Which of the following elements combines directly with nitrogen to form its nitride ?

Options

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

Explanation

To determine which of the given alkali metals combines directly with nitrogen to form its nitride, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Elements**: The elements provided in the question are lithium (Li), sodium (Na), potassium (K), and rubidium (Rb).

2. **Understand Nitride Formation**: A nitride is a compound that contains nitrogen in the form of the nitride ion (N³⁻). For an element to form a nitride, it must be able to react directly with nitrogen.

3. **Analyze Lithium**: Lithium is known to have unique properties compared to other alkali metals. It is the only alkali metal that can directly combine with nitrogen to form lithium nitride (Li₃N).

4. **Consider Other Alkali Metals**: Sodium, potassium, and rubidium do not form stable nitrides. The reason is that their cations (Na⁺, K⁺, Rb⁺) are larger than Li⁺, which results in lower lattice enthalpy when combined with the nitride ion (N³⁻). The larger size of these cations leads to instability in the nitride compounds.

5. **Conclusion**: Based on the analysis, lithium is the only alkali metal that can directly combine with nitrogen to form a stable nitride. Therefore, the answer is lithium (Li).

### Final Answer:
**Lithium (Li)** is the element that combines directly with nitrogen to form its nitride.

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