Based on the group valency of elements state the formula for the following giving justification for each: (i) Oxides of 1st group elements. (ii) Halides of the elements of group 13. (iii) Compounds formed when an element of group 2 combines with an element of group 16.

The elements in a group tend to have similar physical and chemical properties because of their similar outer shell electron structure.

1. Valency of first group elements is 1 and that of oxygen is 2. Hence, oxide of first group is X2O. Eg; H2O is the oxide.
2. Group 13 has a valency of 3 and that of halogen is 1. Hence, halide of group 13 is MX3. Eg; BCl3 is the halide.
3. The valency of the Group 2 elements is +2 and that of Group 16 elements is -2. Group 2 loses electrons and group 16 accepts electrons. Hence, compounds formed are MY. Eg; MgO is the compound.