Question

define valency by taking examples of nitrogen and boron

Answer 1

Valency is the Combination capacity of atom
Nitrogen =3
boron =3

Answer 2

valency is the combining capacity of an element.for example if we take Nitrogen then as we know that atomic no.of nitrogen is 7 so it's electronic configuration will be (2,5)so here we can see that no.of electrons in outermost orbit of nitrogen (valence electrons ) is 5 so when we would subtract it from 8 then what we would get will be valency of nitrogen that is 3