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%)
	247mFm	non-existent							9.2(23) s	α (≈100%); IT (? %)
	247pFm	150(100)#			71 730(170)#			(+7⁄2)
	247qFm	400(150)#			71 980(210)#
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.