Periodicity of valency

The electrons present in the outer most shell are called valence electrons. The valency of an element is defined as its 'combining capacity'.

Variation along the period

The number of valence electrons increase from left to right in a period. So, the valency of the elements first increase from 1 to 4 and then decreases to zero.

Variation down the group

On moving down the group the number of valence electrons remain the same and therefore all the elements in the group exhibit the same valency.