Based on the group valency of elements write the molecular formula of the following compounds giving justification for each:
(1) Oxide of first group elements.
(2) halides of the elements of group thirteen.
(3) compound formed when an element, A of group 2 combines with an element, B of group seventeen.

Hint: Valency is the measure of the combining capacity of atoms or molecules. Therefore, it is the capacity of an atom of a single element to react and combine with particular numbers of atoms of another element.

Complete step by step solution:
Valency in the periodic table across a period first increases and then decreases. There is no change going down a group. For this reason, ionization energy is lower for elements lower down in a group, and polarizability of species is higher for elements lower down in a group.
We need to give justification for:
(A) oxides of first group elements:
The first group elements have a common valency of 1 and the valency of oxygen is 2 and to satisfy the combining capacity of oxygen, 2 elements of the first group are required. Thus, the oxide of the first group elements have the common formula of ${{M}_ {2}} O$.
For example: $N{{a}_ {2}} O, {{K}_ {2}} O$ etc.
(B) halides of the elements of group thirteen:
The valency of group 13 elements is 1 and the valency of oxygen is 2 and to satisfy the combining capacity of group 13 elements, 3 of halogens are required. Thus, the molecular formula will be of form $M{{X}_ {3}} $.
For example: $AlC{{l}_ {3}}, B{{F}_ {3}} $ etc.
(C) compound formed when an element, A of group 2 combines with an element, B of group seventeen.
The valency of group 2 elements is 2 and that of group 17 is 1. Thus, two elements of group 17 are required to combine with group 2 elements.
For example: $MgC{{l}_ {2}}, CaC{{l}_ {2}} $ etc.

Note: A molecular formula consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule. The empirical formula represents the simplest whole-integer ratio of atoms in a compound.