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Among the following, the one having smallest bond angle is

  • A.\(\mathrm{PH_{3}}\)
  • B.\(\mathrm{PF_{3}}\)
  • C.\(\mathrm{NF_{3}}\)
  • D.\(\mathrm{NH_{3}}\)

Answer: A

The smallest bond angle is in PH3, about 93°.

The measured angles are roughly NH3 107°, NF3 102°, PF3 98° and PH3 93°.

Going down the group from N to P, the central atom becomes larger and less electronegative. Its bonding pairs sit farther from the nucleus and repel each other less, and the bonds use orbitals of nearly pure p character, so the angle closes toward 90°. That puts both phosphorus compounds below both nitrogen compounds.

Within the nitrogen pair, fluorine pulls the bonding pairs away from N, reducing their mutual repulsion, so NF3 has a smaller angle than NH3. Between the phosphorus compounds, PH3 is the smaller; PF3 is opened slightly by the bulk of the fluorine atoms and by their lone pairs interacting with phosphorus.

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