Question

state how the valency of element varies in group and in periods

Answer 1

Valency → It is defined as combining capacity of an element.

In groups,
On moving top to down in groups the velocity remains the same.

In periods,
On moving left to right, the velocity first increases than decreases

Answer 2

The valency of an element remains the same down the group (only the no of shells increase)
The valency of an element increases from left to right in a period. (no of shells remain same)