(1) The valency of an element is determined by the number of valence electron in the outermost shell of an atom of an element.
(2) All the elements in a group have the same number of valence electrons. Therefore, elements in the same group should have the same valency. For example, the elements of group I contain only one valence electron: the valency of elements of group I is one. Similarly for group II, the valency is two.