Question

What is the periodic trend in the variation of valency while going down a group? Explain your answer with reference to the group 1, group 2 and group 18.

Answer 1

Group 1	Group 2				Group 18
H 1					He 2
Li 2, 1	Be 2, 2				Ne 2, 8
Na 2, 8, 1	Mg 2, 8, 2				Ar 2, 8, 8
K 2, 8, 8, 1	Ca 2, 8, 8, 2

The number of valence electrons in an element's outermost shell determines the element's valency.

The number of valence electrons is the same for every element in a group. As a result, members belonging to the same group ought to have the same valency. For instance, group 1 elements (H, Li, Na, K, etc.) have a valency of one and only one valence electron. In a similar vein, group 2 elements (Be, Mg, and Ca) have two valence electrons.

With the exception of helium, which has two electrons, the elements in group 18 1 (Ne, Ar) have eight electrons. The octet has zero valency because it is complete.