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

Options

A. (a) Li
B. (b) Na
C. (c) K
D. (d) Rb

Explanation

To determine which alkali metal combines directly with nitrogen to form its nitride, we can follow these steps:

### Step 1: Identify the Alkali Metals
The alkali metals in Group 1 of the periodic table include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).

### Step 2: Understand the Concept of Nitride
A nitride is a compound that contains nitrogen in the form of the nitride ion (N³⁻). This means we are looking for an element that can combine with nitrogen to form a stable nitride.

### Step 3: Analyze Each Alkali Metal
- **Lithium (Li)**: Lithium is known to react with nitrogen to form lithium nitride (Li₃N).
- **Sodium (Na)**: Sodium does not form a stable nitride directly with nitrogen.
- **Potassium (K)**: Potassium also does not form a stable nitride directly with nitrogen.
- **Rubidium (Rb)**: Rubidium does not form a stable nitride directly with nitrogen.
- **Cesium (Cs)**: Cesium does not form a stable nitride directly with nitrogen.

### Step 4: Conclusion
From the analysis, we find that lithium is the only alkali metal that combines directly with nitrogen to form its nitride, specifically lithium nitride (Li₃N).

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

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