The tetrahedral bond angle is approximately 109.5°. We need to identify the molecule with a bond angle smaller than this value.

• Concept: The bond angle in a molecule is influenced by the number of lone pairs of electrons on the central atom. Lone pair-lone pair repulsion > lone pair-bond pair repulsion > bond pair-bond pair repulsion. This increased repulsion by lone pairs compresses the bond angles.
• Why (C) is correct:

  In ${\mathrm{NH}}_3$ (ammonia), the central nitrogen atom has three bond pairs (with hydrogen atoms) and one lone pair of electrons. The steric number is 4 (3 bond pairs + 1 lone pair), suggesting a tetrahedral electron geometry. However, the lone pair exerts greater repulsion on the bond pairs than bond pairs exert on each other. This repulsion compresses the H-N-H bond angles from the ideal tetrahedral angle of 109.5° to approximately 107°.

• Option Analysis:
  • A) ${\mathrm{NH}}_4^{+}$: In the ammonium ion, the central nitrogen atom is bonded to four hydrogen atoms and has no lone pairs. It has a perfect tetrahedral geometry, and the H-N-H bond angle is exactly 109.5°.
  • B) ${\mathrm{SiH}}_4$: In silane, the central silicon atom is bonded to four hydrogen atoms and has no lone pairs. It also has a perfect tetrahedral geometry, and the H-Si-H bond angle is exactly 109.5°.
  • D) ${\mathrm{H}}_2\mathrm{O}$: In water, the central oxygen atom has two bond pairs (with hydrogen atoms) and two lone pairs of electrons. The steric number is 4 (2 bond pairs + 2 lone pairs), suggesting a tetrahedral electron geometry. The two lone pairs exert even greater repulsion than one lone pair, compressing the H-O-H bond angle to approximately 104.5°. While this is smaller than the tetrahedral angle, the question asks for a molecule with a bond angle smaller than tetrahedral, and ${\mathrm{NH}}_3$ is a valid choice. If the question asked for the *smallest* bond angle among the options, ${\mathrm{H}}_2\mathrm{O}$ would be the answer. However, ${\mathrm{NH}}_3$ has a bond angle (107°) that is smaller than the ideal tetrahedral angle (109.5°).

Correct Answer: (C)