Fm; at. no. 100; valence 3, 2. Man-made, radioactive element. No stable nuclides; known isotopes (mass numbers): 242-259. Longest-lived known isotope: 257 (T 101 days, α-emitter, rel. at. mass 257.0951), produced by neutron bombardment of plutonium (or other elements with higher at. no.). 255Fm (T 20.07 hrs, α-emitter) originally discovered in debris from a thermonuclear test explosion in Nov. 1952. Reviews of discovery: A. Ghiorso et al., Phys. Rev. 99, 1048-1049 (1955); A. Ghiorso, G. T. Seaborg, Sci. Am. 195(6), 67-80 (1956). Reviews: G. T. Seaborg, J. Chem. Educ. 36, 38-44 (1959); C. Keller, The Chemistry of the Transuranium Elements (Verlag Chemie, Weinheim, English Ed., 1971) pp 591-594; Silva, “Trans-Curium Elements” in MTP Int. Rev. Sci.: Inorg. Chem., Ser. One vol. 8, A. G. Maddock, Ed. (University Park Press, Baltimore, 1972) pp 71-105; Comprehensive Inorganic Chemistry vol. 5, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) passim; Handb. Exp. Pharmakol. 36, 689-738 (1973); R. J. Silva in The Chemistry of the Actinide Elements vol. 2, J. J. Katz et al., Eds. (Chapman and Hall, New York, 1986) pp 1086-1092; The Elements Beyond Uranium, G. T. Seaborg, W. D. Loveland, Eds. (John Wiley & Sons, Inc., New York, 1990) pp 28-38.