METALS: An element is a metal when the outer valence electrons are free to roam throughout the element, not being bound to any atom within it, giving the element the property of electrical conductivity. This occurs when there is zero energy gap between the valence band and conduction band. Because, by Paulis' exclusion principle, no two electrons are allowed to have exactly the same energy, the free electrons take up a continuous spectrum of energies, called an energy band. It is these free electrons which totally reflect all incident light (light being an electromagnetic wave, which readily interacts with the random seething sea of free electrons representing a wide range of frequencies or colours) preventing light of most frequencies from penetrating more than a few wavelengths into a conducting material. This gives metals their shiny appearance. Examples include sodium, potassium and copper. The metallic nature of metals depends upon how many atoms are together. A cluster of 1500 atoms of silver is non-metallic, yet 10,000 atoms are!
It has recently been discovered that all elements will become metallic when subject to low enough temperatures and high enough pressures. Hydrogen becomes a metallic liquid at a pressure of 2 million atmospheres, when its density is 1.06 g/cc, 14 times the density of normal liquid hydrogen. It is predicted that metallic liquid hydrogen will be superconducting up to a temperature of 50 Kelvin, unusual because all previously known superconductors are solids.
METALLOIDS - SEMI-METALS: A semi-metal arises when the energy gap between the valence band and the conduction band only just overlap. Here, only a few of the valence electrons can enter the conduction band. Examples include arsenic, antimony, bismuth and graphite.
METALLOIDS - SEMI-CONDUCTORS: A semi-conductor arises when the energy gap between the valence band and the conduction band is less that about 2 electron volts. Here, only a few of the valence electrons, those which have sufficient thermal energy to cross the energy gap, can enter the conduction band. Examples include germanium and silicon.
NON-METALS: An element is non-metallic when the energy gap between the valence band and the conduction band is larger than about 2 electron volts. The valence electrons are then tightly bound to the atom (or molecule) unable to roam freely throughout the element. The element is an electrical insulator. Sulphur is a good example.
