Types of Solids and Their Properties

Every solid is a set of particles locked in fixed positions, and what those particles are, and what holds them there, decides everything about how the solid behaves. Ionic solids are arrays of alternating cations and anions held by strong electrostatic attraction in every direction; they have high melting points, are hard but brittle, and conduct only when melted or dissolved, since the ions must be free to move. Molecular solids are separate covalent molecules stacked together by weak intermolecular forces (dispersion forces, dipole-dipole, or hydrogen bonding); melting only needs to separate the molecules, not break bonds inside them, so melting points are low and the solids do not conduct. Covalent network solids, like diamond and SiO2, are one giant structure where every atom is covalently bonded to its neighbors throughout the whole crystal; melting means breaking real covalent bonds everywhere at once, so melting points are extremely high, and most do not conduct since electrons are locked in bonds. Metallic solids are cations sitting in a sea of delocalized electrons; that mobile sea lets metals conduct in any state, bend without shattering, and still melt at moderate-to-high temperatures depending on how strongly the nuclei attract the shared electrons.

The question to ask when you look at a solid is always the same: what are the particles, and what force sits between them? That question links this topic directly to intermolecular forces and bonding earlier in the unit, and it explains conductivity and brittleness later: brittle solids (ionic, covalent network) shatter because shifting the lattice slightly lines up repelling charges, while metals deform because the electron sea just flows around the shift.