2025 H2 Chem P1 Q3 — Chemical bonding Solution

The answer is D (\(\text{UF}_6\) simple molecular, TlI ionic).
• \(\text{UF}_6\): a boiling point of 330 K is low, so little energy is needed to overcome the forces between the particles. \(\text{UF}_6\) is a simple molecular substance with weak instantaneous dipole-induced dipole (id-id) attractions between its molecules.
• TlI: it conducts electricity when molten, so there are mobile ions in the liquid. Together with its fairly high melting point (715 K), this shows a giant ionic lattice.
• A is wrong: \(\text{UF}_6\) is not ionic (an ionic lattice would have a far higher boiling point than 330 K), and TlI is not giant molecular.
• B is wrong: \(\text{UF}_6\) is not ionic, since its boiling point is too low.
• C is wrong: TlI is not giant molecular (giant covalent). A giant covalent substance has no mobile ions, so it would not conduct electricity when molten.