Crystals and Glasses

Contrasting with a molecule or complex ion, that is a finite (often small) assembly of atoms, a solid has no rigid size but can add atoms indefinitely. The bonding arrangements of atoms are supposed to be similar throughout, in a sample of uniform composition. For instance, both glassy and crystalline forms of silica (SiO₂) have structures with each Si surrounded by

four oxygen atoms and each O by two Si. Though, in crystalline solids it is possible to make out a unit cell consisting of a group of atoms that is repeated indefinitely in exact geometric fashion. In fact, all crystals consists defects where this regularity is broken sometimes, but nevertheless crystals are distinct from non-crystalline glasses or solids, in which there is no regular repetition. The critical distinction is that of long-range order. Local chemical bonding arrangements depict short-range order that may be exists even in a glass as in the case of SiO₂. The variation arises from the way these bonds connect together to form an extended network. Glassy types are metastable, prevented by kinetic factors from achieving the very stable (crystalline) arrangement. Very many, probably all solids can be made glassy if they are cooled quickly enough from the liquid or gaseous state. Some substances form glasses particularly easily, usually ones in which atoms are covalently bonded to comparatively few (three or four) neighbours.

Macroscopically in the definite shapes of crystals the distinction can be observed, that reflect the regular atomic arrangements: compare, for instance, the cubic faces of common salt (sodium chloride) crystals with the irregular and frequently curved surfaces of fractured window glass. Microscopically the distinction may be made by X-ray diffraction that depends on the fact that the regular atomic spacing in crystals is identical to the wavelength of X-rays. This is the most powerful method for determining the structures of crystalline substances but cannot be used in similar way for glasses.

A 'complete' requirement of the structure of a glass is not possible but for a crystal it is only essential to give the details of one unit cell. Substances are said to have the 'similar' structure if the arrangement of atoms in a unit cell is necessarily similar, even though the interatomic distances and the dimensions of the cell are different. Structure types are named after a specific instance, often naturally occurring minerals: so we talk of the rocksalt structure of rutile or the NaCl structure of TiO₂. Specifying a particular mineral rather than the compound formula is significant, as some compounds depict polymorphism and can adopt various different crystal forms. TiO₂, for instance, is known also as anatase and brookite, in which the arrangement of atoms is dissimilar from that in rutile.