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.

Explanation

To solve the question regarding the formulas based on the group valency of elements, we will analyze each part step by step.

### (i) Oxides of 1st Group Elements

1. **Identify the Group**: The 1st group elements are known as alkali metals (e.g., Lithium, Sodium, Potassium).
2. **Determine Valency**: The valency of alkali metals is +1.
3. **Identify the Oxide**: The oxide ion (O) has a valency of -2.
4. **Write the Formula**: Let’s denote the alkali metal as M. The formula can be derived by balancing the charges:
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