Four bonding types build fundamentally different unit cells. Ionic NaCl interpenetrates two face-centred sublattices of cations and anions (CN 6, octahedral). Covalent diamond links every atom to 4 tetrahedral neighbours via directional shared-electron bonds — the weakest packing of the four. Metallic FCC atoms behave as hard spheres cemented by a delocalised electron sea, reaching the theoretical maximum sphere-packing density. Molecular crystals stack whole dimers on a simple-cubic grid, held together only by weak Van der Waals contacts (dashed rods) far weaker than the covalent bond inside each dimer (solid rod).
a(T) = a₀ · (1 + α·(T − T₀)) linear thermal expansion
φ(T) = φ₀ · (a₀ / a(T))³ packing efficiency vs. T
CN = nearest neighbours within first coordination shell
- Lattice type — swaps the basis atoms, bond rules, coordination number and packing efficiency for a real unit-cell geometry.
- Temperature — raises the linear expansion coefficient α·ΔT (bigger for weakly-bonded molecular solids, smallest for stiff covalent diamond) and the thermal vibration amplitude of every atom.
- Unit cell edges — outlines one repeating a×a×a cell inside the rendered 3×3×3 supercell.
- Click an atom — highlights it and every atom directly bonded to it, reading out its true coordination number.
Real-world relevance: this is why diamond barely expands when heated (rigid covalent bonds, low α) while a wax or dry-ice molecular crystal expands many times faster and melts at far lower temperature (weak Van der Waals contacts, high α) — the same trade-off engineers account for when picking materials that must hold precise dimensions across a temperature range.