The composition of the atom.

An atom is made up of atomic nucleus and electron shell.

The nucleus of an atom is made up of protons ( p+) and neutrons ( n 0). Most hydrogen atoms have a single proton nucleus.

Number of protons N(p+) is equal to the nuclear charge ( Z) and the ordinal number of the element in the natural series of elements (and in the periodic system of elements).

N(p +) = Z

The sum of the number of neutrons N(n 0), denoted simply by the letter N, and the number of protons Z called mass number and is marked with the letter BUT.

A = Z + N

The electron shell of an atom consists of electrons moving around the nucleus ( e -).

Number of electrons N(e-) in the electron shell of a neutral atom is equal to the number of protons Z at its core.

The mass of a proton is approximately equal to the mass of a neutron and 1840 times the mass of an electron, so the mass of an atom is practically equal to the mass of the nucleus.

The shape of an atom is spherical. The radius of the nucleus is about 100,000 times smaller than the radius of the atom.

Chemical element- type of atoms (set of atoms) with the same nuclear charge (with the same number of protons in the nucleus).

Isotope- a set of atoms of one element with the same number of neutrons in the nucleus (or a type of atoms with the same number of protons and the same number of neutrons in the nucleus).

Different isotopes differ from each other in the number of neutrons in the nuclei of their atoms.

Designation of a single atom or isotope: (E - element symbol), for example: .

The structure of the electron shell of the atom

atomic orbital is the state of an electron in an atom. Orbital symbol - . Each orbital corresponds to an electron cloud.

The orbitals of real atoms in the ground (unexcited) state are of four types: s, p, d and f.

electronic cloud- the part of space in which an electron can be found with a probability of 90 (or more) percent.

Note: sometimes the concepts of "atomic orbital" and "electron cloud" are not distinguished, calling both of them "atomic orbital".

The electron shell of an atom is layered. Electronic layer formed by electron clouds of the same size. Orbitals of one layer form electronic ("energy") level, their energies are the same for the hydrogen atom, but different for other atoms.

Orbitals of the same level are grouped into electronic (energy) sublevels:
s- sublevel (consists of one s-orbitals), symbol - .
p sublevel (consists of three p
d sublevel (consists of five d-orbitals), symbol - .
f sublevel (consists of seven f-orbitals), symbol - .

The energies of the orbitals of the same sublevel are the same.

When designating sublevels, the number of the layer (electronic level) is added to the sublevel symbol, for example: 2 s, 3p, 5d means s- sublevel of the second level, p- sublevel of the third level, d- sublevel of the fifth level.

The total number of sublevels in one level is equal to the level number n. The total number of orbitals in one level is n 2. Accordingly, the total number of clouds in one layer is also n 2 .

Designations: - free orbital (without electrons), - orbital with an unpaired electron, - orbital with an electron pair (with two electrons).

The order in which electrons fill the orbitals of an atom is determined by three laws of nature (formulations are given in a simplified way):

1. The principle of least energy - electrons fill the orbitals in order of increasing energy of the orbitals.

2. Pauli's principle - there cannot be more than two electrons in one orbital.

3. Hund's rule - within the sublevel, electrons first fill free orbitals (one at a time), and only after that they form electron pairs.

The total number of electrons in the electronic level (or in the electronic layer) is 2 n 2 .

The distribution of sublevels by energy is expressed next (in order of increasing energy):

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p ...

Visually, this sequence is expressed by the energy diagram:

The distribution of electrons of an atom by levels, sublevels and orbitals (the electronic configuration of an atom) can be depicted as an electronic formula, an energy diagram, or, more simply, as a diagram of electronic layers ("electronic diagram").

Examples of the electronic structure of atoms:

Valence electrons- electrons of an atom that can take part in the formation of chemical bonds. For any atom, these are all the outer electrons plus those pre-outer electrons whose energy is greater than that of the outer ones. For example: Ca atom has 4 outer electrons s 2, they are also valence; the Fe atom has external electrons - 4 s 2 but he has 3 d 6, hence the iron atom has 8 valence electrons. The valence electronic formula of the calcium atom is 4 s 2, and iron atoms - 4 s 2 3d 6 .

Periodic system of chemical elements of D. I. Mendeleev
(natural system of chemical elements)

Periodic law of chemical elements(modern formulation): the properties of chemical elements, as well as simple and complex substances formed by them, are in a periodic dependence on the value of the charge from atomic nuclei.

Periodic system- graphical expression of the periodic law.

Natural range of chemical elements- a number of chemical elements, arranged according to the increase in the number of protons in the nuclei of their atoms, or, what is the same, according to the increase in the charges of the nuclei of these atoms. The serial number of an element in this series is equal to the number of protons in the nucleus of any atom of this element.

The table of chemical elements is constructed by "cutting" the natural series of chemical elements into periods(horizontal rows of the table) and groupings (vertical columns of the table) of elements with a similar electronic structure of atoms.

Depending on how elements are combined into groups, a table can be long period(elements with the same number and type of valence electrons are collected in groups) and short-term(elements with the same number of valence electrons are collected in groups).

The groups of the short period table are divided into subgroups ( main and side effects), coinciding with the groups of the long-period table.

All atoms of elements of the same period have the same number of electron layers, equal to the number of the period.

The number of elements in the periods: 2, 8, 8, 18, 18, 32, 32. Most of the elements of the eighth period were obtained artificially, the last elements of this period have not yet been synthesized. All periods except the first start with an alkali metal forming element (Li, Na, K, etc.) and end with a noble gas forming element (He, Ne, Ar, Kr, etc.).

In the short period table - eight groups, each of which is divided into two subgroups (main and secondary), in the long period table - sixteen groups, which are numbered in Roman numerals with the letters A or B, for example: IA, IIIB, VIA, VIIB. Group IA of the long period table corresponds to the main subgroup of the first group of the short period table; group VIIB - secondary subgroup of the seventh group: the rest - similarly.

The characteristics of chemical elements naturally change in groups and periods.

In periods (with increasing serial number)

• the nuclear charge increases
• the number of outer electrons increases,
• the radius of the atoms decreases,
• the bond strength of electrons with the nucleus increases (ionization energy),
• electronegativity increases.
• the oxidizing properties of simple substances are enhanced ("non-metallicity"),
• the reducing properties of simple substances ("metallicity") weaken,
• weakens the basic character of hydroxides and the corresponding oxides,
• the acidic character of hydroxides and corresponding oxides increases.

In groups (with increasing serial number)

• the nuclear charge increases
• the radius of atoms increases (only in A-groups),
• the strength of the bond between electrons and the nucleus decreases (ionization energy; only in A-groups),
• electronegativity decreases (only in A-groups),
• weaken the oxidizing properties of simple substances ("non-metallicity"; only in A-groups),
• the reducing properties of simple substances are enhanced ("metallicity"; only in A-groups),
• the basic character of hydroxides and the corresponding oxides increases (only in A-groups),
• the acidic nature of hydroxides and the corresponding oxides weakens (only in A-groups),
• the stability of hydrogen compounds decreases (their reducing activity increases; only in A-groups).

Tasks and tests on the topic "Topic 9. "The structure of the atom. Periodic law and periodic system of chemical elements of D. I. Mendeleev (PSCE)"."

• Periodic Law - Periodic law and structure of atoms Grade 8–9
  You should know: the laws of filling orbitals with electrons (principle of least energy, Pauli's principle, Hund's rule), the structure of the periodic system of elements.

  You should be able to: determine the composition of an atom by the position of an element in the periodic system, and, conversely, find an element in the periodic system, knowing its composition; depict the structure diagram, the electronic configuration of an atom, ion, and, conversely, determine the position of a chemical element in the PSCE from the diagram and electronic configuration; characterize the element and the substances it forms according to its position in the PSCE; determine changes in the radius of atoms, the properties of chemical elements and the substances they form within one period and one main subgroup of the periodic system.

  Example 1 Determine the number of orbitals in the third electronic level. What are these orbitals?
  To determine the number of orbitals, we use the formula N orbitals = n 2 , where n- level number. N orbitals = 3 2 = 9. One 3 s-, three 3 p- and five 3 d-orbitals.

  Example 2 Determine the atom of which element has the electronic formula 1 s 2 2s 2 2p 6 3s 2 3p 1 .
  In order to determine which element it is, you need to find out its serial number, which is equal to the total number of electrons in the atom. In this case: 2 + 2 + 6 + 2 + 1 = 13. This is aluminum.

  After making sure that everything you need is learned, proceed to the tasks. We wish you success.

  
  Recommended literature:
  • O. S. Gabrielyan and others. Chemistry, 11th grade. M., Bustard, 2002;
  • G. E. Rudzitis, F. G. Feldman. Chemistry 11 cells. M., Education, 2001.

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 presence of certain orbits for microparticles (it is impossible to determine the exact position for moving electrons, 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 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 shield 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

An atom is the smallest particle of matter. Its study began in ancient Greece, when the attention of not only scientists, but also philosophers was riveted to the structure of the atom. What is the electronic structure of an atom, and what basic information is known about this particle?

The structure of the atom

Already ancient Greek scientists guessed the existence of the smallest chemical particles that make up any object and organism. And if in the XVII-XVIII centuries. chemists were sure that the atom is an indivisible elementary particle, then at the turn of the 19th-20th centuries, they managed to prove experimentally that the atom is not indivisible.

An atom, being a microscopic particle of matter, consists of a nucleus and electrons. The nucleus is 10,000 times smaller than an atom, but almost all of its mass is concentrated in the nucleus. The main characteristic of the atomic nucleus is that it has a positive charge and is made up of protons and neutrons. Protons are positively charged, while neutrons have no charge (they are neutral).

They are connected to each other by the strong nuclear force. The mass of a proton is approximately equal to the mass of a neutron, but at the same time it is 1840 times greater than the mass of an electron. Protons and neutrons have a common name in chemistry - nucleons. The atom itself is electrically neutral.

An atom of any element can be denoted by an electronic formula and an electronic graphic formula:

Rice. 1. Electron-graphic formula of the atom.

The only element in the Periodic Table that does not contain neutrons is light hydrogen (protium).

An electron is a negatively charged particle. The electron shell consists of electrons moving around the nucleus. Electrons have properties to be attracted to the nucleus, and between each other they are influenced by the Coulomb interaction. To overcome the attraction of the nucleus, the electrons must receive energy from an external source. The farther the electron is from the nucleus, the less energy is needed for this.

Atom Models

For a long time, scientists have sought to understand the nature of the atom. At an early stage, the ancient Greek philosopher Democritus made a great contribution. Although now his theory seems banal and too simple to us, at a time when ideas about elementary particles were just beginning to emerge, his theory about pieces of matter was taken quite seriously. Democritus believed that the properties of any substance depend on the shape, mass and other characteristics of atoms. So, for example, near fire, he believed, there are sharp atoms - therefore, fire burns; water has smooth atoms, so it can flow; in solid objects, in his view, the atoms were rough.

Democritus believed that absolutely everything consists of atoms, even the human soul.

In 1904, J. J. Thomson proposed his model of the atom. The main provisions of the theory boiled down to the fact that the atom was represented as a positively charged body, inside of which there were electrons with a negative charge. Later this theory was refuted by E. Rutherford.

Rice. 2. Thomson's model of the atom.

Also in 1904, the Japanese physicist H. Nagaoka proposed an early planetary model of the atom by analogy with the planet Saturn. According to this theory, electrons are united in rings and revolve around a positively charged nucleus. This theory turned out to be wrong.

In 1911, E. Rutherford, having done a series of experiments, concluded that the atom in its structure is similar to the planetary system. After all, electrons, like planets, move in orbits around a heavy positively charged nucleus. However, this description contradicted classical electrodynamics. Then the Danish physicist Niels Bohr in 1913 introduced the postulates, the essence of which was that the electron, being in some special states, does not radiate energy. Thus, Bohr's postulates showed that classical mechanics is inapplicable to atoms. The planetary model described by Rutherford and supplemented by Bohr was called the Bohr-Rutherford planetary model.

Rice. 3. Bohr-Rutherford planetary model.

Further study of the atom led to the creation of such a section as quantum mechanics, with the help of which many scientific facts were explained. Modern ideas about the atom have developed from the Bohr-Rutherford planetary model. Evaluation of the report

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(Lecture notes)

The structure of the atom. Introduction.

The object of study in chemistry is the chemical elements and their compounds. chemical element A group of atoms with the same positive charge is called. Atom is the smallest particle of a chemical element that retains it Chemical properties. Connecting with each other, atoms of one or different elements form more complex particles - molecules. A collection of atoms or molecules form chemicals. Each individual chemical substance is characterized by a set of individual physical properties, such as boiling and melting points, density, electrical and thermal conductivity, etc.

1. The structure of the atom and the Periodic system of elements

DI. Mendeleev.

Knowledge and understanding of the regularities of the order of filling the Periodic system of elements D.I. Mendeleev allows us to understand the following:

1. the physical essence of the existence in nature of certain elements,

2. the nature of the chemical valency of the element,

3. the ability and "ease" of an element to give or receive electrons when interacting with another element,

4. the nature of the chemical bonds that a given element can form when interacting with other elements, the spatial structure of simple and complex molecules, etc., etc.

The structure of the atom.

An atom is a complex microsystem of elementary particles in motion and interacting with each other.

In the late 19th and early 20th centuries, it was found that atoms are composed of smaller particles: neutrons, protons and electrons. The last two particles are charged particles, the proton carries a positive charge, the electron is negative. Since the atoms of an element in the ground state are electrically neutral, this means that the number of protons in an atom of any element is equal to the number of electrons. The mass of atoms is determined by the sum of the masses of protons and neutrons, the number of which is equal to the difference between the mass of atoms and its serial number in the periodic system of D.I. Mendeleev.

In 1926, Schrodinger proposed to describe the motion of microparticles in the atom of an element using the wave equation he derived. When solving the Schrödinger wave equation for the hydrogen atom, three integer quantum numbers appear: n, ℓ and m ℓ , which characterize the state of an electron in three-dimensional space in the central field of the nucleus. quantum numbers n, ℓ and m ℓ take integer values. Wave function defined by three quantum numbers n, ℓ and m ℓ and obtained as a result of solving the Schrödinger equation is called an orbital. An orbital is a region of space in which an electron is most likely to be found. belonging to an atom of a chemical element. Thus, the solution of the Schrödinger equation for the hydrogen atom leads to the appearance of three quantum numbers, the physical meaning of which is that they characterize three different types of orbitals that an atom can have. Let's take a closer look at each quantum number.

Principal quantum number n can take any positive integer values: n = 1,2,3,4,5,6,7… It characterizes the energy of the electronic level and the size of the electronic "cloud". It is characteristic that the number of the main quantum number coincides with the number of the period in which the given element is located.

Azimuthal or orbital quantum numberℓ can take integer values ​​from ℓ = 0….up to n – 1 and determines the moment of electron motion, i.e. orbital shape. For various numerical values ​​of ℓ, the following notation is used: ℓ = 0, 1, 2, 3, and are denoted by symbols s, p, d, f, respectively for ℓ = 0, 1, 2 and 3. In the periodic table of elements there are no elements with a spin number ℓ = 4.

Magnetic quantum numberm ℓ characterizes the spatial arrangement of electron orbitals and, consequently, the electromagnetic properties of the electron. It can take values ​​from - ℓ to + ℓ , including zero.

The shape or, more precisely, the symmetry properties of atomic orbitals depend on quantum numbers ℓ and m ℓ . "electronic cloud", corresponding to s- orbitals has, has the shape of a ball (at the same time ℓ = 0).

Fig.1. 1s orbital

Orbitals defined by quantum numbers ℓ = 1 and m ℓ = -1, 0 and +1 are called p-orbitals. Since m ℓ in this case has three different values, then the atom has three energetically equivalent p-orbitals (the main quantum number for them is the same and can have the value n = 2,3,4,5,6 or 7). p-Orbitals have axial symmetry and have the form of three-dimensional eights, oriented along the x, y and z axes in an external field (Fig. 1.2). Hence the origin of the symbols p x , p y and p z .

Fig.2. p x , p y and p z -orbitals

In addition, there are d- and f-atomic orbitals, for the first ℓ = 2 and m ℓ = -2, -1, 0, +1 and +2, i.e. five AO, for the second ℓ = 3 and m ℓ = -3, -2, -1, 0, +1, +2 and +3, i.e. 7 AO.

fourth quantum m s called the spin quantum number, was introduced to explain some subtle effects in the spectrum of the hydrogen atom by Goudsmit and Uhlenbeck in 1925. The spin of an electron is the angular momentum of a charged elementary particle of an electron, the orientation of which is quantized, i.e. strictly limited to certain angles. This orientation is determined by the value of the spin magnetic quantum number (s), which for an electron is ½ , therefore, for an electron, according to the quantization rules m s = ± ½. In this regard, to the set of three quantum numbers, one should add the quantum number m s . We emphasize once again that four quantum numbers determine the order in which Mendeleev's periodic table of elements is constructed and explain why there are only two elements in the first period, eight in the second and third, 18 in the fourth, and so on. However, in order to explain the structure of multielectron of atoms, the order in which electronic levels are filled as the positive charge of an atom increases, it is not enough to have an idea about the four quantum numbers that "govern" the behavior of electrons when filling electron orbitals, but you need to know some more simple rules, namely, Pauli's principle, Gund's rule and Klechkovsky's rules.

According to the Pauli principle in the same quantum state, characterized by certain values ​​of four quantum numbers, there cannot be more than one electron. This means that one electron can, in principle, be placed in any atomic orbital. Two electrons can be in the same atomic orbital only if they have different spin quantum numbers.

When filling three p-AOs, five d-AOs and seven f-AOs with electrons, one should be guided not only by the Pauli principle but also by the Hund rule: The filling of the orbitals of one subshell in the ground state occurs with electrons with the same spins.

When filling subshells (p, d, f) the absolute value of the sum of spins must be maximum.

Klechkovsky's rule. According to the Klechkovsky rule, when fillingd and forbital by electrons must be respectedprinciple of minimum energy. According to this principle, electrons in the ground state fill the orbits with minimum energy levels. The sublevel energy is determined by the sum of quantum numbersn + ℓ = E .

Klechkovsky's first rule: first fill those sublevels for whichn + ℓ = E minimal.

Klechkovsky's second rule: in case of equalityn + ℓ for several sublevels, the sublevel for whichn minimal .

Currently, 109 elements are known.

2. Ionization energy, electron affinity and electronegativity.

The most important characteristics of the electronic configuration of an atom are the ionization energy (EI) or ionization potential (IP) and the atom's electron affinity (SE). The ionization energy is the change in energy in the process of detachment of an electron from a free atom at 0 K: A = + + ē . The dependence of the ionization energy on the atomic number Z of the element, the size of the atomic radius has a pronounced periodic character.

Electron affinity (SE) is the change in energy that accompanies the addition of an electron to an isolated atom with the formation of a negative ion at 0 K: A + ē = A - (the atom and ion are in their ground states). In this case, the electron occupies the lowest free atomic orbital (LUAO) if the VZAO is occupied by two electrons. SE strongly depends on their orbital electronic configuration.

Changes in EI and SE correlate with changes in many properties of elements and their compounds, which is used to predict these properties from the values ​​of EI and SE. Halogens have the highest absolute electron affinity. In each group of the periodic table of elements, the ionization potential or EI decreases with increasing element number, which is associated with an increase in atomic radius and with an increase in the number of electron layers, and which correlates well with an increase in the element's reducing power.

Table 1 of the Periodic Table of the Elements gives the values ​​of EI and SE in eV/atom. Note that the exact SE values ​​are known only for a few atoms; their values ​​are underlined in Table 1.

Table 1

The first ionization energy (EI), electron affinity (SE) and electronegativity χ) of atoms in the periodic system.

χ	0.747 2. 1 0 0, 3 7																	1,2 2
χ	0.54 1. 55	-0.3 1. 1 3											0.2 0. 91	1.2 5	-0. 1 0, 55	1.47 0. 59	3.45 0. 64	1 ,60
χ	0. 7 4 1. 89	-0.3 1 . 3 1 1 . 6 0											0. 6	1.63	0.7	2.07	3.61
χ	2.3 6	- 0 .6						1.26(α)				-0.9 1 . 39	0. 18	1.2	0. 6	2.07	3.36
χ	2.4 8											-0.6 1 . 56	0. 2			2.2
χ	2.6 7	2, 2 1						Os

χ - Pauling electronegativity

r- atomic radius, (from "Laboratory and seminar classes in general and inorganic chemistry", N.S. Akhmetov, M.K. Azizova, L.I. Badygina)

The composition of a molecule. That is, by what atoms the molecule is formed, in what quantity, by what bonds these atoms are connected. All this determines the property of the molecule, and, accordingly, the property of the substance that these molecules form.

For example, the properties of water: transparency, fluidity, the ability to cause rust are due precisely to the presence of two hydrogen atoms and one oxygen atom.

Therefore, before proceeding to the study of the properties of molecules (that is, the properties of substances), it is necessary to consider the “building blocks” by which these molecules are formed. Understand the structure of the atom.

How is an atom arranged?

Atoms are particles that, when combined with each other, form molecules.

The atom itself is made up of positively charged nucleus (+) and negatively charged electron shell (-). In general, the atom is electrically neutral. That is, the charge of the nucleus is equal in absolute value to the charge of the electron shell.

The nucleus is formed by the following particles:

• Protons. One proton carries a +1 charge. Its mass is 1 amu (atomic mass unit). These particles are necessarily present in the nucleus.

• Neutrons. The neutron has no charge (charge = 0). Its mass is 1 amu. Neutrons may not be in the nucleus. It is not a required component of the atomic nucleus.

Thus, protons are responsible for the total charge of the nucleus. Since one neutron has a charge of +1, the charge of the nucleus is equal to the number of protons.

The electron shell, as the name implies, is formed by particles called electrons. If we compare the nucleus of an atom with a planet, then electrons are its satellites. Revolving around the nucleus (for now let's imagine that in orbits, but in fact in orbits), they form an electron shell.

• Electron is a very small particle. Its mass is so small that it is taken as 0. But the charge of an electron is -1. That is, the modulus is equal to the charge of the proton, differs in sign. Since one electron carries a charge of -1, the total charge of the electron shell is equal to the number of electrons in it.

One important consequence, since an atom is a particle that does not have a charge (the charge of the nucleus and the charge of the electron shell are equal in absolute value, but opposite in sign), that is, electrically neutral, therefore, the number of electrons in an atom is equal to the number of protons.

How do atoms of different chemical elements differ from each other?

Atoms of different chemical elements differ from each other in the charge of the nucleus (that is, the number of protons, and, consequently, the number of electrons).

How to find out the charge of the nucleus of an atom of an element? The brilliant domestic chemist D. I. Mendeleev, having discovered the periodic law, and having developed a table named after him, gave us the opportunity to do this. His discovery was far ahead of the curve. When it was not yet known about the structure of the atom, Mendeleev arranged the elements in the table in order of increasing nuclear charge.

That is, the serial number of an element in the periodic system is the charge of the nucleus of an atom of a given element. For example, oxygen has a serial number of 8, respectively, the charge of the nucleus of the oxygen atom is +8. Accordingly, the number of protons is 8, and the number of electrons is 8.

It is the electrons in the electron shell that determine the chemical properties of the atom, but more on that later.

Now let's talk about the mass.

One proton is one unit of mass, one neutron is also one unit of mass. Therefore, the sum of neutrons and protons in the nucleus is called mass number. (The electrons do not affect the mass in any way, since we neglect its mass and consider it equal to zero).

The atomic mass unit (a.m.u.) is a special physical quantity for designating small masses of particles that form atoms.

All these three atoms are atoms of one chemical element - hydrogen. Because they have the same nuclear charge.

How will they differ? These atoms have different mass numbers (due to the different number of neutrons). The first atom has a mass number of 1, the second has 2, and the third has 3.

Atoms of the same element that differ in the number of neutrons (and hence mass numbers) are called isotopes.

The presented hydrogen isotopes even have their own names:

• The first isotope (mass number 1) is called protium.
• The second isotope (mass number 2) is called deuterium.
• The third isotope (with a mass number of 3) is called tritium.

Now the next reasonable question is why if the number of neutrons and protons in the nucleus is an integer, their mass is 1 amu, then in the periodic system the mass of an atom is a fractional number. For sulfur, for example: 32.066.

Answer: an element has several isotopes, they differ from each other in mass numbers. Therefore, the atomic mass in the periodic table is the average value of the atomic masses of all isotopes of an element, taking into account their occurrence in nature. This mass, given in the periodic system, is called relative atomic mass.

For chemical calculations, indicators of just such an “averaged atom” are used. Atomic mass is rounded to the nearest integer.

The structure of the electron shell.

The chemical properties of an atom are determined by the structure of its electron shell. The electrons around the nucleus are not arranged anyhow. Electrons are localized in electron orbitals.

Electronic orbital- the space around the atomic nucleus, where the probability of finding an electron is greatest.

An electron has one quantum parameter called spin. If we take the classical definition from quantum mechanics, then spin is the intrinsic angular momentum of the particle. In a simplified form, this can be represented as the direction of rotation of a particle around its axis.

An electron is a particle with a half-integer spin, an electron can have either +½ or -½ spin. Conventionally, this can be represented as a clockwise and counterclockwise rotation.

No more than two electrons with opposite spins can be in one electron orbital.

The generally accepted designation of an electronic dwelling is a cell or a dash. The electron is indicated by an arrow: the up arrow is an electron with a positive spin +½, the down arrow ↓ is an electron with a negative spin -½.

An electron that is alone in an orbital is called unpaired. Two electrons in the same orbital are called paired.

Electronic orbitals are divided into four types depending on the shape: s, p, d, f. Orbitals of the same shape form a sublevel. The number of orbitals at a sublevel is determined by the number of possible locations in space.

1. s orbital.

The s orbital is spherical:

In space, the s-orbital can only be located in one way:

Therefore, the s-sublevel is formed by only one s-orbital.

1. p-orbital.

The p orbital is shaped like a dumbbell:

In space, the p-orbital can only be located in three ways:

Therefore, the p-sublevel is formed by three p-orbitals.

1. d-orbital.

The d-orbital has a complex shape:

In space, the d-orbital can be located in five different ways. Therefore, the d-sublevel is formed by five d-orbitals.

1. f-orbital

The f-orbital has an even more complex shape. In space, the f-orbital can be placed in seven different ways. Therefore, the f-sublevel is formed by seven f-orbitals.

The electron shell of an atom is like a puff pastry. It also has layers. Electrons located on different layers have different energies: on layers closer to the nucleus - less, on those far from the nucleus - more. These layers are called energy levels.

Filling of electron orbitals.

The first energy level has only the s-sublevel:

At the second energy level, there is an s-sublevel and a p-sublevel appears:

At the third energy level, there is an s-sublevel, a p-sublevel, and a d-sublevel appears:

At the fourth energy level, in principle, an f-sublevel is added. But in the school course, f-orbitals are not filled, so we can not depict the f-sublevel:

The number of energy levels in an atom of an element is period number. When filling electron orbitals, the following principles should be followed:

1. Each electron tries to occupy the position in the atom where its energy will be minimal. That is, first the first energy level is filled, then the second, and so on.

To describe the structure of the electron shell, the electronic formula is also used. The electronic formula is a short one-line record of the distribution of electrons by sublevels.

1. At the sublevel, each electron first fills a vacant orbital. And each has spin +½ (up arrow).

And only after there is one electron in each sublevel orbital, the next electron becomes paired - that is, it occupies an orbital that already has an electron:

1. d-sublevel is filled in a special way.

The fact is that the energy of the d-sublevel is higher than the energy of the s-sublevel of the NEXT energy layer. And as we know, the electron tries to take that position in the atom, where its energy will be minimal.

Therefore, after filling the 3p sublevel, the 4s sublevel is filled first, after which the 3d sublevel is filled.

And only after the 3d sublevel is completely filled, the 4p sublevel is filled.

It is the same with the 4th energy level. After the 4p sublevel is filled, the 5s sublevel is filled next, followed by the 4d sublevel. And after it only 5p.

1. And there is one more point, one rule regarding the filling of the d-sublevel.

Then there is a phenomenon called failure. In case of failure, one electron from the s-sublevel of the next energy level literally falls to the d-electron.

Ground and excited states of the atom.

The atoms whose electronic configurations we have now built are called atoms in basic condition. That is, this is a normal, natural, if you like, state.

When an atom receives energy from outside, excitation can occur.

Excitation is the transition of a paired electron to an empty orbital, within the outer energy level.

For example, for a carbon atom:

Excitation is characteristic of many atoms. This must be remembered, because excitation determines the ability of atoms to bind to each other. The main thing to remember is the condition under which excitation can occur: a paired electron and an empty orbital in the outer energy level.

There are atoms that have several excited states:

Electronic configuration of the ion.

Ions are particles that atoms and molecules turn into by gaining or losing electrons. These particles have a charge, because they either "not enough" electrons, or their excess. Positively charged ions are called cations, negative - anions.

The chlorine atom (has no charge) gains an electron. The electron has a charge of 1- (one minus), respectively, a particle is formed that has an excess negative charge. Chlorine anion:

Cl 0 + 1e → Cl –

The lithium atom (also having no charge) loses an electron. An electron has a charge of 1+ (one plus), a particle is formed, with a lack of a negative charge, that is, its charge is positive. lithium cation:

Li 0 – 1e → Li +

Turning into ions, atoms acquire such a configuration that the external energy level becomes "beautiful", that is, completely filled. This configuration is the most thermodynamically stable, so there is a reason for atoms to turn into ions.

And therefore, the atoms of the elements of the VIII-A group (the eighth group of the main subgroup), as stated in the next paragraph, are noble gases, such are chemically inactive. They have the following structure in the ground state: the outer energy level is completely filled. Other atoms, as it were, tend to acquire the configuration of these most noble gases, and therefore turn into ions and form chemical bonds.