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.
Elements of Group 16 have 6 valence electrons. The valency of Group 16 elements is: