The elements of a group show the following important similar characteristics:
(i) Electronic configuration- All elements in a particular group have similar outermost electronic configuration e.g., all elements of group 1, i.e., alkali metals have ns1 configuration in their valency shell. Similarly group 2 elements (alkaline Earths) have ns2 outermost electronic configuration and halogens (group 17) have ns2 np5 configuration (where n is the outermost shell).
(ii) Valency-- The valency of an element depends upon the number of electrons in the outermost shell. So elements of a group show the same valency, e.g., elements of group 1 show +1 valency and group 2 show +2 valency i.e., NaCl, MgCl2 etc.
(iii) Chemical properties-The chemical properties of the elements are related to the number of electrons in the outermost shell of their atoms. Hence, all elements belonging to the same group show similar chemical properties. But the degree of reactivity varies gradually from top to bottom in a group. For example, in group 1, all the elements are highly reactive metals but the degree of reactivity increases from Li to Cs. Similarly elements of group 17, i.e., halogens: F, CI, Br, I are all non-metals and their reactivity goes on decreasing from top to bottom.
