Why does the first period contain only two chemical elements, and the second eight?

This is due to the modern theory of the structure of the atom. Periods can contain a different number of chemical elements, depending on the number of electrons at the outer level. In the first period, atoms of chemical elements have only one energy level, at which 2 electrons can be maximally located, in the second period – two energy levels and a maximum of 8 electrons.

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