Superconductors are material with an electrical resistance of zero below a certain temperature. The electrical resistance of conductive materials tends to drop when the temperature is decreased, but even common conductors such as copper will still show some resistance even very close to absolute zero, partly due to impurities in the metal.
A superconductor, however, will demonstrate an electrical resistance of exactly zero as soon as its temperature is lowered beneath its critical temperature, in spite of the same impurities and defects being present. Beyond this definition, however, the chemical attributes of superconductors can vary widely, as there is more than one type of superconductor.
According to superconductors.org, there are two types of superconductor. Type 1 superconductors, whose electrical resistance falls to zero as soon as it is cooled beneath its critical temperature, and Type 2 superconductors, who have two critical temperatures. In Type 2 superconductors, a material's superconductivity falls into a mixed state when it is cooled beneath the higher critical temperature, and then switches to being a perfect superconductor when cooled beneath the lower critical temperature.
Predicted theoretically by Alexei Alexeyevich Abrikosov, winning him the Nobel Prize for Physics in 2003, Type 2 superconductors tend to be metal alloys or oxide ceramic compounds. They superconduct at higher temperatures and so can carry higher currents and are useful for work with electromagnets. Type 2 superconductors are much less expensive to work with as their higher critical temperatures means they can often be cooled with liquid nitrogen, rather than the far more expensive liquid helium.
Type 1 superconductors are, generally speaking, the pure elemental materials - lead, aluminium, mercury, etc. Most Type 1 superconductors are metals, or transition metals.
The classification into Type 1 and Type 2 superconductors is complicated, however, by the theoretical distinction into conventional and unconventional superconductors. Conventional superconductors conform to the BCS (Bardeen, Cooper and Schrieffer) theory of 1957, which suggests that superconductivity is a phenomenon caused by the condensation of pairs of electrons within a cooled conductive material. Unconventional superconductors, which tend to have higher critical temperatures and to be composed of more complex chemical compounds, do not conform to this theory, and research is ongoing as to the mechanism which could cause their superconductivity.
So the chemical attributes of superconductors, first discovered in 1911, are varied. Some are pure elements, such as mercury, aluminium or lead. Others are carbon allotropes, such as fullerenes, diamond and possibly even graphene, the discovery of which won Manchester University scientists the Nobel Prize in 2010. Other superconductors are alloys, and even ceramics. Magnesium diboride is probably the best known of these as its critical temperature is 39K, making it the conventional superconductor with the highest critical temperature.