(a) An element X has mass number 40 and contains 21 neutrons in its atom. To which group of the periodic table does it belong?
(b) The element X forms a compound $$X_ 2Y$$. Suggest an element that Y might be and give reasons for your choice.
(a) An element X having mass number 40 and contains 21 neutrons in its atom belongs to group 1 (2,8,8,1).
(b) Y might be Oxygen since the valency of oxygen is 2 and X is monovalent so the compound form will be $$X_2Y.$$