Isotopes of fermium

Fermium is an artificial element, and has no stable or naturally occuring isotopes. The first isotope to be discovered was 255100Fm, in fallout from the first test of a hydrogen bomb, codenamed 'Ivy Mike', in 1952. 250100Fm was discovered independently in 1954.

Symbol	Z(p)	N(n)	Mass/u	Excess energy MeV	Binding energy/A MeV	β−-decay energy MeV	Spin	Half life	Decay mode, proportion
	Excitation energy/MeV
242Fm	100	142	242.073 43(43)#	68 400(401)#	7465(2)#	—	0	0.8(2) ms	SF (? %); α (? %)
243Fm	100	143		69 260(220)#			−7⁄2#	210(60) ms	α; β+ (? %); SF (59%#)
244Fm	100	144		69 010(280)			0	3.3(0.5) ms	SF (≈100%); α (0.4%#)
245Fm	100	145		70 220(280)#			+1⁄2#	4.2(13) s	α (? %); β+ (4.2%#); SF (0.13%#)
246Fm	100	146		70 140(40)			0	1.1(2) s	α (? %); β+ (> 10%); SF (4.5–13%); β+SF (10–5%)
247Fm	100	147		71 580(140)#			+5⁄2#	35(4) s	α (≥ 50%); β+ (≤ 50%)
Values marked # are estimated from systematic trends rather than experimentally measured. Spins quoted in parentheses are uncertain in value and/or parity. Sources: Except as otherwise noted, isotopic masses and associated energies are taken from the AME 2003 dataset; nuclear spins and decay properties are taken from NUBASE 2003.