Element: Fermium

This element has only been produced a few atoms at a time, so most of its properties have never been measured - missing values are omitted.

General data

Element name	Fermium
Chemical symbol	Fm
Atomic number Z	100
Year discovered	1952
Standard state (25°C)	Solid
Category	Actinide
Block	f
Atomic mass	[257]
CPK color	#B31FBA

Physical properties

Melting point	1800 K (1527 °C)

Energy properties

First ionization energy	6.5 eV

Atomic structure

Protons	100 (Atomic number)
Neutrons (Fm-257, longest-lived isotope)	157
Electrons	100
Electron configuration	[Rn] 5f¹² 7s²
Oxidation states	+3
Electronegativity (Pauling)	1.3

Ionization energies - NIST ASD (100 states)

State	Charge	Ioniz. energy (eV)	Type
Fm I	0+	6.5 ± 0.07	semi-empirical
Fm II	1+	12.4 ± 0.4	semi-empirical
Fm III	2+	23.2 ± 0.4	semi-empirical
Fm IV	3+	39.3 ± 0.4	semi-empirical
Fm V	4+	55 ± 1.9	calculated
Fm VI	5+	74 ± 2	calculated
Fm VII	6+	93 ± 2	calculated
Fm VIII	7+	120 ± 2.1	calculated
Fm IX	8+	136 ± 2.2	calculated
Fm X	9+	162 ± 2.2	calculated
Fm XI	10+	185 ± 9	calculated
Fm XII	11+	209 ± 10	calculated
Fm XIII	12+	237 ± 12	calculated
Fm XIV	13+	257 ± 12	calculated
Fm XV	14+	276 ± 14	calculated
Fm XVI	15+	300 ± 15	calculated
Fm XVII	16+	326 ± 16	calculated
Fm XVIII	17+	351 ± 17	calculated
Fm XIX	18+	377 ± 19	calculated
Fm XX	19+	402 ± 20	calculated
Fm XXI	20+	430 ± 4	calculated
Fm XXII	21+	453 ± 4	calculated
Fm XXIII	22+	616 ± 4	calculated
Fm XXIV	23+	647 ± 4	calculated
Fm XXV	24+	680 ± 4	calculated
Fm XXVI	25+	716 ± 4	calculated
Fm XXVII	26+	749 ± 4	calculated
Fm XXVIII	27+	782 ± 4	calculated
Fm XXIX	28+	837 ± 4	calculated
Fm XXX	29+	871 ± 4	calculated
Fm XXXI	30+	909 ± 4	calculated
Fm XXXII	31+	944 ± 4	calculated
Fm XXXIII	32+	1110 ± 50	calculated
Fm XXXIV	33+	1150 ± 60	calculated
Fm XXXV	34+	1190 ± 60	calculated
Fm XXXVI	35+	1230 ± 60	calculated
Fm XXXVII	36+	1370 ± 70	calculated
Fm XXXVIII	37+	1420 ± 70	calculated
Fm XXXIX	38+	1550 ± 80	calculated
Fm XL	39+	1600 ± 80	calculated
Fm XLI	40+	1770 ± 90	calculated
Fm XLII	41+	1850 ± 90	calculated
Fm XLIII	42+	1940 ± 100	calculated
Fm XLIV	43+	2030 ± 100	calculated
Fm XLV	44+	2120 ± 110	calculated
Fm XLVI	45+	2210 ± 110	calculated
Fm XLVII	46+	2300 ± 120	calculated
Fm XLVIII	47+	2390 ± 120	calculated
Fm XLIX	48+	2490 ± 120	calculated
Fm L	49+	2590 ± 120	calculated
Fm LI	50+	2680 ± 140	calculated
Fm LII	51+	2760 ± 140	calculated
Fm LIII	52+	2850 ± 150	calculated
Fm LIV	53+	2950 ± 150	calculated
Fm LV	54+	3403 ± 5	calculated
Fm LVI	55+	3480 ± 5	calculated
Fm LVII	56+	3561 ± 5	calculated
Fm LVIII	57+	3647 ± 6	calculated
Fm LIX	58+	3730 ± 6	calculated
Fm LX	59+	3810 ± 7	calculated
Fm LXI	60+	3986 ± 10	calculated
Fm LXII	61+	4070 ± 12	calculated
Fm LXIII	62+	4160 ± 15	calculated
Fm LXIV	63+	4245 ± 19	calculated
Fm LXV	64+	4586 ± 24	calculated
Fm LXVI	65+	4670 ± 30	calculated
Fm LXVII	66+	4760 ± 40	calculated
Fm LXVIII	67+	4840 ± 40	calculated
Fm LXIX	68+	5420 ± 50	calculated
Fm LXX	69+	5510 ± 50	calculated
Fm LXXI	70+	5760 ± 60	calculated
Fm LXXII	71+	5860 ± 60	calculated
Fm LXXIII	72+	9200 ± 70	calculated
Fm LXXIV	73+	9370 ± 90	calculated
Fm LXXV	74+	9570 ± 90	calculated
Fm LXXVI	75+	9770 ± 100	calculated
Fm LXXVII	76+	9970 ± 110	calculated
Fm LXXVIII	77+	10160 ± 120	calculated
Fm LXXIX	78+	10660 ± 120	calculated
Fm LXXX	79+	10860 ± 140	calculated
Fm LXXXI	80+	11080 ± 150	calculated
Fm LXXXII	81+	11280 ± 160	calculated
Fm LXXXIII	82+	11850 ± 170	calculated
Fm LXXXIV	83+	12020 ± 190	calculated
Fm LXXXV	84+	12220 ± 200	calculated
Fm LXXXVI	85+	12390 ± 220	calculated
Fm LXXXVII	86+	14180 ± 240	calculated
Fm LXXXVIII	87+	14400 ± 250	calculated
Fm LXXXIX	88+	14800 ± 300	calculated
Fm XC	89+	15000 ± 300	calculated
Fm XCI	90+	30300 ± 300	calculated
Fm XCII	91+	30800 ± 300	calculated
Fm XCIII	92+	31300 ± 400	calculated
Fm XCIV	93+	31800 ± 400	calculated
Fm XCV	94+	38400 ± 400	calculated
Fm XCVI	95+	39100 ± 400	calculated
Fm XCVII	96+	40000 ± 400	calculated
Fm XCVIII	97+	40482.2 ± 1.1	calculated
Fm XCIX	98+	158171 ± 6	calculated
Fm C	99+	160808 ± 6	calculated

Value type as given by NIST ASD: measured - from evaluated experimental data; calculated - from ab initio calculations (mostly highly ionised atoms that have not been measured; for the simplest ones, e.g. H I, the calculation is more accurate than measurement); semi-empirical - from interpolation, extrapolation or another semi-empirical method.

Photos

Photo of the element: Fermium (Fm) — © images-of-elements.com (CC BY 3.0)

Isotopes (19)

A	N	Half-life	Decay
Fm-241	141	0.73 ms	SF
Fm-242	142	0.8 ms	SF
Fm-243	143	231 ms	α
Fm-244	144	3.12 ms	SF
Fm-245	145	5.4 s	α
Fm-246	146	1.53 s	α
Fm-247	147	31 s	α
Fm-248	148	34.5 s	α
Fm-249	149	1.6 min	EC/β⁺
Fm-250	150	30.9 min	α
Fm-251	151	5.30 h	EC/β⁺
Fm-252	152	25.38 h	α
Fm-253	153	3.00 d	α
Fm-254	154	3.240 h	α
Fm-255	155	20.07 h	α
Fm-256	156	157.1 min	SF
Fm-257	157	100.5 d	α
Fm-258	158	370 μs	SF
Fm-259	159	1.5 s	SF

Data sources

PubChem Periodic Table API - Retrieved on 29 June 2026 - licence: CC0
IUPAC CIAAW - Abridged Standard Atomic Weights 2024 - Table version: 2024 - licence: Public domain
NNDC Wallet Cards 2023 - Data as of 15 October 2024 - licence: Public domain
Images of Elements - Retrieved on 29 June 2026 - licence: CC BY 3.0

More about the data sources