Element: Actinium

General data

Element name	Actinium
Chemical symbol	Ac
Atomic number Z	89
Year discovered	1899
Standard state (25°C)	Solid
Category	Actinide
Block	f
Atomic mass	[227]
CPK color	#70ABFA

Physical properties

Melting point	1324 K (1051 °C)
Boiling point	3471 K (3198 °C)
Density	10.07 g/cm³

Energy properties

First ionization energy	5.17 eV
Electron affinity	no data

Atomic structure

Protons	89 (Atomic number)
Neutrons (Ac-227, longest-lived isotope)	138
Electrons	89
Electron configuration	[Rn] 7s² 6d¹
Oxidation states	+3
Electronegativity (Pauling)	1.1
Atomic radius (van der Waals)	260 pm

Ionization energies - NIST ASD (89 states)

State	Charge	Ioniz. energy (eV)	Type
Ac I	0+	5.380235 ± 0.000012	measured
Ac II	1+	11.75 ± 0.03	semi-empirical
Ac III	2+	17.436 ± 0.006	semi-empirical
Ac IV	3+	44.8 ± 1.4	semi-empirical
Ac V	4+	55 ± 1.9	calculated
Ac VI	5+	67 ± 2	calculated
Ac VII	6+	79 ± 2	calculated
Ac VIII	7+	98.9 ± 2.1	calculated
Ac IX	8+	113.9 ± 2.2	calculated
Ac X	9+	143.9 ± 2.2	calculated
Ac XI	10+	161.1 ± 2.4	calculated
Ac XII	11+	233 ± 2.5	calculated
Ac XIII	12+	255 ± 3	calculated
Ac XIV	13+	279 ± 3	calculated
Ac XV	14+	305 ± 3	calculated
Ac XVI	15+	330 ± 3	calculated
Ac XVII	16+	355 ± 3	calculated
Ac XVIII	17+	390 ± 3	calculated
Ac XIX	18+	416 ± 3	calculated
Ac XX	19+	444 ± 4	calculated
Ac XXI	20+	470 ± 4	calculated
Ac XXII	21+	610 ± 30	calculated
Ac XXIII	22+	640 ± 30	calculated
Ac XXIV	23+	670 ± 30	calculated
Ac XXV	24+	710 ± 40	calculated
Ac XXVI	25+	780 ± 40	calculated
Ac XXVII	26+	820 ± 40	calculated
Ac XXVIII	27+	920 ± 50	calculated
Ac XXIX	28+	950 ± 50	calculated
Ac XXX	29+	1030 ± 50	calculated
Ac XXXI	30+	1100 ± 50	calculated
Ac XXXII	31+	1170 ± 60	calculated
Ac XXXIII	32+	1240 ± 60	calculated
Ac XXXIV	33+	1310 ± 70	calculated
Ac XXXV	34+	1380 ± 70	calculated
Ac XXXVI	35+	1460 ± 70	calculated
Ac XXXVII	36+	1530 ± 80	calculated
Ac XXXVIII	37+	1610 ± 80	calculated
Ac XXXIX	38+	1680 ± 80	calculated
Ac XL	39+	1750 ± 90	calculated
Ac XLI	40+	1820 ± 90	calculated
Ac XLII	41+	1900 ± 100	calculated
Ac XLIII	42+	1970 ± 100	calculated
Ac XLIV	43+	2298 ± 4	calculated
Ac XLV	44+	2362 ± 4	calculated
Ac XLVI	45+	2430 ± 4	calculated
Ac XLVII	46+	2503 ± 4	calculated
Ac XLVIII	47+	2572 ± 4	calculated
Ac XLIX	48+	2639 ± 4	calculated
Ac L	49+	2762 ± 4	calculated
Ac LI	50+	2833 ± 4	calculated
Ac LII	51+	2908 ± 4	calculated
Ac LIII	52+	2980 ± 4	calculated
Ac LIV	53+	3264 ± 5	calculated
Ac LV	54+	3334 ± 5	calculated
Ac LVI	55+	3409 ± 5	calculated
Ac LVII	56+	3479 ± 5	calculated
Ac LVIII	57+	3811 ± 6	calculated
Ac LIX	58+	3893 ± 6	calculated
Ac LX	59+	4093 ± 7	calculated
Ac LXI	60+	4175 ± 4	semi-empirical
Ac LXII	61+	6767 ± 12	calculated
Ac LXIII	62+	6923 ± 15	calculated
Ac LXIV	63+	7088 ± 19	calculated
Ac LXV	64+	7265 ± 24	calculated
Ac LXVI	65+	7430 ± 30	calculated
Ac LXVII	66+	7600 ± 40	calculated
Ac LXVIII	67+	7950 ± 40	calculated
Ac LXIX	68+	8120 ± 50	calculated
Ac LXX	69+	8310 ± 50	calculated
Ac LXXI	70+	8480 ± 60	calculated
Ac LXXII	71+	8970 ± 60	calculated
Ac LXXIII	72+	9120 ± 70	calculated
Ac LXXIV	73+	9290 ± 90	calculated
Ac LXXV	74+	9440 ± 90	calculated
Ac LXXVI	75+	10480 ± 100	calculated
Ac LXXVII	76+	10660 ± 110	calculated
Ac LXXVIII	77+	11030 ± 120	calculated
Ac LXXIX	78+	11200 ± 120	calculated
Ac LXXX	79+	23480 ± 140	calculated
Ac LXXXI	80+	23890 ± 150	calculated
Ac LXXXII	81+	24340 ± 160	calculated
Ac LXXXIII	82+	24760 ± 170	calculated
Ac LXXXIV	83+	28610 ± 190	calculated
Ac LXXXV	84+	29160 ± 200	calculated
Ac LXXXVI	85+	29850 ± 220	calculated
Ac LXXXVII	86+	30293.1 ± 0.5	calculated
Ac LXXXVIII	87+	119945.7 ± 0.6	calculated
Ac LXXXIX	88+	122063.1 ± 2.2	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: Actinium (Ac) — © images-of-elements.com (CC BY 3.0)

Isotopes (33)

A	N	Half-life	Decay
Ac-204	115	7.4 ms	α
Ac-205	116	8 ms	α
Ac-206	117	27 ms	α
Ac-207	118	27 ms	α
Ac-208	119	93 ms	α
Ac-209	120	91 ms	α
Ac-210	121	340 ms	α
Ac-211	122	210 ms	α
Ac-212	123	900 ms	α
Ac-213	124	738 ms	α
Ac-214	125	8.2 s	α
Ac-215	126	170 ms	α
Ac-216	127	360 μs	α
Ac-217	128	69 ns	α
Ac-218	129	1.02 μs	α
Ac-219	130	9.5 μs	α
Ac-220	131	26.4 ms	α
Ac-221	132	52 ms	α
Ac-222	133	4.9 s	α
Ac-223	134	2.12 min	α
Ac-224	135	2.78 h	EC/β⁺
Ac-225	136	9.9190 d	α
Ac-226	137	29.37 h	β⁻
Ac-227	138	21.773 a	β⁻
Ac-228	139	6.15 h	β⁻
Ac-229	140	62.7 min	β⁻
Ac-230	141	122 s	β⁻
Ac-231	142	7.5 min	β⁻
Ac-232	143	119 s	β⁻
Ac-233	144	143 s	β⁻
Ac-234	145	44 s	β⁻
Ac-235	146	62 s	β⁻
Ac-236	147	1 min	β⁻

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