What is valency of an element? How does it vary with respect to hydrogen in the third period.

Explanation

Valency: The combining capacity of an element with another element is called valency.
The number of hydrogen atoms (or) chlorine atoms (or) double the number of oxygen atoms, with which one atom of the element combine is also called valency.
`therefore` Valency= no. of hydrogens
=no. of chlorine atoms
`=2 xx` no. of oxygen atoms present in the molecule

Periodicity of valency:
(1) Each period starts with valency .1. and ends in .0.
eq: `{:("Hydrides",1,2,3,4,3,2,1,0),(,(LiH),(BeH_(2)),(B_(2)H_(6)),(CH_(2)),(NH_(3)),(H_(2)O),HF,Ne ("2nd period")):}`
(2) In a group valency is either equal to the group number (i.e., upto `4^(th)` group) or is equal to (8-group number (i.e., from `5^(th)` group onwards).
Significance: Valency of an element is useful in writing the formulae of compounds.