Question

define valency. what conclusion can be drawn about the relativity of an atom from its valency

Answer 1

valency : the combining capacity of an atom is called valency

Answer 2

valency is the no of electrons present in the outermost shell written in electronic configuration is known as valency.
ex: valency of magnesium= 12
electeonic configuration = 2 , 8 , 2
so here the no of electrons in the outer most shell is 2 so the valency is 2

valency tells us the type of atom like is it a metal, non metal etc
and it is the combining capacity of atom it tells us that can it be bonded with atom or compound are not.