons. This most stable state of the atom is called the ground state.

In fact, the above means that, for example, the placement of the 1st, 2nd, 3rd and 4th electrons on three orbitals of the p-sublevel will be carried out as follows:

The filling of atomic orbitals from hydrogen, which has a charge number of 1, to krypton (Kr) with a charge number of 36, will be carried out as follows:

A similar representation of the order in which atomic orbitals are filled is called an energy diagram. Based on the electronic diagrams of individual elements, you can write down their so-called electronic formulas (configurations). So, for example, an element with 15 protons and, as a result, 15 electrons, i.e. phosphorus (P) will have the following energy diagram:

When translated into an electronic formula, the phosphorus atom will take the form:

15 P = 1s 2 2s 2 2p 6 3s 2 3p 3

Normal-sized digits to the left of the sublevel symbol show the number of the energy level, and superscripts to the right of the sublevel symbol show the number of electrons in the corresponding sublevel.

Below are the electronic formulas of the first 36 elements of D.I. Mendeleev.

period	Item No.	symbol	title	electronic formula
I	1	H	hydrogen	1s 1
	2	He	helium	1s2
II	3	Li	lithium	1s2 2s1
	4	Be	beryllium	1s2 2s2
	5	B	boron	1s 2 2s 2 2p 1
	6	C	carbon	1s 2 2s 2 2p 2
	7	N	nitrogen	1s 2 2s 2 2p 3
	8	O	oxygen	1s 2 2s 2 2p 4
	9	F	fluorine	1s 2 2s 2 2p 5
	10	Ne	neon	1s 2 2s 2 2p 6
III	11	Na	sodium	1s 2 2s 2 2p 6 3s 1
	12	mg	magnesium	1s 2 2s 2 2p 6 3s 2
	13	Al	aluminum	1s 2 2s 2 2p 6 3s 2 3p 1
	14	Si	silicon	1s 2 2s 2 2p 6 3s 2 3p 2
	15	P	phosphorus	1s 2 2s 2 2p 6 3s 2 3p 3
	16	S	sulfur	1s 2 2s 2 2p 6 3s 2 3p 4
	17	Cl	chlorine	1s 2 2s 2 2p 6 3s 2 3p 5
	18	Ar	argon	1s 2 2s 2 2p 6 3s 2 3p 6
IV	19	K	potassium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 1
	20	Ca	calcium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2
	21	sc	scandium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1
	22	Ti	titanium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 2
	23	V	vanadium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 3
	24	Cr	chromium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 s on the d sublevel
	25	Mn	manganese	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 5
	26	Fe	iron	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6
	27	co	cobalt	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 7
	28	Ni	nickel	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 8
	29	Cu	copper	1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 10 s on the d sublevel
	30	Zn	zinc	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10
	31	Ga	gallium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 1
	32	Ge	germanium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 2
	33	As	arsenic	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 3
	34	Se	selenium	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 4
	35	Br	bromine	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5
	36	kr	krypton	1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6

As already mentioned, in their ground state, electrons in atomic orbitals are arranged according to the principle of least energy. Nevertheless, in the presence of empty p-orbitals in the ground state of an atom, often, when excess energy is imparted to it, the atom can be transferred to the so-called excited state. So, for example, a boron atom in its ground state has an electronic configuration and an energy diagram of the following form:

5 B = 1s 2 2s 2 2p 1

And in the excited state (*), i.e. when imparting some energy to the boron atom, its electronic configuration and energy diagram will look like this:

5 B* = 1s 2 2s 1 2p 2

Depending on which sublevel in the atom is filled last, chemical elements are divided into s, p, d or f.

Finding s, p, d and f-elements in the table D.I. Mendeleev:

• s-elements have the last s-sublevel to be filled. These elements include elements of the main (on the left in the table cell) subgroups of groups I and II.
• For p-elements, the p-sublevel is filled. The p-elements include the last six elements of each period, except for the first and seventh, as well as elements of the main subgroups of III-VIII groups.
• d-elements are located between s- and p-elements in large periods.
• The f-elements are called lanthanides and actinides. They are placed at the bottom of the table by D.I. Mendeleev.

As you know, everything material in the Universe consists of atoms. An atom is the smallest unit of matter that carries its properties. In turn, the structure of an atom is made up of a magical trinity of microparticles: protons, neutrons and electrons.

Moreover, each of the microparticles is universal. That is, you cannot find two different protons, neutrons or electrons in the world. All of them are absolutely similar to each other. And the properties of the atom will depend only on the quantitative composition of these microparticles in the general structure of the atom.

For example, the structure of a hydrogen atom consists of one proton and one electron. Next in complexity, the helium atom is made up of two protons, two neutrons, and two electrons. A lithium atom is made up of three protons, four neutrons and three electrons, etc.

Structure of atoms (from left to right): hydrogen, helium, lithium

Atoms combine into molecules, and molecules combine into substances, minerals and organisms. The DNA molecule, which is the basis of all life, is a structure assembled from the same three magical building blocks of the universe as the stone lying on the road. Although this structure is much more complex.

Even more amazing facts are revealed when we try to take a closer look at the proportions and structure of the atomic system. It is known that an atom consists of a nucleus and electrons moving around it along a trajectory that describes a sphere. That is, it cannot even be called a movement in the usual sense of the word. The electron is rather located everywhere and immediately within this sphere, creating an electron cloud around the nucleus and forming an electromagnetic field.

Schematic representations of the structure of the atom

The nucleus of an atom consists of protons and neutrons, and almost the entire mass of the system is concentrated in it. But at the same time, the nucleus itself is so small that if you increase its radius to a scale of 1 cm, then the radius of the entire structure of the atom will reach hundreds of meters. Thus, everything that we perceive as dense matter consists of more than 99% of the energy bonds between physical particles alone and less than 1% of the physical forms themselves.

But what are these physical forms? What are they made of, and how material are they? To answer these questions, let's take a closer look at the structures of protons, neutrons, and electrons. So, we descend one more step into the depths of the microcosm - to the level of subatomic particles.

What is an electron made of?

The smallest particle of an atom is an electron. An electron has mass but no volume. In the scientific view, the electron does not consist of anything, but is a structureless point.

An electron cannot be seen under a microscope. It is observed only in the form of an electron cloud, which looks like a fuzzy sphere around the atomic nucleus. At the same time, it is impossible to say with accuracy where the electron is located at a moment in time. Devices are capable of capturing not the particle itself, but only its energy trace. The essence of the electron is not embedded in the concept of matter. It is rather like an empty form that exists only in and through movement.

No structure has yet been found in the electron. It is the same point particle as the quantum of energy. In fact, an electron is energy, however, this is its more stable form than the one represented by photons of light.

At the moment, the electron is considered indivisible. This is understandable, because it is impossible to divide something that has no volume. However, there are already developments in the theory, according to which the composition of an electron contains a trinity of such quasiparticles as:

• Orbiton - contains information about the orbital position of the electron;
• Spinon - responsible for the spin or torque;
• Holon - carries information about the charge of an electron.

However, as we see, quasi-particles have absolutely nothing in common with matter, and carry only information.

Photographs of atoms of different substances in an electron microscope

Interestingly, an electron can absorb energy quanta, such as light or heat. In this case, the atom moves to a new energy level, and the boundaries of the electron cloud expand. It also happens that the energy absorbed by an electron is so great that it can jump out of the atomic system and continue its movement as an independent particle. At the same time, it behaves like a photon of light, that is, it seems to cease to be a particle and begins to exhibit the properties of a wave. This has been proven in an experiment.

Young's experiment

In the course of the experiment, a stream of electrons was directed onto a screen with two slits cut into it. Passing through these slits, the electrons collided with the surface of another projection screen, leaving their mark on it. As a result of this “bombardment” by electrons, an interference pattern appeared on the projection screen, similar to that which would appear if waves, but not particles, passed through two slits.

Such a pattern occurs due to the fact that the wave, passing between the two slots, is divided into two waves. As a result of further movement, the waves overlap each other, and in some areas they cancel each other out. As a result, we get many stripes on the projection screen, instead of one, as it would be if the electron behaved like a particle.

The structure of the nucleus of an atom: protons and neutrons

Protons and neutrons make up the nucleus of an atom. And despite the fact that in the total volume the core occupies less than 1%, it is in this structure that almost the entire mass of the system is concentrated. But at the expense of the structure of protons and neutrons, physicists are divided in opinion, and at the moment there are two theories at once.

• Theory #1 - Standard

The Standard Model says that protons and neutrons are made up of three quarks connected by a cloud of gluons. Quarks are point particles, just like quanta and electrons. And gluons are virtual particles that ensure the interaction of quarks. However, neither quarks nor gluons have been found in nature, so this model is subject to severe criticism.

• Theory #2 - Alternative

But according to the alternative unified field theory developed by Einstein, the proton, like the neutron, like any other particle of the physical world, is an electromagnetic field rotating at the speed of light.

Electromagnetic fields of man and the planet

What are the principles of the structure of the atom?

Everything in the world - subtle and dense, liquid, solid and gaseous - is just the energy states of countless fields that permeate the space of the Universe. The higher the energy level in the field, the thinner and less perceptible it is. The lower the energy level, the more stable and tangible it is. In the structure of the atom, as well as in the structure of any other unit of the Universe, lies the interaction of such fields - different in energy density. It turns out that matter is only an illusion of the mind.