The pronounced change from non-metallic behaviour and also increase in the basicity of oxides from nitrogen to bismuth in group `15` is principally due to increasing size of the atoms. The ionisation potential of nitrogen is very high on account of its small size. However, ionisation potential decreases regularly on descending the group.
Which trihalide is most ionic among the following ?

Options

A. `NCl_3`
B. `PCl_3`
C. `BiF_3`
D. `SbF_3`

Explanation

To determine which trihalide is the most ionic among the group 15 elements, we need to analyze the electronegativity differences between the group 15 elements (nitrogen, phosphorus, arsenic, antimony, and bismuth) and the halogens (fluorine, chlorine, bromine, and iodine).

### Step-by-Step Solution:

1. **Understanding Ionic Character**:
The ionic character of a compound is influenced by the difference in electronegativity between the two elements involved in the bond. A larger difference in electronegativity typically results in a more ionic bond.

2. **Identify the Trihalides**:
The trihalides of interest from group 15 are:
- Nitrogen trifluoride (NF₃)
- Phosphorus trifluoride (PF₃)
- Arsenic trifluoride (AsF₃)
- Antimony trifluoride (SbF₃)
- Bismuth trifluoride (BiF₃)

3. **Electronegativity Values**:
The electronegativity values (Pauling scale) are approximately:
- Nitrogen (N) = 3.04
- Phosphorus (P) = 2.19
- Arsenic (As) = 2.18
- Antimony (Sb) = 1.89
- Bismuth (Bi) = 1.93
- Fluorine (F) = 3.98

4. **Calculate Electronegativity Differences**:
- For NF₃: |3.04 - 3.98| = 0.94
- For PF₃: |2.19 - 3.98| = 1.79
- For AsF₃: |2.18 - 3.98| = 1.80
- For SbF₃: |1.89 - 3.98| = 2.09
- For BiF₃: |1.93 - 3.98| = 2.05

5. **Determine the Most Ionic Compound**:
The compound with the highest electronegativity difference will exhibit the most ionic character. From the calculations:
- The difference for SbF₃ is the largest at 2.09, followed closely by BiF₃ at 2.05.

6. **Conclusion**:
Based on the electronegativity differences, **SbF₃ (Antimony trifluoride)** is the most ionic among the trihalides of group 15.

### Final Answer:
The most ionic trihalide among the given options is **SbF₃**.

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