Crystal Structures and Solid Properties

Core Idea

Solids form repeating 3D patterns of atoms or ions. Ionic solids have alternating cations and anions in fixed arrangements. Metallic solids have atoms in close-packed arrays. Covalent network solids have all atoms bonded throughout. Molecular solids have discrete molecules held by intermolecular forces. Crystal type determines physical properties like hardness and melting point.

Explainer

When a liquid cools into a solid, the particles arrange themselves into a repeating three-dimensional pattern called a crystal lattice. The smallest repeating unit of this pattern is the unit cell — think of it as the tile that, when copied in all directions, builds the entire crystal. From your work with ionic and metallic bonding, you already know the forces holding these particles together. Crystal structure is where those forces become visible as architecture.

Ionic solids like sodium chloride arrange alternating cations and anions so that every positive ion is surrounded by negative ions and vice versa, maximizing electrostatic attraction while minimizing repulsion. The result is a rigid, brittle lattice with high melting points — it takes enormous energy to pull all those opposite charges apart. When you strike an ionic crystal, layers shift so that like charges suddenly face each other, and the crystal shatters along clean planes. Ionic solids do not conduct electricity as solids because ions are locked in place, but they conduct when melted or dissolved because the ions become free to move.

Metallic solids take a different approach. Metal atoms pack together as tightly as possible — often in face-centered cubic or hexagonal close-packed arrangements — with their valence electrons delocalized into a shared "electron sea." This delocalization, which you studied in metallic bonding, explains why metals conduct electricity and heat so well: electrons flow freely through the lattice. It also explains malleability — when layers of metal atoms slide past each other, the electron sea simply redistributes around the new arrangement, maintaining cohesion rather than shattering.

Covalent network solids like diamond and quartz are built from atoms connected by continuous covalent bonds extending throughout the entire crystal. There are no discrete molecules — the whole crystal is essentially one giant molecule. This makes them extraordinarily hard and gives them very high melting points, because breaking the solid means breaking strong covalent bonds. Molecular solids like ice or sugar, by contrast, consist of individual molecules held together only by weak intermolecular forces (hydrogen bonds, dipole-dipole, or London dispersion). The covalent bonds within each molecule are strong, but the forces between molecules are weak, so molecular solids have low melting points and are soft. The key insight is that a solid's physical properties — melting point, hardness, electrical conductivity, brittleness — are direct consequences of which type of bonding holds the crystal together.