Elements in the same group show the same valency

i.                     Valency is the number of electrons given, taken or shared by an atom to complete its outermost shell.

ii.                    All the elements in the same group have the same number of electrons in their outermost shells.

iii.                  Thus, all these elements take, give or share the same number of electrons.

Hence, elements in the same group show the same valency.