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

Explanation

To solve the question "Which one of the alkali metals forms only the normal oxide, M₂O, on heating in air?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Alkali Metals and Oxides**:
- Alkali metals are known to react with oxygen in the air to form oxides. The types of oxides formed can vary depending on the alkali metal involved.

2. **Types of Oxides**:
- There are three main types of oxides that alkali metals can form:
- **Normal Oxides (M₂O)**: where the oxidation state of oxygen is -2.
- **Peroxides (M₂O₂)**: where the oxidation state of oxygen is -1.
- **Superoxides (MO₂)**: where the oxidation state of oxygen is -1/2.

3. **Analyzing Each Alkali Metal**:
- **Lithium (Li)**:
- When lithium reacts with oxygen, it forms lithium oxide (Li₂O), which is a normal oxide.
- **Sodium (Na)**:
- Sodium reacts with oxygen to form sodium peroxide (Na₂O₂), not a normal oxide.
- **Potassium (K)**, **Rubidium (Rb)**, and **Cesium (Cs)**:
- These metals react with oxygen to form superoxides (e.g., KO₂), which are not normal oxides.

4. **Conclusion**:
- Among the alkali metals, only lithium forms the normal oxide (Li₂O) when heated in air. Therefore, the answer to the question is **Lithium**.

### Final Answer:
Lithium (Li) is the alkali metal that forms only the normal oxide, M₂O, on heating in air.

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