Based on the group valency of elements write the molecular formula of the following compounds giving justification for each :
(i) Oxide of first group elements.
(ii) Halide of the elements of group thirteen, and
(iii) Compounds formed when an element, `A` of group 2 combines with an element, `B` of group 17.

Explanation

(i) Let the first group elements be 'X'. Its valency is 1. So, the molecular formula of its
oxide is `X_(2)O.`

(II) Let us take halide of Chlorine with valency 1 and Group 13 element Aluminium (Al)
with valency 3. Its molecular formula is `AlCl_(3).`

Thus, halides of group 13 elements have a common molecular formula `MX_(3),` where M
is metal and X is the halogen element.
(iii) Molecular formula of compound is `AB_(2).`