Compose electronic formulas for elements 1st 3rd period. Electronic formulas

The arrangement of electrons on energy shells or levels is written using electronic formulas of chemical elements. Electronic formulas or configurations help represent the atomic structure of an element.

Atomic structure

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

Electrons are at different energy levels. The further 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 orbital or orbital cloud. This is the space in which the electron moves.

Rice. 1. General structure atom.

Orbitals can have different geometric configurations:

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

The first energy level of any atom always contains an s-orbital with two electrons (the 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 presence of electrons in them and can be filled or vacant.

Writing a formula

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

each energy level corresponds serial number, denoted by an Arabic numeral;
the number is followed by a letter indicating 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 you write down the electronic formula. The number of energy levels corresponds to the period number. The charge of an atom and the number of electrons is indicated by the atomic number of the element. 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.

Let us remember that the first energy level contains an s orbital with two electrons, and the second contains s and p orbitals. All that remains is to fill out 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 long periodic table formulas are also indicated in each cell of the element.

Rice. 3. Table of electronic formulas.

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

The task of compiling an electronic formula for a chemical element is not the easiest.

So, the algorithm for compiling electronic formulas of elements is as follows:

First we write down the chemical sign. element, where at the bottom left of the sign we indicate its serial number.
Next, by the number of the period (from which the element) we determine the number of energy levels and draw such a number of arcs next to the sign of the chemical element.
Then, according to the group number, the number of electrons in the outer level is written under the arc.
At the 1st level, the maximum possible is 2, at the second there are already 8, at the third - as many as 18. We begin to put numbers under the corresponding arcs.
The number of electrons at the penultimate level must be calculated as follows: the number of electrons already assigned is subtracted from the element’s serial number.
It remains to turn our diagram into an electronic formula:

Here are the electronic formulas of some chemical elements:

1. We write the chemical element and its serial number. The number shows the number of electrons in the atom.
2. Let's make a formula. To do this, you need to find out the number of energy levels; the basis for the determination is the period number of the element.
3. We divide the levels into sub-levels.
Below you can see an example of how to correctly compose electronic formulas of chemical elements.

You need to create electronic formulas of chemical elements in this way: you need to look at the number of the element in the periodic table, thus finding out how many electrons it has. Then you need to find out the number of levels, which is equal to the period. Then the sublevels are written and filled in:

First of all, you need to determine the number of atoms according to the periodic table.

To compile the electronic formula, you will need the Mendeleev periodic system. Find your chemical element there and look at the period - it will be equal to the number of energy levels. The group number will correspond numerically to the number of electrons in the last level. The number of an element will be quantitatively equal to the number of its electrons. You also clearly need to know that the first level has a maximum of 2 electrons, the second - 8, and the third - 18.

These are the main points. In addition, on the Internet (including our website) you can find information with a ready-made electronic formula for each element, so you can test yourself.

Compiling electronic formulas of chemical elements is a very complex process; you can’t do it without special tables, and you need to use a whole bunch of formulas. Briefly, to compile you need to go through these stages:

It is necessary to draw up an orbital diagram in which there will be a concept of how electrons differ from each other. The diagram highlights orbitals and electrons.

Electrons are filled in levels, from bottom to top, and have several sublevels.

So first we find out the total number of electrons of a given atom.

We fill out the formula according to a certain scheme and write it down - this will be the electronic formula.

For example, for Nitrogen this formula looks like this, first we deal with electrons:

And write down the formula:

To understand the principle of compiling the electronic formula of a chemical element, first you need to determine the total number of electrons in an atom by the number in the periodic table. After this, you need to determine the number of energy levels, taking as a basis the number of the period in which the element is located.

The levels are then broken down into sublevels, which are filled with electrons based on the Principle of Least Energy.

You can check the correctness of your reasoning by looking, for example, here.

By composing the electronic formula of a chemical element, you can find out how many electrons and electron layers are in a particular atom, as well as the order of their distribution among the layers.

First, we determine the atomic number of the element according to the periodic table; it corresponds to the number of electrons. The number of electron layers indicates the period number, and the number of electrons per last layer atom corresponds to the group number.

first we fill the s-sublevel, and then the p-, d- b f-sublevels;
according to Klechkovsky's rule, electrons fill orbitals in order of increasing energy of these orbitals;
according to Hund's rule, electrons within one sublevel occupy free orbitals one at a time and then form pairs;
According to the Pauli principle, there are no more than 2 electrons in one orbital.

The electronic formula of a chemical element shows how many electron layers and how many electrons are contained in the atom and how they are distributed among the layers.
To compose the electronic formula of a chemical element, you need to look at the periodic table and use the information obtained for this element. The atomic number of an element in the periodic table corresponds to the number of electrons in an atom. The number of electronic layers corresponds to the period number, the number of electrons in the last electronic layer corresponds to the group number.
It must be remembered that the first layer contains a maximum of 2 electrons 1s2, the second - a maximum of 8 (two s and six p: 2s2 2p6), the third - a maximum of 18 (two s, six p, and ten d: 3s2 3p6 3d10).
For example, the electronic formula of carbon: C 1s2 2s2 2p2 (serial number 6, period number 2, group number 4).
Electronic formula for sodium: Na 1s2 2s2 2p6 3s1 (serial number 11, period number 3, group number 1).
To check whether the electronic formula is written correctly, you can look at the website www.alhimikov.net.
At first glance, compiling an electronic formula for chemical elements may seem like a rather complicated task, but everything will become clear if you adhere to the following scheme:
- first we write the orbitals
- We insert numbers in front of the orbitals that indicate the number of the energy level. Don't forget the formula for determining maximum quantity electrons at energy level: N=2n2
How can you find out the number of energy levels? Just look at the periodic table: this number is equal to the number of the period in which the element is located.
- Above the orbital icon we write a number that indicates the number of electrons that are in this orbital.
For example, the electronic formula for scandium will look like this.

When writing electronic formulas for atoms of elements, indicate energy levels (values of the main quantum number n in the form of numbers - 1, 2, 3, etc.), energy sublevels (orbital quantum number values l in the form of letters - s, p, d, f) and the number at the top indicate the number of electrons in a given sublevel.

The first element in the table is D.I. Mendeleev is hydrogen, therefore the charge of the nucleus of the atom N equals 1, an atom has only one electron per s-sublevel of the first level. Therefore, the electronic formula of the hydrogen atom has the form:

The second element is helium; its atom has two electrons, so the electronic formula of the helium atom is 2 Not 1s 2. The first period includes only two elements, since the first energy level is filled with electrons, which can only be occupied by 2 electrons.

The third element in order - lithium - is already in the second period, therefore, its second energy level begins to be filled with electrons (we talked about this above). The filling of the second level with electrons begins with s-sublevel, therefore the electronic formula of the lithium atom is 3 Li 1s 2 2s 1 . The beryllium atom is completed filling with electrons s-sublevel: 4 Ve 1s 2 2s 2 .

In subsequent elements of the 2nd period, the second energy level continues to be filled with electrons, only now it is filled with electrons R-sublevel: 5 IN 1s 2 2s 2 2R 1 ; 6 WITH 1s 2 2s 2 2R 2 … 10 Ne 1s 2 2s 2 2R 6 .

The neon atom completes filling with electrons R-sublevel, this element ends the second period, it has eight electrons, since s- And R-sublevels can only contain eight electrons.

The elements of the 3rd period have a similar sequence of filling the energy sublevels of the third level with electrons. The electronic formulas of the atoms of some elements of this period are as follows:

11 Na 1s 2 2s 2 2R 6 3s 1 ; 12 Mg 1s 2 2s 2 2R 6 3s 2 ; 13 Al 1s 2 2s 2 2R 6 3s 2 3p 1 ;

14 Si 1s 2 2s 2 2R 6 3s 2 3p 2 ;…; 18 Ar 1s 2 2s 2 2R 6 3s 2 3p 6 .

The third period, like the second, ends with an element (argon), which is completely filled with electrons R-sublevel, although the third level includes three sublevels ( s, R, d). According to the above order of filling energy sublevels in accordance with Klechkovsky's rules, the energy of sublevel 3 d more sublevel 4 energy s, therefore, the potassium atom next to argon and the calcium atom behind it are filled with electrons 3 s– sublevel of the fourth level:

19 TO 1s 2 2s 2 2R 6 3s 2 3p 6 4s 1 ; 20 Sa 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 .

Starting from the 21st element - scandium, sublevel 3 in the atoms of the elements begins to be filled with electrons d. The electronic formulas of the atoms of these elements are:

21 Sc 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 1 ; 22 Ti 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 2 .

In the atoms of the 24th element (chromium) and the 29th element (copper), a phenomenon called “leakage” or “failure” of an electron is observed: an electron from the outer 4 s– sublevel “falls” by 3 d– sublevel, completing filling it halfway (for chromium) or completely (for copper), which contributes to greater stability of the atom:

24 Cr 1s 2 2s 2 2R 6 3s 2 3p 6 4s 1 3d 5 (instead of...4 s 2 3d 4) and

29 Cu 1s 2 2s 2 2R 6 3s 2 3p 6 4s 1 3d 10 (instead of...4 s 2 3d 9).

Starting from the 31st element - gallium, the filling of the 4th level with electrons continues, now - R– sublevel:

31 Ga 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 10 4p 1 …; 36 Kr 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 10 4p 6 .

This element ends the fourth period, which already includes 18 elements.

A similar order of filling energy sublevels with electrons occurs in the atoms of elements of the 5th period. For the first two (rubidium and strontium) it is filled s– sublevel of the 5th level, for the next ten elements (from yttrium to cadmium) is filled d– sublevel of the 4th level; The period is completed by six elements (from indium to xenon), the atoms of which are filled with electrons R– sublevel of the external, fifth level. There are also 18 elements in a period.

For elements of the sixth period, this order of filling is violated. At the beginning of the period, as usual, there are two elements whose atoms are filled with electrons s– sublevel of the external, sixth, level. The next element behind them, lanthanum, begins to fill with electrons d– sublevel of the previous level, i.e. 5 d. This completes the filling with electrons 5 d-sublevel stops and the next 14 elements - from cerium to lutetium - begin to fill f-sublevel of the 4th level. These elements are all included in one cell of the table, and below is an expanded row of these elements, called lanthanides.

Starting from the 72nd element - hafnium - to the 80th element - mercury, filling with electrons continues 5 d-sublevel, and the period ends, as usual, with six elements (from thallium to radon), the atoms of which are filled with electrons R– sublevel of the external, sixth, level. This is the largest period, including 32 elements.

In the atoms of the elements of the seventh, incomplete, period, the same order of filling sublevels is visible as described above. We let students write the electronic formulas of atoms of elements of the 5th – 7th periods themselves, taking into account everything said above.

Note:In some textbooks A different order of writing the electronic formulas of atoms of elements is allowed: not in the order of their filling, but in accordance with the number of electrons given in the table at each energy level. For example, the electronic formula of the arsenic atom may look like: As 1s 2 2s 2 2R 6 3s 2 3p 6 3d 10 4s 2 4p 3 .

Composition of the atom.

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

The nucleus of an atom consists of protons ( p+) and neutrons ( n 0). Most hydrogen atoms have a nucleus consisting of one proton.

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 table of elements).

N(p +) = Z

Sum of neutrons N(n 0), denoted simply by the letter N, and number of protons Z called mass number and is designated by the letter A.

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 almost equal to the mass of the nucleus.

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

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

Isotope- a collection of atoms of the same element with the same number of neutrons in the nucleus (or a type of atom 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 an individual atom or isotope: (E - element symbol), for example: .

Structure of the electron shell of an atom

Atomic orbital- state of an electron in an atom. The symbol for the orbital is . Each orbital has a corresponding electron cloud.

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 “atomic orbital”.

The electron shell of an atom is layered. Electronic layer formed by electron clouds same size. The 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 type 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 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 at one level is equal to the level number n. The total number of orbitals at one level is equal to n 2. Accordingly, total number clouds in one layer is also equal 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 (the formulations are given in simplified terms):

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

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

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

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

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

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

This sequence is clearly expressed by an energy diagram:

The distribution of an atom's electrons across levels, sublevels, and orbitals (electronic configuration of an atom) can be depicted as an electron formula, an energy diagram, or, more simply, as a diagram of electron layers ("electron 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: the Ca atom has 4 outer electrons s 2, they are also valence; the Fe atom has 4 outer electrons s 2 but he has 3 d 6, therefore the iron atom has 8 valence electrons. Valence electronic formula of the calcium atom is 4 s 2, and iron atoms - 4 s 2 3d 6 .

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

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

Periodic table- graphic expression of the periodic law.

Natural series of chemical elements- a series of chemical elements arranged according to the increasing number of protons in the nuclei of their atoms, or, what is the same, according to the increasing charges of the nuclei of these atoms. The atomic 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 the way you combine elements into groups, the table may be long-period(elements with the same number and type of valence electrons are collected into groups) and short period(elements with the same number of valence electrons are collected in groups).

The short-period table groups are divided into subgroups ( main And side), 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 period number.

Number of elements in 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 begin 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 there are eight groups, each of which is divided into two subgroups (main and secondary), in the long-period table there are 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)

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

In groups (with increasing serial number)

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),
the oxidizing properties of simple substances weaken ("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 corresponding oxides increases (only in A-groups),
weakens the acidic character of hydroxides and corresponding oxides (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. "Structure of the atom. Periodic law and periodic system of chemical elements by D. I. Mendeleev (PSHE) "."

Periodic law - Periodic law and structure of atoms grades 8–9
You must know: the laws of filling orbitals with electrons (the principle of least energy, the Pauli principle, Hund's rule), the structure of the periodic table of elements.
You must be able to: determine the composition of an atom by the position of the element in the periodic table, and, conversely, find an element in the periodic system, knowing its composition; depict the structure diagram, 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, 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 electron 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 which element's atom has electronic formula 1 s 2 2s 2 2p 6 3s 2 3p 1 .
In order to determine what element it is, you need to find out its atomic number, which is equal to the total number of electrons of the atom. IN in this case: 2 + 2 + 6 + 2 + 1 = 13. This is aluminum.
After making sure that everything you need has been learned, proceed to completing the tasks. We wish you success.

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

Knowledge of the possible states of an electron in an atom, Klechkovsky's rule, Pauli's principle and Hund's rule make it possible to consider the electronic configuration of an atom. Electronic formulas are used for this.

The electron formula denotes the state of an electron in an atom, indicating with a number the main quantum number characterizing its state, and with a letter indicating the orbital quantum number. The number indicating how many electrons are in a given state is written to the right above the letter indicating the shape of the electron cloud.

For the hydrogen atom (n = 1, l = 0, m = 0) the electronic formula will be: 1s 1. Both electrons of the next element helium are not characterized the same values n, l, m and differ only in their spins. The electronic formula of the helium atom is ls 2. The electron shell of the helium atom is complete and very stable. Helium is a noble gas.

For elements of the 2nd period (n = 2, l = 0 or l = 1), first the 2s-state is filled, and then the p-sublevel of the second energy level.

Electronic formula of the lithium atom: ls 2 2s 1. The 2s 1 electron is weaker bound to the atomic nucleus (Fig. 6), so the lithium atom can easily give it up (as you obviously remember, this process is called oxidation), turning into the Li + ion.

Rice. 6.
Sections of 1s- and 2s-electron clouds by a plane passing through the nucleus

In the beryllium atom, the fourth electron also occupies the 2s state: ls 2 2s 2. The two outer electrons of the beryllium atom are easily detached - in this case, Be is oxidized into the Be 2+ cation.

The boron atom has an electron in the 2p state: ls 2 2s 2 2p 1. Next, for carbon, nitrogen, oxygen and fluorine atoms (in accordance with Hund’s rule), the 2p sublevel is filled, which ends at the noble gas neon: ls 2 2s 2 2p 6.

If they want to emphasize that electrons at a given sublevel occupy quantum cells individually, in the electronic formula the designation of the sublevel accompanies the index. For example, the electronic formula of the carbon atom

For elements of the 3rd period, the Zs-state (n = 3, l = 0) and the Zp-sublevel (n = 3, l - 1) are filled, respectively. The 3d sublevel (n = 3, l = 2) remains free:

Sometimes in diagrams depicting the distribution of electrons in atoms, only the number of electrons at each energy level is indicated, that is, abbreviated electronic formulas of atoms of chemical elements are written, in contrast to the full electronic formulas given above, for example:

For elements of large periods (4th and 5th), in accordance with the Klechkovsky rule, the first two electrons of the outer electron layer occupy the 4s-state (n = 4, l = 0) and 5s-states (n = 5, l = 0):

Starting from the third element of each long period, the next ten electrons arrive at the previous 3d and 4d sublevels, respectively (for elements side subgroups):

As a rule, when the previous d-sublevel is filled, then the outer (4p- and 5p-respectively) p-sublevel will begin to fill:

For elements of large periods - the 6th and incomplete 7th - energy levels and sublevels are filled with electrons, as a rule, like this: the first two electrons go to the outer s-sublevel, for example:

the next one electron (in La and Ac) goes to the previous d-sublevel:

Then the next 14 electrons enter the third outer energy level in the 4f and 5f sublevels of lanthanides and actinides, respectively:

Then the second outside energy level (d-sublevel) of the elements of the side subgroups will begin to build up again:

Only after the d-sublevel is completely filled with ten electrons will the outer p-sublevel be filled again:

In conclusion, let's look again different ways displaying the electronic configurations of atoms of elements by periods of D.I. Mendeleev’s table.

Let's consider the elements of the 1st period - hydrogen and helium.

Electronic formulas of atoms show the distribution of electrons across energy levels and sublevels.

Graphic electronic formulas of atoms show the distribution of electrons not only across levels and sublevels, but also across quantum cells (atomic orbitals).

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

Hydrogen and helium are s-elements; the ls-sublevel of these atoms is filled with electrons.

For all elements of the 2nd period, the first electron layer is filled, and electrons fill the 2s and 2p states 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 complete - it has 8 electrons.

table 2
Structure electronic shells atoms of elements of the 2nd period

Lithium Li, beryllium Be - s-elements.

Boron B, carbon C, nitrogen N, oxygen O, fluorine F, neon Ne are p-elements; the p-sublevel of these atoms is filled with electrons.

For atoms of elements of the 3rd period, the first and second electronic layers are completed, so the third electronic layer is filled, in which electrons can occupy 3s-, 3p- and 3d-states (Table 3).

Table 3
Structure of electronic shells of atoms of elements of the 3rd period

The 3s sublevel is being completed at the magnesium atom. Sodium Na and magnesium Mg are s-elements.

In aluminum and the elements following it, the 3p sublevel is filled with electrons.

An argon atom has 8 electrons in its outer layer (third electron layer). 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 3rd period have an unfilled 3d state.

All elements from aluminum Al to argon Ar are p-elements.

The s- and p-elements form the main subgroups in the Periodic Table.

For atoms of the elements of the 4th period - potassium and calcium - a fourth energy level appears, the 48th sublevel is filled (Table 4), since, according to Klechkovsky’s rule, it has lower energy than the 3d sublevel.

Table 4
Structure of electronic shells of atoms of elements of the 4th period

To simplify the graphical electronic formulas of atoms of elements of the 4th period:

Potassium K and calcium Ca are s-elements included in the main subgroups. In atoms from scandium Sc to zinc Zn, the 3d sublevel is filled with electrons. These are 3d elements. They are included in secondary subgroups, their outermost electronic layer is filled, and they are classified as transition elements.

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

In the zinc atom, the third energy level is complete; all sublevels are filled in it - 3s, 3p and 3d, with a total of 18 electrons.

The elements following zinc continue to fill the fourth energy level, the 4p sublevel.

Elements from gallium Ga to krypton Kr are p-elements.

The Kr krypton atom has an outer layer (fourth) that is complete and has 8 electrons. But in total in the fourth electron layer, as you know, there can be 32 electrons; the krypton atom still has unfilled 4d and 4f states.

For elements of the 5th period, in accordance with Klechkovsky’s rule, sublevels are filled in the following order: 5s ⇒ 4d ⇒ 5r. And there are also exceptions associated with the “failure” of electrons in 41 Nb, 42 Mo, 44 Ru, 45 Rh, 46 Pd, 47 Ag.

In the 6th and 7th periods, f-elements appear, i.e. elements for which the 4f- and 5f-sublevels of the third outside energy level are being filled, respectively.

4f elements are called lanthanides.

5f-elements are called actinides.

The order of filling electronic sublevels in atoms of elements of the 6th period: 55 Cs and 56 Ba - bs 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 - br-elements. But here, too, there are elements in which the order of filling the energy sublevels is “disturbed,” which, for example, is associated with the greater energy stability of half and fully filled f-sublevels, i.e. 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):

Rice. 7.
Division of the Periodic Table (table) into blocks of elements

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 groups III-VIII;
d-elements; the d-sublevel of the pre-external level of the atom is filled with electrons; d-elements include elements of secondary subgroups of groups I-VIII, i.e. elements of plug-in decades of large periods located between s- and p-elements. They are also called transition elements;
f-elements; the f-sublevel of the third outer level of the atom is filled with electrons; These include lanthanides and actinides.

Questions and tasks for § 3

Make diagrams of the electronic structure, electronic formulas and graphic electronic formulas of atoms of the following chemical elements:
Write the electronic formula for element No. 110 using the symbol for the appropriate noble gas.
What is an electron "dip"? Give examples of elements in which this phenomenon is observed, write down their electronic formulas.
How is the belonging of a chemical element to a particular electron family determined?
Compare the electronic and graphical electronic formulas of the sulfur atom. Which Additional information does the last formula contain?