There are `2, 8` and `8` elements in the first, second and third periods of periodic table respectively. Explain.

Explanation

The first period `(1s)` is completed with two electrons. The second period `(2s, 2p)` is completed with `2` and `6` electrons respectively in `2s` and `2p`.
Likewise the third period `(3s, 3p)` is completed with `2` and `6` electrons. Thus, `2, 8` and `8` elements are present in the first, second and third periods respectively.