(a) The number of valence electrons in element X is 2 because this element belongs to group 2.
(b) The valency of the group 2 element X is 2.
(c) Valence electrons of group 15:
Group number = Valence electrons + 10
15 = Valence electrons + 10
Valence electrons = 15 - 10 = 5
(d) Valency of the group 15 element Y is 3, that is, (8 - 5).