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(alkaline earth). The general electron formula for the last energy level for these

2
elements is described by the formula ns , where n is the period number.
The elements of the first and the second groups of main subgroups belong to

the s-elements. The electrons of the elements III-VIII main subgroups fill in the p-

sublevels. The general electron formula for the last energy level for these elements

2
is described by the formula ns np 1 - 6 , where n is the period number. These
elements are called p-elements. The electrons of the elements of the subgroups fill

in the d-sublevel of the penultimate energy level (starting with the elements of the

2
fourth period). The general formula for them is ns (n-1) d 1 ÷ 10 , where n is the
period number. These elements are called d-elements. For example, the full

electron formula for Cobalt is as follows:

2
2
6
2
7
2
6
27Co 1s 2s 2p 3s 3p 4s 3d
For d-elements, valence electrons are the last-level electrons and partially d-
electrons of the penultimate energy level.
th
st
Starting with the 58 chemical element (after Lanthanum) and up to the 71 the
electrons fill in the second external energy level of the f-sublayer. These elements

have similar physical and chemical properties, which is determined by the same
structure of the last electron shells of their atoms. They are called lanthanides.

The electron configuration of all lanthanides differs by the number of electrons at

the inner 4f-sublayer - from zero in Lanthanum to the maximum possible number

14 in Lutetium. For example, the full electron formula of Gadolinium is as

follows:

1
10
2
6
10
6
2
2
6
7
2
6
2
2
64Gd 1s 2s 2p 3s 3p 4s 3d 4р 5s 4d 5p 6s 5d 4f

For f-elements valence electrons are the last-level electrons and the d-electron
of the penultimate energy level (underlined). Therefore, lanthanides in compounds
are often trivalent. Similarly, the 5f-sublevel electrons are filled with elements
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