Mendelevium, a transuranium chemical element. All isotopes of mendelevium are radioactive. The principal isotope, mendelevium 256, has a half-life of about 1 1/2 hours and is produced by bombarding einsteinium 253 with alpha particles. The longest-lived isotope is mendelevium 258, with a half-life of about 60 days. The element was discovered in 1955 by Albert Ghiorso, Bernard G. Harvey, G. R. Choppin, S. G. Thompson, and Glenn T. Seaborg of the University of California. It was named for Dmitri Mendeleev, discoverer of the periodic law.

Symbol: Md. Atomic number: 101. Atomic weight of most stable isotope: about 258. Mendelevium has four known isotopes—Md-255 through Md-258— and belongs to the actinide series of the Periodic Table.