Alums are not formed by which alkali metal ?

Options

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

Answer

To determine which alkali metal does not form alums, we need to understand the composition and structure of alums.

### Step-by-Step Solution:

1. **Understanding Alums**: Alums are double sulfate salts with the general formula \( \text{AMS}_2\text{O}_8 \cdot 12\text{H}_2\text{O} \), where \( A \) is a monovalent cation (like potassium, sodium, or cesium) and \( M \) is a trivalent cation (like aluminum or chromium).

2. **Identifying Alkali Metals**: The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). However, francium is highly radioactive and not commonly encountered.

3. **Analyzing the Role of Alkali Metals in Alums**: For a compound to be classified as an alum, it must contain a monovalent cation (A) and a trivalent cation (M). The common alkali metals that can serve as the monovalent cation in alums are sodium (Na), potassium (K), and cesium (Cs).

4. **Examining Lithium**: Lithium (Li) is unique among the alkali metals because it is smaller in size compared to the other alkali metals. Due to its small ionic radius and high charge density, lithium cannot effectively accommodate the crystal structure required for alums.

5. **Conclusion**: Therefore, lithium does not form alums, while sodium, potassium, and cesium can form alums with trivalent cations.

### Final Answer:
**Alums are not formed by lithium.**

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