Which of the following alkali metal doesn't from ethynide on
reaction with ethyne ?

Options

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

Answer

To solve the question of which alkali metal does not form ethynide upon reaction with ethyne, we can follow these steps:

### Step-by-Step Solution:

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

2. **Understand the Reaction with Ethyne**: Ethyne, also known as acetylene, has the formula C₂H₂. Alkali metals typically react with ethyne to form ethynides (or acetylides), which are compounds containing the acetylide ion (C≡C⁻).

3. **Analyze the Reactivity of Alkali Metals with Ethyne**:
- Sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) can react with ethyne to form their respective ethynides.
- Lithium (Li), however, behaves differently due to its unique properties.

4. **Explain Lithium's Anomalous Behavior**: Lithium has a much smaller atomic and ionic size compared to the other alkali metals. This small size leads to different chemical behavior, causing lithium not to react with ethyne in the same way as the other alkali metals.

5. **Conclusion**: Since lithium does not form ethynide upon reaction with ethyne while the other alkali metals do, we conclude that the answer to the question is lithium (Li).

### Final Answer:
**Lithium does not form ethynide on reaction with ethyne.**

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