Question

How does valency vary across a group or period? And why?

Answer 1

The valency of an element is dependent on the number of electrons in the outermost shell of the electron. 
In a group all constituting elements will have the same valency because they all have the same number of electrons in the outer shells.
Across the a period, the number however increase by 1. Therefore in group 1 is valency of 1, in two valency of 2, in three valency of 3..and so on.

Answer 2

Answer: because the no of electrons in the Valence shell after some period

Explanation: