A radioactive metallic transuranic element belonging to the actinoids; a.n. 101; mass number of the first discovered nuclide 256 (half-life 1.3 hours). Several short-lived isotopes have now been synthesized. The element was first identified by Albert Ghiorso, Glenn Seaborg (1912–99), and associates in 1955.

https://www.webelements.com/mendelevium/ Information from the WebElements site