Assertion : `BO_(3^(-3))` and `SO_(3^(-2))` are not isostructural
Reason : In `SO_(3^(-2-))` sulphur has one lone pair of electron

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that BO₃³⁻ (borate ion) and SO₃²⁻ (sulfite ion) are not isostructural.

- **BO₃³⁻**: The central atom is boron (B). Boron has an atomic number of 5, and its electronic configuration is 1s² 2s² 2p¹. In the formation of the borate ion, boron undergoes sp² hybridization, resulting in a trigonal planar geometry.

- **SO₃²⁻**: The central atom is sulfur (S). Sulfur has an atomic number of 16, and its electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁴. In the formation of the sulfite ion, sulfur undergoes sp³ hybridization, leading to a tetrahedral arrangement. However, due to the presence of a lone pair of electrons, the actual shape of the sulfite ion is trigonal pyramidal.

Thus, since the geometries of BO₃³⁻ (trigonal planar) and SO₃²⁻ (trigonal pyramidal) are different, the assertion is **true**.

### Step 2: Analyze the Reason
The reason states that in SO₃²⁻, sulfur has one lone pair of electrons.

- This statement is also **true**. The presence of a lone pair on sulfur does contribute to the difference in geometry between the two ions. However, the assertion's validity does not depend solely on this reason.

### Step 3: Conclusion
The assertion is true, but the reason is not the only factor that explains why they are not isostructural. Therefore, while both statements are true, the reason does not adequately justify the assertion.

### Final Answer
- Assertion: True
- Reason: True, but not a sufficient explanation.