Solution

$NH_4^+$: 4 bond pairs, no lone pair, regular tetrahedron, bond angle $109^\circ 28'$.
$NH_3$: 3 bp + 1 lp, the lone pair compresses the angle to about $107^\circ$.
$PCl_3$: 3 bp + 1 lp, angle about $100^\circ$ (P is larger and less electronegative).
$SCl_2$: 2 bp + 2 lp, angle about $103^\circ$.
So the bond angle is maximum in $NH_4^+$.

Concept Effect of lone pairs on bond angle

Chemical Bonding and Molecular Structure in past papers

108 questions from this chapter have appeared across 27 exam years.