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 will analyze the properties of alums and the characteristics of the alkali metals provided in the options.

### Step-by-Step Solution:

1. **Understanding Alums**:
- Alums are double sulfate salts with the general formula \( A_2(SO_4) \cdot M(SO_4) \cdot 12H_2O \), where \( A \) is a monovalent cation (e.g., potassium, sodium, or cesium) and \( M \) is a trivalent cation (e.g., aluminum or chromium).
- Alums are commonly used for water purification and have a specific crystal structure.

2. **Identifying Alkali Metals**:
- The alkali metals in the options are lithium (Li), sodium (Na), potassium (K), and cesium (Cs).
- These metals are known for their reactivity and form various compounds, including salts.

3. **Crystal Structure Consideration**:
- Alums crystallize in an octahedral structure, which requires specific ionic sizes to fit into the crystal lattice.
- The size of the cation is crucial for fitting into this structure.

4. **Comparing Sizes of Alkali Metals**:
- Lithium has a smaller ionic radius compared to sodium, potassium, and cesium.
- The ionic radii increase as we move down the group in the periodic table (Li < Na < K < Cs).

5. **Conclusion**:
- Due to the small size of lithium, it cannot effectively fit into the octahedral crystal structure required for alums.
- Therefore, lithium does not form alums, while sodium, potassium, and cesium can.

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

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