Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2024 / Paper 1 / 6 / e

HgS and ${\mathrm{C}}_3{\mathrm{N}}_4$ are semiconductors; $\mathrm{Hg}(\mathrm{SCN}{)}_2$ is a wider-gap, insulating material under the stated conditions. None is a metallic conductor. Their finite electronvolt-scale gaps and very low measured conductivities exclude a partially filled, highly conducting metallic band picture.

The insulator/semiconductor boundary is conventional rather than a sharp universal gap cutoff. Calling the $3.2\mathrm{eV}$ material a wide-bandgap semiconductor is also physically reasonable; the table alone does not establish a unique naming boundary. Conductivity is $\sigma =e(n{\mu }_n+p{\mu }_p)$, so its ordering need not follow gap size alone: carrier density, defects, trapping and mobility also matter.