X-ray Crystallography:
is the science of determining the arrangement of atoms within a crystal from the manner in which a beam of X-rays is scattered from the electrons within the crystal. The method produces a three-dimensional picture of the density of electrons within the crystal, from which the mean atomic positions, their chemical bonds, their disorder and sundry other information can be derived. By definition, a crystal is a solid in which a particular arrangement of atoms (its unit cell) is repeated indefinitely along three principal directions known as the basis (or lattice) vectors, which are not necessarily perpendicular. A wide variety of materials can form crystals — such as salts, metals, minerals, semiconductors, as well as various inorganic, organic and biological molecules — which has made X-ray crystallography fundamental to many scientific fields.