Which of the following alkali metal does not form alum ?

Options

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

Explanation

To determine which alkali metal does not form alum, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Alum**:
- Alums are double salts with the general formula \( AM(SO_4)_2 \cdot 12H_2O \), where \( A \) is a monovalent cation (like potassium or ammonium) and \( M \) is a trivalent cation (like aluminum or chromium).

2. **Identify the Alkali Metals**:
- The alkali metals in question are lithium (Li), sodium (Na), potassium (K), and rubidium (Rb).

3. **Analyze the Size of Alkali Metals**:
- As we move down the group of alkali metals, the size of the cations increases. Lithium is the smallest among them, followed by sodium, potassium, and rubidium.

4. **Relate Size to Alum Formation**:
- The structure of alums can only accommodate cations of a specific size. The small size of lithium makes it difficult for it to fit into the crystalline structure of alum.

5. **Conclusion**:
- Since lithium cannot be accommodated in the alum structure due to its small size, it does not form alum. Therefore, the answer to the question is lithium.

### Final Answer:
- **Lithium (Li)** does not form alum.