Inorganic ionic solids (e.g. NaCl) are held together by the electrostatic (Coulombic) attraction between oppositely-charged ions, arranged in a repeating crystal lattice.
E_lattice ~ -k * q1*q2 / r
Melting: k_B*T >> |E_lattice| -> ions gain mobility
σ ∝ n_free * q * μ_ion
- Temperature — raises the average kinetic (vibrational) energy of each ion; past the melting point, thermal energy overcomes the lattice's electrostatic binding energy.
- Lattice size — how many ion pairs sit along each cube edge of the crystal shown.
- Lattice energy — scales the Coulombic binding strength, i.e. how "stubborn" this particular ionic compound is (higher = higher real-world melting point, like MgO vs NaCl).
Once ions detach from their lattice sites (melting or dissolving), they become mobile charge carriers — this is exactly why molten or dissolved ionic compounds conduct electricity while the solid crystal does not.