Which of the following alkali metal does not form alum ?

Options

A. Li
B. Na
C. K
D. All of these

Answer

To determine which alkali metal does not form alum, we need to understand what alums are and the characteristics of the alkali metals.

### Step-by-Step Solution:

1. **Definition of Alum**: Alums are double sulfate salts that typically consist of two different metal ions: one monovalent (monopositive) metal ion and one trivalent (tripositive) metal ion. The general formula for alums can be represented as M1M2(SO4)2·12H2O, where M1 is the monovalent cation and M2 is the trivalent cation.

2. **Common Trivalent Cations**: The trivalent cations commonly used in alums are aluminum (Al³⁺) and chromium (Cr³⁺).

3. **Monovalent Cations in Alums**: The monovalent cations that can be used to form alums include potassium (K⁺), sodium (Na⁺), and ammonium (NH4⁺).

4. **Properties of Alkali Metals**: Among the alkali metals (Li, Na, K, Rb, Cs), lithium (Li) is unique due to its small ionic size compared to the other alkali metals.

5. **Crystal Structure Consideration**: Alums crystallize in an octahedral structure. The size of the monovalent cation is crucial for fitting into this structure. Lithium ion (Li⁺) is too small to be accommodated in the octahedral crystal structure of alums.

6. **Conclusion**: Since lithium cannot fit into the crystal structure necessary for forming alums, it does not form alum. Therefore, the alkali metal that does not form alum is **lithium (Li)**.

### Final Answer:
Lithium does not form alum.