(Dmitri Mendeleev) Mendelevium, the ninth transuranium element of the actinide series discovered, was first identified by Ghiorso, Harvey, Choppin, Thompson, and Seaborg in early in 1955 during the bombardment of the isotope 253Es with helium ions in the Berkeley 60-inch cyclotron. The isotope produced was 256Md, which has a half-life of 76 min. This first identification was notable in that 256Md was synthesized on a one-atom-at-a-time basis.
Fourteen isotopes are now recognized. 258Md has a half-life of 2 months. This isotope has been produced by the bombardment of an isotope of einsteinium with ions of helium. Eventually enough 258Md should be made to determine its physical properties.
256Md has been used to elucidate some of the chemical properties of mendelevium in aqueous solution.
Experiments seem to show that the element possesses a moderately stable dipositive (II) oxidation state in addition to the tripositive (III) oxidation state, which is characteristic of the actinide elements.