Lithium is the only alkali metal to form nitride directly. Explain .

Explanation

### Step-by-Step Solution:

1. **Understanding Alkali Metals**:
- Alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). Among these, lithium is unique in its ability to form nitride directly.

2. **Size of Lithium**:
- Lithium is the smallest alkali metal. The atomic size of alkali metals increases down the group due to the addition of electron shells. This means that lithium has a higher charge density because it has a smaller radius.

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