Concept: Bond Angle Comparison in Group 15 Trihalides

To determine which molecule has the smallest bond angle among PCl₃, NCl₃, SbCl₃, and AsCl₃, we analyze the effect of central atom size on bond angle in pyramidal molecules. All these molecules have a trigonal pyramidal geometry with one lone pair on the central atom.

Step-by-Step Explanation:

Step 1: Recall the general trend. For molecules with the same geometry and number of lone pairs (AX₃E type), bond angle decreases as the size of the central atom increases. Larger atoms have more diffuse electron clouds, leading to weaker bond pair-bond pair repulsion and greater lone pair-bond pair repulsion, which compresses the bond angle.

Step 2: Identify the central atoms and their positions in Group 15: Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb). Atomic size increases down the group: N < P < As < Sb.

Step 3: Apply the trend. Since Sb is the largest central atom, SbCl₃ should have the smallest bond angle due to maximum lone pair-bond pair repulsion and minimal bond pair-bond pair repulsion.

Step 4: Verify with known data. Experimental bond angles are approximately: NCl₃ ≈ 107°, PCl₃ ≈ 100°, AsCl₃ ≈ 98.5°, SbCl₃ ≈ 97°. This confirms SbCl₃ has the smallest angle.

Final Answer: SbCl₃ has the smallest bond angle.

Related Topics:

VSEPR Theory (Valence Shell Electron Pair Repulsion)
Molecular Geometry and Bond Angles
Periodic Trends: Atomic Size and Electronegativity
Lone Pair-Bond Pair Repulsion vs. Bond Pair-Bond Pair Repulsion

Key Formulae and Theory:

No specific formula, but important concepts:

VSEPR Theory: Electron pairs around a central atom arrange themselves to minimize repulsion. Lone pairs occupy more space than bond pairs.

Bond Angle Order: For Group 15 trihalides (EX₃), bond angle decreases down the group: NX₃ > PX₃ > AsX₃ > SbX₃.

Reason: As atomic size increases, bond pairs are farther from the nucleus, reducing bond pair-bond pair repulsion. Lone pair-bond pair repulsion becomes dominant, compressing the bond angle.