Based on the group valency of elements state the formula for the following giving justification for each:
i] Oxides of Ist group elements,
ii] Halides of the elements of group 13, and
iii] Compounds formed when an element of group 2 combines with elements of group 16 .

Explanation

i] First-group elements have a valency +1 and oxygen has valency-2. Hence, their oxides have the formula `M_2O`
ii] Group 13 elements have valency +3 and chlorine has valency -1. Hence, their halides have formula `MCI_3`
iii) Elements of group 2 have a valency +2 while elements of group 16 have a valency -2. Hence, their compounds have the formula MX.