Definition of Crystal

A crystal is a solid composed of atoms, ions, or molecules arranged in a pattern that is periodic in three dimensions. [ASTM F1241]

The Unit Cell

Crystals are usually presented in terms of their unit cell. The unit cell is the smallest part of a crystal that, if repeated regularly by translation in three dimensions, creates the whole crystal. (Note: only translation of the unit cell is permitted, not inversion, rotation, or reflection.)

The unit cell of a crystal is defined by the crystal's lattice points.

Unit cell of sodium chloride. Image by Ben Mills.

Unit cell of sodium chloride - alternative representation. Image by Ben Mills

Examples of Crystals

Nonmetal Elements

The carbon atoms in diamond are held together in a covalent lattice by network covalent bonding. Iodine crystals are made of I₂ molecules held together in a crystal lattice by van der Waals forces. These forces are weak compared with covalent bonds, leading to a low melting point for iodine.

Crystalline carbon - diamond in rock.

Crystalline iodine. Image by Ben Mills.

Metal Elements

Solid metal crystals are held together by metallic bonding: i.e. a lattice of positively charged metal ions is held together by sharing delocalized conduction electrons.

Crystalline native gold. Image by Aram Dulyan.

Crystalline bismuth. Image by Heinrich Pniok.

Metalloid Elements

Crystalline silicon is held together in a covalent lattice by network covalent bonding. Tellurium crystals are formed from spiral chains of covalently bonded atoms in a hexagonal lattice.

Crystalline silicon. Image by Enricoros.

Crystalline native tellurium. Image by Christian Rewitzer.

Ionic Compounds

Ionic solid compounds are held together in a lattice by ionic bonds between anions and cations.

Copper(II) sulfate hydrate crystals CuSO₄·5H₂O.
Image by Crystal Titan.

Three varieties of beryl and an emerald . Beryl and emerald's formula is Be₃Al₂(SiO₃)₆. The different colors are caused by traces of different elements.

Covalent Compounds 1

The covalent solid compounds below are held together in a covalent lattice by network covalent bonding.

Amethyst, a variety of quartz - crystalline SiO₂.
Image by Didier Descouens.

A monocrystal of carborundum - crystalline SiC. Image by David Monniaux.

Covalent Compounds 2

The water ice crystals below are held together in a lattice by polar covalent bonds.

Crystalline H₂O.

Crystalline water.

Anisotropy

Differences in crystals when viewed along different axes can lead to anisotropy in crystalline materials.

Crystal Structure

The precise lattice constructions of crystals are found using X-ray diffraction.

Non-Crystalline Solids

Not all solids are crystalline. Solids also exist in amorphous and glass forms: unlike crystals, these forms have no long-range ordering in their structures.