Concept:When the central atom is highly electronegative, bond pairs are pulled closer to the central nucleus. This causes strong bond pair-bond pair (BP-BP) repulsion, which forces the bond angle to widen.
Formula:$$ \text{Bond Angle} \propto \text{Electronegativity of Central Atom} $$
Solution:- Oxygen (EN = 3.5) is much more electronegative than Sulfur (EN = 2.5).
- In \( \text{H}_2\text{O} \), the bonding electrons are pulled very close to the Oxygen nucleus. Because they are confined in a tight space, they repel each other strongly, keeping the angle relatively wide (104.5°).
- In \( \text{H}_2\text{S} \), the bonding electrons are further from the Sulfur nucleus. The BP-BP repulsion is weaker, allowing the lone pairs to crush the bond angle down to about 92°.
Why other options are incorrect:- Option A: Size plays a role, but electronegativity is the direct electronic driver of the bond pair positioning.
- Option C: Both Oxygen and Sulfur contain two lone pairs, so this is not the distinguishing factor.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.