A-Level How atoms bond and how that governs a substance's behaviour: worked solution

Question

Using ideas of structure and bonding, account for why CaC2 exists as a solid under room conditions whereas C2H2 is a gas.

CaC2 is an ionic compound: it has a giant ionic lattice of Ca2+ ions and C2²- (carbide) ions held together by strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is needed to overcome these strong forces, so it has a high melting point and is a solid at room temperature. C2H2 (ethyne) is a simple molecular substance: the atoms are held by strong covalent bonds WITHIN each molecule, but neighbouring molecules are attracted only by weak intermolecular forces (instantaneous dipole-induced dipole). Only a little energy is needed to separate the molecules, so it has a very low boiling point and is a gas at room temperature.