Question

what is the periodic trend in the variation of valency while going down a group explain your answer with reference to group 1 group 2 and group 18

Answer 1

1.The valency of an element is determined by the number of valence electron in the outermost shell of an element.
2.All the elements in a group have the same number of valence elements .Therefore ,elements in the same group should have the same valency for example the elements of group 1(H,Li,Na,K etc)have contain only one Valence Electrons the valency of elements of group 1 is one .Similarly for group 2(Be,Mg,Ca) contain two Valence Electrons the valency of elements elements of groups 2 is two.
3.The elements of group 18 1 (NE, At) contain 8 electrons (exception Helium contain 2 electrons). Since the oxide is completed their valency is zero.

Answer 2

Valence numbers do not vary as you go down a group, since the periodic table is arranged by atomic number and chemical properties. All members of a group have the same number of valence electrons. Group I has 1, Group II has 2, Group III - XII has 2, since only s or p electrons are counted as valence electrons, Group XIII has 3, Group XIV has 4, Group XV has 5, Group XVI has 6, Group XVII has 7 and Group XVIII (noble gases) have 8.