S2.1 The ionic model

Ionic compounds have high melting points due to strong electrostatic forces in the lattice. They are non-conductors in the solid state (ions fixed in position) but conductors when molten or dissolved (ions free to move).

The correct option states: melting point ~995°C (high — consistent with a strong ionic lattice), no conduction as a solid (ions fixed), but conduction in aqueous solution (ions dissociate and become mobile).

All three characteristics — high melting point, non-conductor as solid, conductor in solution — match the ionic model perfectly. This is the correct answer.

We need the option that best fits the properties of an ionic compound: high melting point, non-conductor as solid, conductor when molten or dissolved.

"Melts at 78°C and does not conduct when melted" — a melting point of 78°C is far too low for an ionic compound, and ionic compounds do conduct when molten. This describes a simple molecular compound.

"Conducts in the solid state and has a metallic lustre" — ionic solids do not conduct in the solid state. Conduction in the solid state combined with metallic lustre is characteristic of a metal.

"A gas at room temperature and dissolves in non-polar solvents" — ionic compounds are solids at room temperature and dissolve in polar solvents like water, not non-polar ones.

Option C — high melting point, no solid-state conductivity, and conductivity in aqueous solution — is entirely consistent with the ionic model.

Sulfur (S) is in Group 16 and has 16 protons and, as a neutral atom, 16 electrons. The ${\text{S}}^{2-}$ ion has gained 2 electrons, giving it 18 electrons total.

Filling the subshells in order: $1s^{2}2s^{2}2p^{6}3s^{2}3p^6$. This accounts for $2+2+6+2+6=18$ electrons.

The configuration $1s^{2}2s^{2}2p^{6}3s^{2}3p^6$ is identical to that of argon (Ar), which has 18 electrons. ${\text{S}}^{2-}$ is isoelectronic with argon.

The correct configuration is $1s^{2}2s^{2}2p^{6}3s^{2}3p^6$, matching a full third shell and the argon noble gas configuration.

We need the electron configuration of ${\text{S}}^{2-}$, which has 16 + 2 = 18 electrons.

$1s^{2}2s^{2}2p^{6}3s^{2}3p^4$ has $2+2+6+2+4=16$ electrons — this is the configuration of neutral sulfur, not the ${\text{S}}^{2-}$ ion.

$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^2$ has 20 electrons and incorrectly places electrons in the 3d subshell before the 4s. This does not match any common ion of sulfur.

$1s^{2}2s^{2}2p^{6}3s^{2}3p^5$ has 17 electrons — this would represent ${\text{S}}^{-}$ (gaining one electron), not ${\text{S}}^{2-}$.

Option C, $1s^{2}2s^{2}2p^{6}3s^{2}3p^6$, has exactly 18 electrons and represents the argon configuration achieved by ${\text{S}}^{2-}$.