How many elements are there in `4^(th)` period? Prove it.

Explanation

In fourth period, 18 elements are present. In this period electrons are entering into fourth energy level, ie, n=4. It starts with the filling of 4s-orbitals. However, after the 4s, but before the 4p orbitals, there are five 3d-orbitals also to be filled. Thus, nine orbitals (one 4s, five 3d and three 4p) have to be filled. These nine orbitals can accommodate `(9xxx2=18)` 18 electrons. Hence, `4^(th)` period contain 18 elements in it.