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.
Examples of Crystals
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.
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 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.
Ionic solid compounds are held together in a lattice by ionic bonds between anions and cations.
Image by Crystal Titan.
Covalent Compounds 1
The covalent solid compounds below are held together in a covalent lattice by network covalent bonding.
Image by Didier Descouens.
Covalent Compounds 2
The water ice crystals below are held together in a lattice by polar covalent bonds.
Differences in crystals when viewed along different axes can lead to anisotropy in crystalline materials.
The precise lattice constructions of crystals are found using X-ray diffraction.
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.