2022 H2 Chem P1 Q2 — Chemical bonding Solution

The answer is C.

Br has one more electron shell than Cl, so its outer shell electrons are further from the nucleus and more shielded from the nuclear charge. The shared pair of electrons in the Br-Cl bond is therefore attracted more strongly by the Cl nucleus than by the Br nucleus. Electron density shifts towards Cl, giving \(\delta-\) on Cl and \(\delta+\) on Br, so the bond is polar.

• A is wrong: Cl is more electronegative than Br, not the other way round.
• B is wrong: the statement is true (both are Group 17 elements), but a shared configuration does not explain why the two atoms attract the bonding pair differently.
• D is wrong: the repulsion between outer shell electrons is smaller in Br than in Cl, because Br is the larger atom and its outer shell occupies a larger volume, so the electrons are more spread out. In any case, this repulsion does not explain the polarity of the bond.