Q.14 Among the following, the one having smallest bond angle is
(A) PH3
(B) PF3
(C) NF3
(D) NH3
PH3 has the smallest bond angle among PH3, PF3, NF3, and NH3. All four molecules share a trigonal pyramidal shape due to sp³ hybridization and one lone pair on the central atom, but differences in central atom size and electronegativity of ligands lead to varying bond pair-lone pair repulsions.
Bond Angles Overview
Experimental bond angles deviate from the ideal tetrahedral 109.5° due to lone pair repulsions compressing the structure.
| Molecule | Bond Angle | Key Factor |
|---|---|---|
| PH₃ | ~93.5° | Large P atom reduces bond pair density; lone pair in nearly pure s-orbital |
| PF₃ | ~97° | F electronegativity pulls electrons, increasing bp-bp repulsion over lp-bp |
| NF₃ | ~102° | High N electronegativity concentrates bp near N; F reduces lp-bp repulsion |
| NH₃ | ~107° | Smaller N-H bonds increase lp-bp repulsion more than in heavier analogs |
PH₃ shows the smallest angle as phosphorus’s larger size diffuses bonding electrons, minimizing repulsions and yielding p-orbital-like 90° bonds.
Option Analysis
(A) PH₃: Correct—lowest angle (~93.5°) from weak lp-bp repulsion due to long P-H bonds and poor hybridization.
(B) PF₃: ~97°; higher than PH₃ as electronegative F shifts electron density toward bonds, enhancing bp-bp repulsion.
(C) NF₃: ~102°; F pulls electrons from N, reducing lp-bp dominance compared to NH₃.
(D) NH₃: Largest (~107°); compact N-H bonds amplify lp-bp repulsion.
In competitive exams like CSIR NET Life Sciences, questions on smallest bond angle PH3 PF3 NF3 NH3 test VSEPR theory mastery. All exhibit trigonal pyramidal geometry (AX₃E), yet bond angles vary: PH₃ (~93.5°) < PF₃ (~97°) < NF₃ (~102°) < NH₃ (~107°). PH₃ wins as smallest due to phosphorus’s larger size.
VSEPR and Hybridization Factors
sp³ hybridization predicts 109.5°, but lone pair repels bonds. Central atom size matters:
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PH₃/PF₃: Large P (3p orbitals) yields diffuse bonds; lone pair stays s-like (~90° base).
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NF₃/NH₃: Small N (2p) enhances hybridization; tighter bonds.
Ligand electronegativity flips trends:
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H (low χ): lp-bp > bp-bp → smaller angles down group.
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F (high χ): bp-bp > lp-bp → larger angles (NH₃ > NF₃; PH₃ < PF₃).
Exam Tips for CSIR NET
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Memorize: PH₃ (93°), PF₃ (97°), NF₃ (102°), NH₃ (107°).
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Trend: Bond angle decreases down group for hydrides; increases with electronegative ligands.
Answer: (A) PH₃
