Crystals and Crystal Chemistry

A crystal is a solid in which the atoms, ions or molecules are arranged in a regular, repeating pattern that extends in three dimensions. This orderly internal structure, called the crystal lattice, distinguishes crystalline solids from amorphous solids such as glass, in which the particles have no long-range order. The regular arrangement gives crystals their flat faces, sharp angles and many of their physical properties.

The lattice is described by repeating units that fill space without gaps. The possible distinct ways such repeating arrangements can be built in three dimensions are limited, and they are classified into the Bravais lattices, a fixed set of lattice types grouped by their symmetry. The branch of science that studies these structures is crystallography, which uses techniques such as X-ray diffraction, in which the regular spacing of atoms scatters X-rays into a pattern that reveals the arrangement inside. From such patterns chemists deduce exactly how atoms are positioned in a material.

Crystal chemistry examines how the chemical nature of the atoms and the bonds between them determine the structure a substance adopts and the properties that result, including hardness, cleavage and electrical behaviour. Part of this analysis involves determining oxidation states, the effective charges assigned to atoms in a compound, which help explain bonding and stoichiometry. Liquid crystals occupy a middle ground: they flow like liquids yet retain some of the ordered alignment of crystals, a combination exploited in display technology. The study of crystals thus connects the microscopic arrangement of atoms to the visible and useful properties of materials.

Frequently asked questions

What makes a solid a crystal?

A crystal has its atoms, ions or molecules arranged in a regular, repeating three-dimensional pattern called a lattice, unlike amorphous solids such as glass that lack long-range order.

What are Bravais lattices?

They are the limited set of distinct repeating arrangements that can fill three-dimensional space, classified by symmetry, into which all crystal structures fall.

What is a liquid crystal?

A liquid crystal is a state of matter that flows like a liquid but keeps some of the ordered molecular alignment of a crystal, a property used in electronic displays.