U3.2 · Properties of SolidsPractice · 固体的性质

X conducts as a solid and is malleable: metallic (delocalized electrons). Y has a high melting point, is hard and brittle, and conducts only when melted: ionic, since the ions are locked in the lattice until it melts. Z melts low, is soft, and never conducts: molecular, since melting only overcomes intermolecular forces.

Both are covalent network solids. In diamond each C bonds tetrahedrally to 4 other C atoms, so every valence electron is held in a C–C bond and nothing charged can move. In graphite each C bonds to 3 others in a plane, and the fourth valence electron is delocalized over the whole layer, free to move within it, so graphite conducts. Graphite has no ions and isn’t a metal.

A molecular solid is made of molecules held together by intermolecular forces. Melting leaves the molecules intact and only overcomes those relatively weak forces, so melting points are generally low. The molecules are neutral, so even the melt has no mobile ions and doesn’t conduct. Ice and sucrose are solids at room temperature, so “all gases” is wrong.

When NaCl melts, its lattice breaks down and $\ce{Na+}$ and $\ce{Cl-}$ ions can move, so the melt conducts by moving ions. Copper is metallic: the copper cations stay fixed in the lattice, and the current is carried by the delocalized valence electrons.

In a covalent network solid, every atom is joined by covalent bonds into one continuous structure, with no separate molecules. Diamond (C–C) and quartz (Si–O) both qualify. Dry ice and ice are molecular; NaCl and MgO are ionic; graphite is a covalent network, but $\ce{I2}$ is molecular.