Why are certain substances metallic?

Metallic properties are by no means confined to elemental substances alone. A variety of materials, both inorganic and organic, show metallic properties. Some of these exotic substances exhibit electrical conductivities comparable to those of elemental metals like copper. A large number of systems traverse the transition from the metallic state to the nonmetallic state when there is a change in temperature, pressure or composition. Metal oxides provide a wide range of materials exhibiting metallic behaviour or going through the metal to non-metal (M-NM) transition. Alkali metal-ammonia solutions, with which chemists are all too familiar, probably constitute one of the earliest and most widely studied examples of the M-NM transition. However, a proper recognition of the metallization of ammonia in the context of the variety of solid systems exhibiting M-NM transitions has only been possible recently. Another interesting class of substances is that of expanded metals such as Hg and Cs which become non-metallic when the density is reduced below a critical value. Several organic solids, metal-chain compounds and polymers are not only metallic, but also become superconducting at low temperatures. With such a galaxy of chemical substances exhibiting metallic behaviour, the fundamental, recurring question of vital interest is "what makes a metal?". In this contribution, we shall examine operational criteria as well as criteria derived from models to answer this question. A related question of equal interest to chemists is “how many atoms are necessary to bring about metallic properties?”.