Boron can be obtained by various methods but not by :

Options

A. thermal decomposition of `B_(2)H_(6)`
B. pyrolysis of `BI_(3)` (van Arkel)
C. reducing `BCl_(3) " with " H_(2)`
D. electrolysis of fused `BCl_(3)`

Answer

To determine which method cannot be used to obtain boron, we will analyze each option provided in the question step by step.

### Step 1: Analyze the first method - Thermal Decomposition of B2H6
- **Process**: B2H6 (diborane) is heated at high temperatures (300-500 °C).
- **Reaction**:
\[ \text{B}_2\text{H}_6 \rightarrow 2 \text{B} + 3 \text{H}_2 \]
- **Conclusion**: This method produces boron. Therefore, this option is not the answer.

### Step 2: Analyze the second method - Pyrolysis of Bi3
- **Process**: Bi3 (boron triiodide) is heated at high temperatures (450 °C).
- **Reaction**:
\[ 2 \text{Bi}_3 \rightarrow 2 \text{B} + 3 \text{I}_2 \]
- **Conclusion**: This method also produces boron. Therefore, this option is not the answer.

### Step 3: Analyze the third method - Reducing BCl3 with H2
- **Process**: BCl3 (boron trichloride) is reduced with hydrogen at high temperatures (800-1000 °C).
- **Reaction**:
\[ 2 \text{BCl}_3 + 3 \text{H}_2 \rightarrow 2 \text{B} + 6 \text{HCl} \]
- **Conclusion**: This method produces boron as well. Therefore, this option is not the answer.

### Step 4: Analyze the fourth method - Electrolysis of fused BCl3
- **Process**: Electrolysis is performed on fused BCl3.
- **Conclusion**: Electrolysis is generally used for extracting highly electropositive metals (like Na, K, Mg, Ca, Al) and not for boron. Therefore, this method does not yield boron.

### Final Answer
Based on the analysis, the method that cannot be used to obtain boron is:
**Electrolysis of fused BCl3 (Option D)**.

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