(a) (i) Atomic radius decreases.
(ii) Atomic radius increases.
(b) Atomic number of element X = 12
electronic configuration = 2, 8, 2
K L M
Atomic number of element Y = 16
Electronic configuration = 2, 8, 6
K L M
The period number of an element is equal to the number of electron shells in its atom. These two elements have 3 electron shells, therefore they belong to 3rd period.
They will form ionic bond because X is a metal and Y is a non-metal. X loses two electrons which will be gained by Y.