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located in a periodic system after Uranium (ordinal numbers 90-103). They are

called actinides. The general formula of lanthanides and actinides is

0-1
2
ns (n-1)d (n-2)f 1-14 ,
where n is the period number.

Based on the electron configuration of the elements, we can conclude that the

smaller number of the electrons located at the external energy level is, the more

metallic properties are shown by an element.

In addition, the size of the atoms has a great influence on the properties of the

elements. In periods the size of the atoms decreases, which is explained by the

increase in the electrostatic interaction of the nucleus with the external electrons.

Element Li Be B C N O F

Radius of the atom by 134 107 89 77 70 66 64

Pauling, p.m.

Within the main subgroups the radii of the atoms with increasing number of

ordinal numbers also increase.

Element Li Na K Rb Cs Fr

The radius of the atom by 155 189 236 248 268 280

Belov, p.m.

Within the main subgroups, the "shielding" effect is manifested, due to the

presence of the bigger number of electrons between the nucleus and the electrons

of the external energy level. Therefore, with the increase of the ordinal number in

the subgroups, the metallic properties are amplified.

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