5. STRUCTURE OF ELECTRON SHELLS OF ATOMS

In the section "Data on Atomic Structure" you have got acquainted with planetary model of atom according to which around positive kernel electrons which form electron shells of atom move. Electrons differ with the energy, than further from kernel electrons are located the they possess large supply of energy. Always in atom there are electrons With close values of energy which form electron layers in atom cover.
The number of electron layers is equal to number of the period in periodic system. The maximum number of electrons on layer can be found by means of ^{formula N=2n2} where N-number of electrons on layer, n-number of layer. Having made simple calculations, we will find out that on the first electron layer there will be 2 electrons, on the second-8, third-18, fourth-32. But on external electron layer there cannot be more than 8 electrons, and for first coat no more than 2. небходимо to consider it at distribution of electrons on power layers.
Let's review several examples of filling of electron shells.

After filling of the second layer to eight electrons the third layer appears.

The third electron layer is filled to the maximum number-18 of electrons. After that the external fourth layer to 8 electrons begins to be filled.

The external electron layer is filled at elements of the main subgroups, and the number of electrons corresponds to number of group. At elements of secondary subgroups preexternal layers are filled, and on external layer there are most often two electrons. At d-elements the preexternal layer, and at f-elements the layer third outside is filled.
Distribution of electrons on levels can be written down by means of Hund's cells:

Here levels are divided into subtotals. You probably have noticed that in periodic system elements are subdivided into s-, p-, d-, f-elements. What subtotal is filled with the last, this element also concerns to such group. For example, the last electron occupies one of r-orbitals, this element belongs to r-elements.
When filling cells electrons are designated by shooters (one shooter another is directed up, down, it is connected with the fact that electrons rotate around the axis - one clockwise, another - counterclockwise), and in one cell there can be no more than two electrons.

Order of filling of cells.

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

When filling cells it is necessary to consider that cells of subtotals are filled on one electron in the beginning, then to them place other electrons.

  

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