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

Explanation

Valency: Valency is the combining capacity. It is the number of atoms of hydrogen or chlorine or any monovalent atom, with which one atom of the element combines.
Generally all the elements in a group shw the same valency. In case of s-block of elements, valency = group number.
In case of p-block of elements,
Valency = group number, or (8-group no.)
Variation of valency w.r.t hydrogen, in a period: In representative elements, the valency increases from 1 to 4 and then decreases to 1 from left to right.
Ex: Valency of 3rd period of elements:
`{:("name of element",Na,Mg,Al,Si,P,S,Cl),("Valency w.r.t.H",1(NaH),2(MgH_(2)),3(AlH_(3)),4(SiH_4),5(PH_3),6(H_2S),7(HCl)):}`.