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and it determines its atomic mass.

The atomic mass is indicated by the upper index before the element symbol.

For example means that the element atomic nucleus with an ordinal number

17 contains 17 protons and 35-17 = 18 neutrons. It is known that there are atoms

that have the same charge of nuclei, but different mass due to different number of
neutrons in the nucleus. Such atoms are isotopes. For example, except there

exists atoms , that is, they also have charges of nuclei +17, but 37-17 = 20

neutrons.

The atomic mass of the element, given in the periodic table, is the average

value of the masses of isotopes occurring in nature. The averaging is carried out in

accordance with the prevalence of an isotope in nature. Therefore, the atomic mass

of an element is not an integer, and for Chlorine, for example, the average atomic

mass is 35.45. The atoms with the same mass numbers, but different charges of the

nuclei, and therefore the different number of electrons at the external and

penultimate energy levels, which determine their chemical activity are isobars.

For example, atoms, , , are isobars.

2.2 The structure of the atom and the periodicity of the properties of

chemical elements

The nucleus is surrounded by an electron cloud. There are shells which can be

singled out in the electron cloud, and there are several possible orbits for each.

The filled orbits create an electronic configuration that is peculiar to each chemical

element. The structures of the electron configuration of atoms are represented by

electron formulas, which indicate the distribution of electrons by energy levels and

sublevels (levels are indicated by the numbers 1, 2, 3 ..., sublevels - by the letters s,

p, d, f).

The number of electrons at the sublevel is denoted by the number recorded by

3
the right upper index (for example, p ). The number of energy levels coincides
with the period where the chemical element is located. At the first energy level

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