2018 H2 Chem P1 Q3 — Chemical bonding Solution

The answer is D (2 only).
A molecule is polar only if it has polar bonds whose dipoles do not cancel. All the bonds here are polar because the electronegativities are different, so the shape decides.
• 1, \(\text{CSe}_2\): C has two double bonds and no lone pair, so the molecule is linear (Se=C=Se, 180°). The two equal C=Se dipoles point in opposite directions and cancel, so it is non-polar.
• 2, \(\text{SeCl}_2\): Se has 2 bond pairs and 2 lone pairs, so the molecule is bent (like water). The two Se-Cl dipoles do not cancel and there is a resultant dipole, so it is polar.
• 3, \(\text{CCl}_4\): C has 4 bond pairs and no lone pair, so the molecule is tetrahedral. The four equal C-Cl dipoles cancel, so it is non-polar.
Only molecule 2 is polar.
• A (1 and 3) is wrong: both \(\text{CSe}_2\) and \(\text{CCl}_4\) are symmetrical and non-polar.
• B (1 only) is wrong: linear \(\text{CSe}_2\) is non-polar, and it leaves out the polar \(\text{SeCl}_2\).
• C (2 and 3) is wrong: tetrahedral \(\text{CCl}_4\) is non-polar.