A chemical element, Md, atomic number 101, the twelfth member of the actinide series of elements. Mendelevium does not occur in nature; it was discovered and is prepared by artificial nuclear transmutation of a lighter element. Known isotopes of mendelevium have mass numbers from 248 to 258 and half-lives from a few seconds to about 55 days. They are all produced by charged-particle bombardments of more abundant isotopes. The amounts of mendelevium which are produced and used for studies of chemical and nuclear properties are usually less than about a million atoms; this is of the order of a million times less than a weighable amount. Studies of the chemical properties of mendelevium have been limited to a tracer scale. The behavior of mendelevium in ion-exchange chromatography shows that it exists in aqueous solution primarily in the 3 + oxidation state characteristic of the actinide elements. However, it also has a dipositive (2+) and a monopositive (1+) oxidation state.