Why dos the number of elements vary from period to period?

Answer 1

This is because of how the electrons are packed inside each atom. There are four known electron orbitals, the s, p, d, and f orbitals. Each can hold a different number of electrons: the s can hold 2; the p can hold 6; the d can hold 10; and the f can hold 14.

As you go down the Periodic Table, different orbitals are filled as you go. On the first period, only the s orbital is filled, so there are only two elements. On the second and third periods, only the s and p orbitals are filled, so there are eight elements each on those. On the fourth and fifth, the s, p, and d orbitals are filled, so there are eighteen elements on those. On the sixth and seventh periods, the s, p, d, and f orbitals are filled, so there are thirty-two elements on those.