Question

what is valency of elements ? 

Answer 1

A valency of a element is the measure of electrons it needs or needs to loose to obtain a stable structure (a noble gas structure,such as helium,neon,argon...)    

Answer 2

valency means combining capacity..
for example h₂o
here valency of hydrogen is 1 and valency of oxygen is 2.
because hydrogen combines with one atom of oxygen so valency of hydrozen is 1.
and oxygen combines with 2 atoms of hydrogen so valency of oxygen is 2.