Question

How does the valency vary in a period on going from left or right

Answer 1

It depends upon the metals and non-metals

And also s,p,d,f block elemens

Answer 2

Valency first increases from 1 to 4 and then decreases from 4 to 0 along a period from left to right. This happens beacuse electron donating power increases and metallic character increases and then electron attracting power increases and non-metallic character increases.