Based on the group valency of elements write the molecular formula of the following compounds giving justification for each :
Oxide of first group elements.

Explanation

To determine the molecular formula of the oxides of first group elements (alkali metals), we need to consider the group valency of these elements. The first group elements include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

### Step-by-Step Solution:

1. **Identify the Group Valency:**
- The first group elements have a valency of +1. This means that each alkali metal can lose one electron to form a cation with a charge of +1.

2. **Identify the Oxide Ion:**
- The oxide ion (O²⁻) has a charge of -2. This means that it can accept two electrons to achieve a stable electron configuration.

3. **Combining the Cations and Anions:**
- To form a neutral compound, the total positive charge must balance the total negative charge.
- Since the alkali metals have a +1 charge and the oxide ion has a -2 charge, we need two alkali metal ions to balance one oxide ion.

4. **Writing the Molecular Formula:**
- For each alkali metal (Li, Na, K, Rb, Cs), the molecular formula for the oxide will be:
- Lithium oxide: Li₂O
- Sodium oxide: Na₂O
- Potassium oxide: K₂O
- Rubidium oxide: Rb₂O
- Cesium oxide: Cs₂O

5. **Justification:**
- Each formula indicates that two atoms of the alkali metal combine with one atom of oxygen to form a stable compound. This is consistent with the charge balance required for neutrality in the compound.

### Summary of Molecular Formulas:
- Lithium oxide: Li₂O
- Sodium oxide: Na₂O
- Potassium oxide: K₂O
- Rubidium oxide: Rb₂O
- Cesium oxide: Cs₂O