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) Compound formed when an element, A of group 2 combines with an element B of group seventeen.

The molecular formula of a compound can be determined by the group valency of elements. (i) Oxide of first group elements: The elements of group 1 have a valency of 1. Oxygen has a valency of 2. Therefore, the molecular formula of the oxide of first group elements is M2O, where M is the element of group 1. (ii) Halide of the elements of group thirteen: The elements of group 13 have a valency of 3. Halogens have a valency of 1. Therefore, the molecular formula of the halide of the elements of group thirteen is MX3, where M is the element of group 13 and X is the halogen. (iii) Compound formed when an element, A of group 2 combines with an element B of group seventeen: The elements of group 2 have a valency of 2. The elements of group 17 have a valency of 1. Therefore, the molecular formula of the compound formed when an element, A of group 2 combines with an element B of group seventeen is AB2, where A is the element of group 2 and B is the element of group 17.