Concept: Bond angles are influenced by the hybridization of the central atom, the number of lone pairs on the central atom, and the electronegativity of the surrounding atoms. Lone pairs exert greater repulsion than bond pairs, leading to a decrease in bond angles. Steric hindrance can also play a role.

Why (A) is incorrect:

Let's analyze the bond angles for each species:

• NH${}_3$ (Ammonia): The central nitrogen atom is sp${}^3$ hybridized. It has three bond pairs and one lone pair. Due to the lone pair-bond pair repulsion, the bond angle is distorted from the ideal tetrahedral angle (109.5°) to approximately 107°.
• NH${}_2^{-}$ (Amide ion): The central nitrogen atom is sp${}^3$ hybridized. It has two bond pairs and two lone pairs. The presence of two lone pairs leads to even greater lone pair-lone pair and lone pair-bond pair repulsions, further reducing the bond angle. The bond angle in NH${}_2^{-}$ is approximately 104.5°.

Therefore, the correct order of bond angle should be NH${}_3$ > NH${}_2^{-}$ (107° > 104.5°). The given order NH${}_3$ < NH${}_2^{-}$ is incorrect.

Option Analysis:

• A) NH${}_3$ < NH${}_2^{-}$: This statement is incorrect. As explained above, NH${}_3$ has one lone pair and a bond angle of ~107°, while NH${}_2^{-}$ has two lone pairs and a bond angle of ~104.5°. Thus, NH${}_3$ > NH${}_2^{-}$.
• B) CCl${}_4$ = CH${}_4$: This statement is correct. Both CCl${}_4$ and CH${}_4$ have a central carbon atom that is sp${}^3$ hybridized with four bond pairs and no lone pairs. Both are perfectly tetrahedral with bond angles of 109.5°.
• C) BF${}_3$ = AlCl${}_3$: This statement is correct. Both BF${}_3$ and AlCl${}_3$ have a central atom (B or Al) that is sp${}^2$ hybridized with three bond pairs and no lone pairs. Both are trigonal planar with bond angles of 120°.
• D) NH${}_4^{+}$ > NH${}_3$: This statement is correct. NH${}_4^{+}$ has a central nitrogen atom that is sp${}^3$ hybridized with four bond pairs and no lone pairs, resulting in an ideal tetrahedral angle of 109.5°. NH${}_3$ has one lone pair, which reduces its bond angle to ~107°. Thus, NH${}_4^{+}$ > NH${}_3$.

Correct Answer: (A)