Atom- an electrically neutral particle consisting of a positively charged nucleus and negatively charged electrons. At the center of an atom is a positively charged nucleus. It occupies an insignificant part of the space inside the atom; all the positive charge and almost the entire mass of the atom are concentrated in it.

The nucleus consists of elementary particles - proton and neutron; Electrons move around the atomic nucleus in closed orbitals.

Proton (r)- an elementary particle with a relative mass of 1.00728 atomic mass units and a charge of +1 conventional unit. The number of protons in the atomic nucleus is equal to the serial number of the element in the Periodic system of D.I. Mendeleev.

Neutron (n)- an elementary neutral particle with a relative mass of 1.00866 atomic mass unit (a.m.u.).

The number of neutrons in the nucleus N is determined by the formula:

where A is the mass number, Z is the charge of the nucleus, equal to the number of protons (serial number).

Usually, the parameters of the nucleus of an atom are written as follows: the charge of the nucleus is placed at the bottom left of the symbol of the element, and the mass number is placed at the top, for example:

This record shows that the nuclear charge (hence the number of protons) for a phosphorus atom is 15, the mass number is 31, and the number of neutrons is 31 - 15 = 16. Since the masses of the proton and neutron differ very little from each other, the mass the number is approximately equal to the relative atomic mass of the nucleus.

Electron (e -)- an elementary particle with a mass of 0.00055 a. e.m. and conditional charge –1. The number of electrons in an atom is equal to the charge of the atomic nucleus (the serial number of the element in the Periodic system of D.I. Mendeleev).

Electrons move around the nucleus in strictly defined orbits, forming the so-called electron cloud.

The region of space around the atomic nucleus, where the electron is most likely to be found (90% or more), determines the shape of the electron cloud.

The electron cloud of the s-electron has a spherical shape; the s-energy sublevel can have a maximum of two electrons.

The electron cloud of the p-electron is dumbbell-shaped; Three p-orbitals can hold a maximum of six electrons.

Orbitals are depicted as a square, above or below which they write the values ​​of the main and secondary quantum numbers that describe this orbital. Such a record is called a graphic electronic formula, for example:

In this formula, arrows denote an electron, and the direction of the arrow corresponds to the direction of the spin - the intrinsic magnetic moment of the electron. Electrons with opposite spins ↓ are called paired.

The electronic configurations of atoms of elements can be represented as electronic formulas, in which the symbols of the sublevel are indicated, the coefficient in front of the symbol of the sublevel shows its belonging to this level, and the degree of the symbol shows the number of electrons of this sublevel.

Table 1 shows the structure of the electron shells of atoms of the first 20 elements of the Periodic Table of Chemical Elements of D.I. Mendeleev.

Chemical elements in whose atoms the s-sublevel of the outer level is replenished with one or two electrons are called s-elements. Chemical elements in whose atoms the p-sublevel (from one to six electrons) is filled are called p-elements.

The number of electron layers in an atom of a chemical element is equal to the period number.

In accordance with Hund's rule electrons are located in orbitals of the same type of the same energy level in such a way that the total spin is maximum. Consequently, when filling the energy sublevel, each electron first of all occupies a separate cell, and only after that their pairing begins. For example, for a nitrogen atom, all p-electrons will be in separate cells, and for oxygen, their pairing will begin, which will completely end in neon.

isotopes called atoms of the same element, containing in their nuclei the same number of protons, but a different number of neutrons.

Isotopes are known for all elements. Therefore, the atomic masses of elements in the periodic system are the average value of the mass numbers of natural mixtures of isotopes and differ from integer values. Thus, the atomic mass of a natural mixture of isotopes cannot serve as the main characteristic of an atom, and, consequently, of an element. Such a characteristic of an atom is the nuclear charge, which determines the number of electrons in the electron shell of the atom and its structure.

Let's take a look at a few typical tasks in this section.

Example 1 Which element atom has the electronic configuration 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 ?

This element has one 4s electron in its outer energy level. Therefore, this chemical element is in the fourth period of the first group of the main subgroup. This element is potassium.

This answer can be arrived at in a different way. Adding the total number of all electrons, we get 19. The total number of electrons is equal to the atomic number of the element. Potassium is number 19 in the periodic table.

Example 2 The highest oxide RO 2 corresponds to the chemical element. The electronic configuration of the external energy level of the atom of this element corresponds to the electronic formula:

1. ns 2 np 4
2. ns 2 np 2
3. ns 2 np 3
4. ns 2 np 6

According to the formula of the highest oxide (look at the formulas of the highest oxides in the Periodic system), we establish that this chemical element is in the fourth group of the main subgroup. These elements have four electrons in their outer energy level - two s and two p. Therefore, the correct answer is 2.

Training tasks

1. The total number of s-electrons in a calcium atom is

1) 20
2) 40
3) 8
4) 6

2. The number of paired p-electrons in a nitrogen atom is

1) 7
2) 14
3) 3
4) 4

3. The number of unpaired s-electrons in a nitrogen atom is

1) 7
2) 14
3) 3
4) 4

4. The number of electrons in the outer energy level of an argon atom is

1) 18
2) 6
3) 4
4) 8

5. The number of protons, neutrons and electrons in the atom 9 4 Be is

1) 9, 4, 5
2) 4, 5, 4
3) 4, 4, 5
4) 9, 5, 9

6. Distribution of electrons over electron layers 2; eight; 4 - corresponds to the atom located in (in)

1) 3rd period, IA group
2) 2nd period, IVA group
3) 3rd period, IVA group
4) 3rd period, VA group

7. The chemical element located in the 3rd period of the VA group corresponds to the scheme of the electronic structure of the atom

1) 2, 8, 6
2) 2, 6, 4
3) 2, 8, 5
4) 2, 8, 2

8. A chemical element with the electronic configuration 1s 2 2s 2 2p 4 forms a volatile hydrogen compound, the formula of which is

1) EN
2) EN 2
3) EN 3
4) EN 4

9. The number of electron layers in an atom of a chemical element is

1) its serial number
2) group number
3) the number of neutrons in the nucleus
4) period number

10. The number of external electrons in the atoms of chemical elements of the main subgroups is

1) the serial number of the element
2) group number
3) the number of neutrons in the nucleus
4) period number

11. Two electrons are in the outer electron layer of the atoms of each of the chemical elements in the series

1) He, Be, Ba
2) Mg, Si, O
3) C, Mg, Ca
4) Ba, Sr, B

12. A chemical element whose electronic formula is 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 forms an oxide of the composition

1) Li 2 O
2) MgO
3) K2O
4) Na 2 O

13. The number of electron layers and the number of p-electrons in a sulfur atom is

1) 2, 6
2) 3, 4
3) 3, 16
4) 3, 10

14. The electronic configuration ns 2 np 4 corresponds to the atom

1) chlorine
2) sulfur
3) magnesium
4) silicon

15. The valence electrons of the sodium atom in the ground state are at the energy sublevel

1) 2s
2) 2p
3) 3s
4) 3p

16. The nitrogen and phosphorus atoms have

1) the same number of neutrons
2) the same number of protons
3) the same configuration of the outer electron layer

17. Calcium atoms have the same number of valence electrons

1) potassium
2) aluminum
3) beryllium
4) boron

18. The carbon and fluorine atoms have

1) the same number of neutrons
2) the same number of protons
3) the same number of electronic layers
4) the same number of electrons

19. At the carbon atom in the ground state, the number of unpaired electrons is

1) 1
3) 3
2) 2
4) 4

20. In the oxygen atom in the ground state, the number of paired electrons is

The conditional image of the distribution of electrons in the electron cloud by levels, sublevels and orbitals is called the electronic formula of the atom.

Rules based on|based on| which | which | make up | hand over | electronic formulas

1. Principle of minimum energy: the less energy the system has, the more stable it is.

2. Klechkovsky's rule: the distribution of electrons over the levels and sublevels of the electron cloud occurs in ascending order of the sum of the principal and orbital quantum numbers (n + 1). In the case of equality of values ​​(n + 1), the sublevel that has the smaller value of n is filled first.

1 s 2 s p 3 s p d 4 s p d f 5 s p d f 6 s p d f 7 s p d f Level number n 1 2 2 3 3 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 7 Orbital 1* 0 0 1 0 1 2 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 quantum number

n+1| 1 2 3 3 4 5 4 5 6 7 5 6 7 8 6 7 8 9 7 8 9 10

Klechkovsky series

1* - see table No. 2.

3. Hund's rule: when the orbitals of one sublevel are filled, the lowest energy level corresponds to the placement of electrons with parallel spins.

Drafting|Submitting| electronic formulas

Potential row: 1 s 2 s p 3 s p d 4 s p d f 5 s p d f 6 s p d f 7 s p d f

(n+1|) 1 2 3 3 4 5 4 5 6 7 5 6 7 8 6 7 8 9 7 8 9 10

Klechkovsky series

Filling order Electroni 1s 2 2s 2 p 6 3s 2 p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 10 6p 6 7s 2 5f 14 ..

(n+l|) 1 2 3 3 4 4 5 5 5 6 6 6 7 7 7 7 8.

Electronic formula

(n+1|) 1 2 3 3 4 5 4 5 6 7 5 6 7 8 6 7 8 9 7 8 9 10

Informativeness of electronic formulas

1. The position of the element in the periodic|periodic| system.

2. Possible degrees| element oxidation.

3. The chemical nature of the element.

4. Composition|warehouse| and connection properties of the element.

The position of the element in the periodic|Periodic|D.I. Mendeleev’s system:

a) period number, in which the element is located, corresponds to the number of levels on which the electrons are located;

b) group number, to which this element belongs, is equal to the sum of valence electrons. Valence electrons for atoms of s- and p-elements are electrons of the outer level; for d-elements, these are the electrons of the outer level and the unfilled sublevel of the previous level.

in) electronic family is determined by the symbol of the sublevel to which the last electron enters (s-, p-, d-, f-).

G) subgroup is determined by belonging to the electronic family: s - and p - elements occupy the main subgroups, and d - elements - secondary, f - elements occupy separate sections in the lower part of the periodic system (actinides and lanthanides).

2. Possible degrees| element oxidation.

Oxidation state is the charge that an atom acquires when it gives or gains electrons.

Atoms that donate electrons acquire a positive charge, which is equal to the number of electrons donated (electron charge (-1)

Z E 0 – ne  Z E + n

The atom that donated electrons becomes cation(positive charged ion). The process of removing an electron from an atom is called ionization process. The energy needed to carry out this process is called ionization energy ( Eion, eB).

The first to separate from the atom are electrons of the outer level, which do not have a pair in the orbital - unpaired. In the presence of free orbitals within the same level, under the action of external energy, the electrons that formed pairs at this level are unpaired, and then separated all together. The process of depairing, which occurs as a result of the absorption of a portion of energy by one of the electrons of the pair and its transition to the highest sublevel, is called arousal process.

The largest number of electrons that an atom can donate is equal to the number of valence electrons and corresponds to the number of the group in which the element is located. The charge that an atom acquires after losing all its valence electrons is called the highest degree of oxidation atom.

After release|dismissal| valence level external becomes|becomes| level which|what| preceded valence. This is a level completely filled with electrons, and therefore | and therefore | energy resistant.

Atoms of elements that have from 4 to 7 electrons at the external level achieve an energetically stable state not only by giving up electrons, but also by adding them. As a result, a level (.ns 2 p 6) is formed - a stable inert gas state.

An atom that has attached electrons acquires negativedegreeoxidation- a negative charge, which is equal to the number of received electrons.

Z E 0 + ne  Z E - n

The number of electrons that an atom can attach is equal to the number (8 –N|), where N is the number of the group in which|what| the element is located (or the number of valence electrons).

The process of attaching electrons to an atom is accompanied by the release of energy, which is called c affinity to the electron (Esrodship,eV).

The Swiss physicist W. Pauli in 1925 established that in an atom in one orbital there can be no more than two electrons that have opposite (antiparallel) spins (translated from English as “spindle”), that is, they have properties that can be conditionally represented itself as the rotation of an electron around its imaginary axis: clockwise or counterclockwise. This principle is called the 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.

Figure 5 shows a diagram of the division of energy levels into sublevels.

The S-orbital, as you already know, is spherical. The electron of the hydrogen atom (s = 1) is located in this orbital and is unpaired. Therefore, its electronic formula or electronic configuration will be written as follows: 1s 1. In electronic formulas, the energy level number is indicated by the number in front of the letter (1 ...), the sublevel (orbital type) is indicated by the Latin letter, and the number that is written to the upper right of the letter (as an exponent) shows the number of electrons in the sublevel.

For a helium atom, He, having two paired electrons in the same s-orbital, this formula is: 1s 2 .

The electron shell of the helium atom is complete and very stable. Helium is a noble gas.

The second energy level (n = 2) has four orbitals: one s and three p. Second-level s-orbital electrons (2s-orbitals) have a higher energy, since they are at a greater distance from the nucleus than 1s-orbital electrons (n ​​= 2).

In general, for every value of n, there is one s-orbital, but with a corresponding amount of electron energy in it and, therefore, with a corresponding diameter, growing as the value of n increases.

The R-orbital is shaped like a dumbbell or a figure eight. All three p-orbitals are located in the atom mutually perpendicularly along the spatial coordinates drawn through the nucleus of the atom. It should be emphasized again that each energy level (electronic layer), starting from n = 2, has three p-orbitals. As the value of n increases, the electrons occupy p-orbitals located at large distances from the nucleus and directed along the x, y, and z axes.

For elements of the second period (n = 2), first one β-orbital is filled, and then three p-orbitals. Electronic formula 1l: 1s 2 2s 1. The electron is weaker bound to the nucleus of the atom, so the lithium atom can easily give it away (as you obviously remember, this process is called oxidation), turning into a Li + ion.

In the beryllium atom Be 0, the fourth electron is also located in the 2s orbital: 1s 2 2s 2 . The two outer electrons of the beryllium atom are easily detached - Be 0 is oxidized to the Be 2+ cation.

At the boron atom, the fifth electron occupies a 2p orbital: 1s 2 2s 2 2p 1. Further, the atoms C, N, O, E are filled with 2p orbitals, which ends with the noble gas neon: 1s 2 2s 2 2p 6.

For the elements of the third period, the Sv- and Sp-orbitals are filled, respectively. Five d-orbitals of the third level remain free:

Sometimes in diagrams depicting the distribution of electrons in atoms, only the number of electrons at each energy level is indicated, that is, they write down the abbreviated electronic formulas of atoms of chemical elements, in contrast to the full electronic formulas given above.

For elements of large periods (fourth and fifth), the first two electrons occupy the 4th and 5th orbitals, respectively: 19 K 2, 8, 8, 1; 38 Sr 2, 8, 18, 8, 2. Starting from the third element of each large period, the next ten electrons will go to the previous 3d- and 4d-orbitals, respectively (for elements of secondary subgroups): 23 V 2, 8, 11, 2; 26 Tr 2, 8, 14, 2; 40 Zr 2, 8, 18, 10, 2; 43 Tr 2, 8, 18, 13, 2. As a rule, when the previous d-sublevel is filled, the outer (4p- and 5p, respectively) p-sublevel will begin to fill.

For elements of large periods - the sixth and the incomplete seventh - electronic levels and sublevels are filled with electrons, as a rule, as follows: the first two electrons will go to the outer β-sublevel: 56 Ba 2, 8, 18, 18, 8, 2; 87Gr 2, 8, 18, 32, 18, 8, 1; the next one electron (for Na and Ac) to the previous (p-sublevel: 57 La 2, 8, 18, 18, 9, 2 and 89 Ac 2, 8, 18, 32, 18, 9, 2.

Then the next 14 electrons will go to the third energy level from the outside in the 4f and 5f orbitals, respectively, for lanthanides and actinides.

Then the second outside energy level (d-sublevel) will begin to build up again: for elements of secondary subgroups: 73 Ta 2, 8.18, 32.11, 2; 104 Rf 2, 8.18, 32, 32.10, 2 - and, finally, only after the complete filling of the current level with ten electrons will the outer p-sublevel be filled again:

86 Rn 2, 8, 18, 32, 18, 8.

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: the Pauli principle, according to which there can be no more than two electrons in a cell (orbitals, but with antiparallel spins), and F. Hund's rule, according to which electrons occupy free cells (orbitals), are located in they are first one at a time and at the same time have the same spin value, and only then they pair, but the spins in this case, according to the Pauli principle, will already be oppositely directed.

In conclusion, let us once again consider the mapping of the electronic configurations of the atoms of the elements over the periods of the D. I. Mendeleev system. Schemes of the electronic structure of atoms show the distribution of electrons over electronic layers (energy levels).

In a helium atom, the first electron layer is completed - it has 2 electrons.

Hydrogen and helium are s-elements; these atoms have an s-orbital filled with electrons.

Elements of the second period

For all elements of the second period, the first electron layer is filled and the electrons fill the e- 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 (Table 2).

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

Table 2 The structure of the electron shells of atoms of elements of the second period

The end of the table. 2

Li, Be are β-elements.

B, C, N, O, F, Ne are p-elements; these atoms have p-orbitals filled with electrons.

Elements of the third period

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

Table 3 The structure of the electron shells of atoms of elements of the third period

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

There are 8 electrons in the outer layer (the third electron layer) in the argon atom. As an outer layer, it is complete, but in total, in the third electron layer, as you already know, there can be 18 electrons, which means that 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.

A fourth electron layer appears at the potassium and calcium atoms, and the 4s sublevel is filled (Table 4), since it has a lower energy than the 3d sublevel. To simplify the graphical electronic formulas of atoms of the elements of the fourth period: 1) let's conditionally denote the graphical electronic formula of argon as follows:
Ar;

2) we will not depict the sublevels that are not filled for these atoms.

Table 4 The structure of the electron shells of atoms of the elements of the fourth period

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, their pre-external electron layer is 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 4n- 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, the 4p sublevel, continues to be filled: Elements from Ga to Kr are p-elements.

The outer layer (fourth) of the krypton atom is complete and has 8 electrons. But just in the fourth electron layer, as you know, there can be 32 electrons; the 4d and 4f sublevels of the krypton atom still remain unfilled.

The elements of the fifth period are filling the sublevels in the following order: 5s-> 4d -> 5p. And there are also exceptions associated with the "failure" of electrons, in 41 Nb, 42 MO, etc.

In the sixth and seventh periods, elements appear, that is, elements in which the 4f and 5f sublevels of the third outside electronic layer are being filled, respectively.

The 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 1 - 5d element; 58 Ce - 71 Lu - 4f elements; 72 Hf - 80 Hg - 5d elements; 81 Tl - 86 Rn - 6p 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, that is, nf 7 and nf 14.

Depending on which sublevel of the atom is filled with electrons last, all elements, as you already understood, are divided into four electronic families or blocks (Fig. 7).

1) s-Elements; the β-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;

2) 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;

3) 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, that is, elements of intercalated decades of large periods located between s- and p-elements. They are also called transition elements;

4) f-elements, the f-sublevel of the third outside level of the atom is filled with electrons; these include lanthanides and actinides.

1. What would happen if the Pauli principle was not respected?

2. What would happen if Hund's rule was not respected?

3. Make diagrams of the electronic structure, electronic formulas and graphic electronic formulas of atoms of the following chemical elements: Ca, Fe, Zr, Sn, Nb, Hf, Ra.

4. Write the electronic formula for element #110 using the symbol for the appropriate noble gas.

5. What is the “failure” of an electron? Give examples of elements in which this phenomenon is observed, write down their electronic formulas.

6. How is the belonging of a chemical element to one or another electronic family determined?

7. Compare the electronic and graphic electronic formulas of the sulfur atom. What additional information does the last formula contain?

The location of electrons on energy shells or levels is recorded using electronic formulas of chemical elements. Electronic formulas or configurations help to represent the structure of an element's atom.

The structure of the atom

The atoms of all elements consist of a positively charged nucleus and negatively charged electrons that are located around the nucleus.

The electrons are at different energy levels. The farther an electron is from the nucleus, the more energy it has. The size of the energy level is determined by the size of the atomic orbit or orbital cloud. This is the space in which the electron moves.

Rice. 1. The general structure of the atom.

Orbitals can have different geometric configurations:

• s-orbitals- spherical;
• p-, d and f-orbitals- dumbbell-shaped, lying in different planes.

At the first energy level of any atom, there is always an s-orbital with two electrons (an exception is hydrogen). Starting from the second level, the s- and p-orbitals are at the same level.

Rice. 2. s-, p-, d and f-orbitals.

Orbitals exist regardless of the location of electrons on them and can be filled or vacant.

Formula entry

Electronic configurations of atoms of chemical elements are written according to the following principles:

• each energy level corresponds to a serial number, denoted by an Arabic numeral;
• the number is followed by a letter denoting the orbital;
• a superscript is written above the letter, corresponding to the number of electrons in the orbital.

Recording examples:

• calcium -

  1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 ;

• oxygen -

  1s 2 2s 2 2p 4 ;

• carbon-

  1s 2 2s 2 2p 2 .

The periodic table helps to write down the electronic formula. The number of energy levels corresponds to the number of the period. The number of the element indicates the charge of an atom and the number of electrons. The group number indicates how many valence electrons are in the outer level.

Let's take Na as an example. Sodium is in the first group, in the third period, at number 11. This means that the sodium atom has a positively charged nucleus (contains 11 protons), around which 11 electrons are located at three energy levels. There is one electron in the outer level.

Recall that the first energy level contains an s-orbital with two electrons, and the second contains s- and p-orbitals. It remains to fill the levels and get the full record:

11 Na) 2) 8) 1 or 1s 2 2s 2 2p 6 3s 1 .

For convenience, special tables of electronic formulas of the element have been created. In the long periodic table, the formulas are also indicated in each cell of the element.

Rice. 3. Table of electronic formulas.

For brevity, elements are written in square brackets, the electronic formula of which coincides with the beginning of the element formula. For example, the electronic formula of magnesium is 3s 2, neon is 1s 2 2s 2 2p 6. Therefore, the full formula for magnesium is 1s 2 2s 2 2p 6 3s 2 . 4.6. Total ratings received: 195.

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 (electronic configuration of an atom) can be depicted in the form of an electronic formula, an energy diagram, or, more simply, in the form of an electron layer diagram ("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.