A molecular substance contains discrete molecules with strong covalent bonds inside each molecule and weaker intermolecular attractions between molecules. A nonmolecular substance has no separate molecular units: ions, atoms, or metal cores form a continuous lattice. A chemical formula for an ionic or network solid gives a composition ratio, not a molecule.

Lattice sites are occupied by molecules. Dispersion forces, dipole-dipole attraction, or hydrogen bonding hold the molecules together. Because melting usually separates molecules without breaking their internal covalent bonds, many molecular solids have relatively low melting points and are soft or volatile. They usually do not conduct because they lack mobile charged particles. Strong hydrogen bonding can raise melting and boiling points, so predictions should compare substances of similar size and shape.

Alternating cations and anions form an extended three-dimensional lattice. Strong electrostatic attraction gives high melting points. Ionic crystals are hard but brittle: shifting a layer can place like charges together, producing strong repulsion and fracture. They do not conduct as solids because ions are fixed, but they conduct when molten or dissolved. Solubility depends on the competition between lattice attraction and solvation by the liquid.