`Li_(2)SO_(4)` is not isomorphous with sodium sulphate:

Options

A. Due to small size of lithium
B. Due to high coordination number of lithium
C. Due to high ionisation energy of lithium
D. None of the above.

Answer

To determine why lithium sulfate (Li₂SO₄) is not isomorphous with sodium sulfate (Na₂SO₄), we need to analyze the structures and characteristics of both compounds.

### Step-by-Step Solution:

1. **Definition of Isomorphism**:
- Isomorphism refers to the phenomenon where two different compounds have the same crystal structure. For two compounds to be isomorphous, they must have similar ionic sizes and coordination numbers.

2. **Chemical Composition**:
- Lithium sulfate has the formula Li₂SO₄, while sodium sulfate has the formula Na₂SO₄. The anion (sulfate, SO₄²⁻) is the same in both compounds, but the cations (Li⁺ and Na⁺) are different.

3. **Comparison of Cation Sizes**:
- Lithium ion (Li⁺) is the smallest cation among the alkali metals, whereas sodium ion (Na⁺) is larger than lithium. The ionic radius of Li⁺ is approximately 0.76 Å, while that of Na⁺ is about 1.02 Å.

4. **Effect of Cation Size on Crystal Structure**:
- The smaller size of Li⁺ leads to a different crystal packing compared to Na⁺. This difference in size affects the coordination number and the overall geometry of the crystal lattice.

5. **Conclusion**:
- Due to the significantly smaller size of the lithium ion compared to the sodium ion, lithium sulfate does not adopt the same crystal structure as sodium sulfate. Therefore, Li₂SO₄ is not isomorphous with Na₂SO₄.

### Final Answer:
Lithium sulfate (Li₂SO₄) is not isomorphous with sodium sulfate (Na₂SO₄) due to the small size of the lithium ion, which leads to a different crystal structure.

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