Which one of the alkali metals forms only the normal oxide, `M_(2)O`, on heating in air ?

Options

A. Li
B. Na
C. Rb
D. K

Answer

To determine which alkali metal forms only the normal oxide \( M_2O \) when heated in air, we need to analyze the properties of each alkali metal provided in the options: lithium, sodium, rubidium, and potassium.

### Step 1: Understanding Alkali Metals and Their Oxides
Alkali metals are known to react with oxygen to form oxides. The general formula for the oxides formed by alkali metals can vary:
- Normal oxides: \( M_2O \)
- Peroxides: \( M_2O_2 \)
- Superoxides: \( MO_2 \)

### Step 2: Analyzing Each Alkali Metal
1. **Lithium (Li)**:
- Lithium is the smallest alkali metal and has a high charge density due to its small size.
- It forms only the normal oxide \( Li_2O \) when heated in air.

2. **Sodium (Na)**:
- Sodium can form both normal oxide \( Na_2O \) and peroxide \( Na_2O_2 \) when heated in air, especially in the presence of excess oxygen.

3. **Potassium (K)**:
- Potassium also forms both normal oxide \( K_2O \) and superoxide \( KO_2 \) when heated in air.

4. **Rubidium (Rb)**:
- Rubidium behaves similarly to potassium and can form both normal oxide \( Rb_2O \) and superoxide \( RbO_2 \).

### Step 3: Conclusion
From the analysis above, we conclude that lithium is the only alkali metal that forms only the normal oxide \( Li_2O \) when heated in air.

### Final Answer
**Lithium (Li)** forms only the normal oxide \( M_2O \) when heated in air.

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