Prove that valency is a periodic property.

Explanation

Variation in period: The number of valence electrons increases from 1 to 8 on moving across a period. The valency of the elements with respect to hydrogen and chlorine increases from 1 to 4 and then decreases from 4 to zero.
Variation in group: On moving down a group, the number of valence electrons remains same. All the elements in a group exhibit same valency. For example, all the elements of group 1 have valency equal to 1.
Hence, valency is a periodic property.