Question

The correct increasing order for bond angles among \( \text{BF}_3, \, \text{PF}_3, \, \text{and} \, \text{CF}_3 \) is:

• A. \( \text{PF}_3 \, < \, \text{BF}_3 \, < \, \text{CF}_3 \)
• B. \( \text{BF}_3 \, < \, \text{PF}_3 \, < \, \text{CF}_3 \)
• C. \( \text{CF}_3 \, < \, \text{PF}_3 \, < \, \text{BF}_3 \)
• D. \( \text{BF}_3 \, = \, \text{PF}_3 \, < \, \text{CF}_3 \)

Answer 1

The Correct Option is C

To determine the increasing order of bond angles among \( \text{BF}_3, \, \text{PF}_3, \text{and} \, \text{CF}_3 \), we need to understand the molecular geometry and electronic effects influencing these compounds:

1. \(\text{BF}_3\) (Boron Trifluoride) has a trigonal planar structure due to the \( \text{sp}^2 \) hybridization of the boron atom. This leads to bond angles of 120°.
2. \(\text{PF}_3\) (Phosphorus Trifluoride) exhibits a trigonal pyramidal shape, with phosphorus undergoing \( \text{sp}^3 \) hybridization. The presence of a lone pair causes the bond angle to decrease from the ideal tetrahedral angle (109.5°) to approximately 97°.
3. \(\text{CF}_3\) (represented as part of a radical such as \( \text{CF}_3^+ \) in chemistry) typically has a trigonal planar arrangement similar to \( \text{BF}_3 \), but due to the presence of differing electronic factors (such as resonance or inductive effects involving fluorine), the bond angle can be slightly different from 120°.

Based on these observations, we conclude:

• The bond angle in \( \text{BF}_3 \) is the largest because there are no lone pair repulsions.
• The bond angle in \( \text{PF}_3 \) is smaller due to the lone pair repulsion.
• The bond angle in \( \text{CF}_3 \) is influenced by both the resonance and inductive effects, which make it smaller than the ideal angles in \( \text{BF}_3 \) and \( \text{PF}_3 \) structures.

Thus, the increasing order of bond angles is:

\(\text{CF}_3 \, < \, \text{PF}_3 \, < \, \text{BF}_3\)

Answer 2

BF$_3$: Planar structure with 120$^\circ$ bond angles ($sp^2$ hybridization).
PF$_3$: Tetrahedral geometry distorted by lone pair on phosphorus, bond angle $<$ 109.5$^\circ$.
CF$_3$: Tetrahedral geometry with strong electron-withdrawing fluorine atoms, bond angle $\sim$ 104$^\circ$.
The order of bond angles is CF$_3$ $<$ PF$_3$ $<$ BF$_3$.