Chemical Bonding, Giant vs. Simple Lattice Property Tables & Allotropes (Graphite/Diamond)

Classify each substance by its melting point and electrical conductivity: (1) $\text{X}$ has a high melting point ($801\,^\circ\text{C}$) and conducts only when molten or aqueous (where ions become mobile), which is the signature of a giant ionic lattice (such as $\text{NaCl}$). (2) $\text{Y}$ has a very high melting point ($1710\,^\circ\text{C}$) and never conducts, which is a giant covalent structure (such as $\text{SiO}_2$). (3) $\text{Z}$ sublimes at $-78\,^\circ\text{C}$ and never conducts, as only weak intermolecular forces are overcome, so $\text{Z}$ is a simple molecular structure (~110s).