Ac 89

Actinium (Ac)

actinide

Period: 7 Block: s

Solid

Standard Atomic Weight

Electron configuration

[Rn] 7s² 6d¹

Melting point

1050.85 °C (1324 K)

Boiling point

3197.85 °C (3471 K)

Density

1.007000e+4 kg/m³

Oxidation states

Electronegativity (Pauling)

1.1

Ionization energy (1st)

Discovery year

1899

Atomic radius

195 pm

Details

Name origin Greek: akis, aktinos (ray).

Discovery country France

Discoverers André Debierne

Actinium is a radioactive actinide metal and the element that gives the actinide series its name. Natural actinium is present only in minute amounts, chiefly as ²²⁷Ac in uranium decay chains. Its chemistry is dominated by the +3 oxidation state and resembles that of lanthanum more than the later, more strongly 5f-influenced actinides. Because all isotopes are radioactive and scarce, most knowledge comes from tracer-scale chemistry rather than ordinary bulk handling.

Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha particles. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees C. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons.

In April of 2012, Los Alamos National Laboratory announced a new medical isotope project that shows promise for rapidly producing major quantities of a new cancer-treatment agent, actinium 225 (Ac-225). Both a press release and a video are available.

Actinium was discovered in 1899 by André-Louis Debierne, a French chemist, while experimenting with new methods of separating rare earth oxides. Friedrich Otto Giesel independently discovered actinium in 1902. Actinium is a rare element that is present in uranium ores in tiny amounts, but it is usually cheaper and easier to create actinium when it is needed by bombarding radium with neutrons in a nuclear reactor.

Actinium's most stable isotope, actinium-227, has a half-life of 21.77 years. It decays into francium-223 through alpha decay or into thorium-227 through beta decay.

From the Greek aktis, aktinos, meaning beam or ray. Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals.

Read about Actinium on Wikipedia

Images

Properties

Physical

Atomic radius (empirical) 195 pm

Covalent radius 215 pm

Van der Waals radius 260 pm

Density

Molar volume 0.02254 L/mol

Phase at STP solid

Melting point 1050.85 °C

Boiling point 3197.85 °C

Specific heat capacity 0.12 J/(g·K)

Molar heat capacity 27.2 J/(mol·K)

Crystal structure fcc

Chemical

Electronegativity (Pauling) 1.1

Electron affinity

Ionization energy (1st)

Ionization energy (2nd)

Ionization energy (3rd)

Ionization energy (4th)

Ionization energy (5th)

Oxidation states +3

Valence electrons 3

Electron configuration

Electron configuration (semantic)

Thermodynamic

Heat of fusion 0.14510027 eV

Heat of vaporization 4.145722 eV

Heat of sublimation 4.456651 eV

Heat of atomization 4.456651 eV

Atomization enthalpy

Nuclear

Stable isotopes 0

Mass number (most stable) 227

Discovery year 1899

Abundance

Abundance (Earth's crust) 5.500e-10 mg/kg

Reactivity

N/A

Crystal Structure

Lattice constant a 531 pm

Electronic Structure

Electrons per shell 2, 8, 18, 32, 18, 9, 2

Identifiers

CAS number 7440-34-8

Term symbol

InChI InChI=1S/Ac

InChI Key QQINRWTZWGJFDB-UHFFFAOYSA-N

Electron Configuration Measured

Ion charge

Protons 89

Electrons 89

Charge Neutral

Configuration Ac: 6d¹ 7s²

[Rn] 6d¹ 7s²

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s²

Orbital diagram

↑↓

2/2

↑↓

2/2

↑↓

6/6

↑↓

2/2

↑↓

6/6

↑↓

2/2

↑↓

10/10

↑↓

6/6

↑↓

2/2

↑↓

10/10

↑↓

6/6

↑↓

2/2

↑↓

14/14

↑↓

10/10

↑↓

6/6

↑↓

2/2

↑

1/10 1↑

Total electrons: 89 Unpaired: 1

Atomic model

Protons 89

Neutrons 123

Electrons 89

Mass number 212

Stability Radioactive

Isotopes change neutron count, mass, and stability — not the electron configuration of a neutral atom.

Schematic atomic model, not to scale.

Atomic Fingerprint

Emission / Absorption Spectrum

Emission Visible: 380–750 nm

Isotope Distribution

No stable isotopes.

Mass number	Atomic mass (u)	Natural abundance	Half-life
212 Radioactive	212.007813 ± 0.000055	N/A	895 ms
213 Radioactive	213.006609 ± 0.000056	N/A	738 ms
216 Radioactive	216.008743 ± 0.000012	N/A	440 us
210 Radioactive	210.009436 ± 0.000062	N/A	350 ms
211 Radioactive	211.007732 ± 0.000057	N/A	213 ms

Measured

Phase / State

Solid 25 °C (298.15 K)

Reason: 1025.8 °C below melting point (1050.85 °C)

Melting point 1050.85 °C

Boiling point 3197.85 °C

Below melting by 1025.8 °C

0 K Current temperature: 25 °C 6000 K

Phase timeline

Schematic, not to scale

Solid

Liquid

Gas

Melting

Boiling

25°C

Solid

Liquid

Gas

Current

Phase transition points

Melting point Literature

1050.85 °C

Boiling point Literature

3197.85 °C

Current phase Calculated
Solid

Transition energies

Heat of fusion Literature

0.14510027 eV

Energy required to melt 1 mol at melting point

Heat of vaporization Literature

4.145722 eV

Energy required to vaporize 1 mol at boiling point

Heat of sublimation Literature

4.456651 eV

Energy required to sublime 1 mol at sublimation point

Density

Reference density Literature

1.007000e+4 kg/m³

At standard conditions

Current density Calculated

1.007000e+4 kg/m³

At standard conditions

Atomic Spectra

Showing 10 of 89 Atomic Spectra. Sorted by ion charge (ascending).

Lines Holdings ?

Ion	Charge	Total lines	Transition probabilities	Level designations
Ac I	0	153	95	113
Ac II	+1	345	294	319
Ac III	+2	11	11	11
Ac IV	+3	6	0	0

NIST Lines Holdings →

Levels Holdings ?

Ion	Charge	Levels
Ac I	0	53
Ac II	+1	84
Ac III	+2	8
Ac IV	+3	2
Ac V	+4	2
Ac VI	+5	2
Ac VII	+6	2
Ac VIII	+7	2
Ac IX	+8	2
Ac X	+9	2

NIST Levels Holdings →

89 Ac 227

Actinium — Atomic Orbital Visualizer

[Rn]7s26d1

Energy levels 2 8 18 32 18 9 2

Oxidation states +3

HOMO 6d n=6 · l=2 · m=-2

Actinium — Atomic Orbital Visualizer Preview

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89 Ac 227

Actinium — Crystal Structure Visualizer

Face-Centered Cubic · Pearson cF4

Experimental

Pearson cF4

Coord. № 12

Packing 74.000%

Actinium — Crystal Structure Visualizer Preview

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Ionic Radii

Charge	Coordination	Spin	Radius
+3	6	N/A	112.00000000000001 pm
+3	9	N/A	122 pm

Compounds

227.028 u

225.023 u

227.028 u

228.031 u

226.026 u

224.022 u

223.019 u

Isotopes (5)

Mass number	Atomic mass (u)	Natural abundance	Half-life	Decay mode
212 Radioactive	212.007813 ± 0.000055	N/A	895 ms	α ≈100%β+ ?
213 Radioactive	213.006609 ± 0.000056	N/A	738 ms	α ≈100%β+ ?
216 Radioactive	216.008743 ± 0.000012	N/A	440 us	α =100%β+ ?
210 Radioactive	210.009436 ± 0.000062	N/A	350 ms	α ≈100%β+ ?
211 Radioactive	211.007732 ± 0.000057	N/A	213 ms	α ≈100%β+ ?

212 Radioactive

Atomic mass (u) 212.007813 ± 0.000055

Natural abundance N/A

Half-life 895 ms

Decay mode

α ≈100%β+ ?

213 Radioactive

Atomic mass (u) 213.006609 ± 0.000056

Natural abundance N/A

Half-life 738 ms

Decay mode

α ≈100%β+ ?

216 Radioactive

Atomic mass (u) 216.008743 ± 0.000012

Natural abundance N/A

Half-life 440 us

Decay mode

α =100%β+ ?

210 Radioactive

Atomic mass (u) 210.009436 ± 0.000062

Natural abundance N/A

Half-life 350 ms

Decay mode

α ≈100%β+ ?

211 Radioactive

Atomic mass (u) 211.007732 ± 0.000057

Natural abundance N/A

Half-life 213 ms

Decay mode

α ≈100%β+ ?

Spectral Lines

Showing 50 of 213 Spectral Lines. Only spectral lines with measured intensity are shown by default.

Wavelength (nm)	Intensity	Ion stage	Type	Transition	Accuracy	Source
391.4468 nm	24000	Ac II	emission	6d.7s 3D → 5f.7s 3F*	Measured	NIST
417.99772 nm	17000	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2D*	Measured	NIST
481.2218 nm	16000	Ac II	emission	6d.7s 3D → 7s.7p 3P*	Measured	NIST
438.64 nm	15000	Ac II	emission	6d.7s 1D → 6d.7p 1D*	Measured	NIST
397.736 nm	13000	Ac II	emission	6d2 3F → 6d.7p 3P*	Measured	NIST
445.2188 nm	12000	Ac II	emission	6d.7s 3D → 6d.7p 3F*	Measured	NIST
388.55592 nm	9000	Ac I	emission	6d.7s2 2D → 6d.7s.(3D).7p 2P*	Measured	NIST
401.9622 nm	8700	Ac II	emission	6d2 3F → 5f.6d 1G*	Measured	NIST
544.6367 nm	8700	Ac II	emission	6d2 3F → 6d.7p 3D*	Measured	NIST
435.9118 nm	7300	Ac II	emission	6d.7s 3D → 7s.7p 3P*	Measured	NIST
446.27307 nm	6900	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2F*	Measured	NIST
471.65807 nm	6900	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2F*	Measured	NIST
495.8233 nm	6800	Ac II	emission	6d.7s 1D → 6d.7p 1P*	Measured	NIST
384.304 nm	6600	Ac I	emission	6d.7s2 2D → 6d.7s.(3D).7p 2P*	Measured	NIST
392.0101 nm	6500	Ac II	emission	6d2 3P → 5f.6d 1D*	Measured	NIST
418.31199 nm	6400	Ac I	emission	6d.7s2 2D → 6d.7s.(3D).7p 2F*	Measured	NIST
439.67158 nm	5800	Ac I	emission	6d.7s2 2D → 6d.7s.(3D).7p 2F*	Measured	NIST
419.43971 nm	5300	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2P*	Measured	NIST
494.5181 nm	5300	Ac II	emission	6d2 3F → 6d.7p 3D*	Measured	NIST
496.0869 nm	4900	Ac II	emission	6d2 3F → 6d.7p 3D*	Measured	NIST
461.39285 nm	4000	Ac I	emission	6d.7s2 2D → 6d.7s.(3D).7p 2F*	Measured	NIST
527.15603 nm	3800	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2F*	Measured	NIST
406.31064 nm	3700	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2D*	Measured	NIST
407.8693 nm	3700	Ac II	emission	6d.7s 3D → 6d.7p 1P*	Measured	NIST
403.4629 nm	3500	Ac I	emission	6d.7s2 2D → 6d2.(3F).7p 4G*	Measured	NIST
420.9682 nm	3500	Ac II	emission	6d.7s 3D → 5f.7s 1F*	Measured	NIST
669.5231 nm	3300	Ac II	emission	6d2 3F → 6d.7p 3F*	Measured	NIST
474.0522 nm	2600	Ac II	emission	6d2 1G → 6d.7p 1F*	Measured	NIST
515.6541 nm	2100	Ac II	emission	6d2 3F → 5f.7s 3F*	Measured	NIST
383.53206 nm	1900	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2P*	Measured	NIST
488.9102 nm	1800	Ac II	emission	6d2 3F → 5f.7s 3F*	Measured	NIST
480.7843 nm	1700	Ac II	emission	6d.7s 1D → 6d.7p 3F*	Measured	NIST
534.47384 nm	1700	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4G*	Measured	NIST
400.5469 nm	1500	Ac II	emission	6d2 1G → 5f.6d 3G*	Measured	NIST
536.2615 nm	1400	Ac II	emission	6d2 3F → 6d.7p 1D*	Measured	NIST
522.8309 nm	1300	Ac I	emission	6d.7s2 2D → 6d.7s.(3D).7p 4D*	Measured	NIST
454.408 nm	1200	Ac II	emission	6d2 1D → 7s.7p 1P*	Measured	NIST
461.01055 nm	1100	Ac I	emission	6d.7s2 2D → 6d.7s.(1D).7p 2D*	Measured	NIST
422.599 nm	1000	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4D*	Measured	NIST
387.7035 nm	980	Ac II	emission	6d.7p 3P* → 5f.7p 3D	Measured	NIST
420.89072 nm	960	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4D*	Measured	NIST
469.05284 nm	960	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4F*	Measured	NIST
521.5399 nm	960	Ac II	emission	6d2 3P → 6d.7p 3P*	Measured	NIST
486.88523 nm	890	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4F*	Measured	NIST
526.4481 nm	890	Ac I	emission	7s2.7p 2P* → 7s2.8s 2S	Measured	NIST
440.21056 nm	860	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4F*	Measured	NIST
447.18106 nm	850	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4F*	Measured	NIST
421.80204 nm	740	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3P).7p 2D*	Measured	NIST
462.16811 nm	710	Ac I	emission	6d2.(3F).7s 4F → 6d2.(3F).7p 4F*	Measured	NIST
426.8209 nm	570	Ac II	emission	6d2 3F → 6d.7p 3P*	Measured	NIST

Extended Properties

Covalent Radii (Extended)

Covalent radius (Pyykkö)

Covalent radius (Pyykkö, double)

Covalent radius (Pyykkö, triple)

Van der Waals Radii

Alvarez

UFF

MM3

Atomic & Metallic Radii

Atomic radius (Rahm)

Numbering Scales

Mendeleev

Pettifor

Glawe

Electronegativity Scales

Ghosh

Polarizability & Dispersion

Dipole polarizability

Dipole polarizability (unc.)

Phase Transitions & Allotropes

Melting point	1323.15 K
Boiling point	3473.15 K

Oxidation State Categories

+3 main

Advanced Reference Data

Crystal Radii Detail (2)

Charge	CN	Spin	r_crystal (pm)	Origin
3	VI		126	from r^3 vs V plots,
3	IX	—	136

Isotope Decay Modes (56)

Isotope	Mode	Intensity
205	A	100%
205	B+	—
206	A	100%
206	B+	—
207	A	100%
208	A	100%
208	B+	—
209	A	100%
209	B+	—
210	A	100%

X‑ray Scattering Factors (516)

Energy (eV)	f₁	f₂
10	—	1.19484
10.1617	—	1.15672
10.3261	—	1.11982
10.4931	—	1.0841
10.6628	—	1.04952
10.8353	—	1.01604
11.0106	—	0.98234
11.1886	—	0.94669
11.3696	—	0.91233
11.5535	—	0.88152

Additional Data

Estimated Crustal Abundance

The estimated element abundance in the earth's crust.

5.5×10-10 milligrams per kilogram

References (1)

[5] Actinium https://education.jlab.org/itselemental/ele089.html

Estimated Oceanic Abundance

The estimated element abundance in the earth's oceans.

Not Applicable

References (1)

[5] Actinium https://education.jlab.org/itselemental/ele089.html

References

(9)

1 PubChem

Data deposited in or computed by PubChem

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2 Atomic Mass Data Center (AMDC), International Atomic Energy Agency (IAEA)

The half-life and atomic mass data was provided by the Atomic Mass Data Center at the International Atomic Energy Agency.

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3 IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW)

Actinium

Element data are cited from the Atomic weights of the elements (an IUPAC Technical Report). The IUPAC periodic table of elements can be found at https://iupac.org/what-we-do/periodic-table-of-elements/. Additional information can be found within IUPAC publication doi:10.1515/pac-2015-0703 Copyright © 2020 International Union of Pure and Applied Chemistry.

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4 IUPAC Periodic Table of the Elements and Isotopes (IPTEI)

The information are cited from Pure Appl. Chem. 2018; 90(12): 1833-2092, https://doi.org/10.1515/pac-2015-0703.

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License note: Copyright (c) 2020 International Union of Pure and Applied Chemistry. The International Union of Pure and Applied Chemistry (IUPAC) contribution within Pubchem is provided under a CC-BY-NC-ND 4.0 license, unless otherwise stated.

5 Jefferson Lab, U.S. Department of Energy

Actinium

Thomas Jefferson National Accelerator Facility (Jefferson Lab) is one of 17 national laboratories funded by the U.S. Department of Energy. The lab's primary mission is to conduct basic research of the atom's nucleus using the lab's unique particle accelerator, known as the Continuous Electron Beam Accelerator Facility (CEBAF). For more information visit https://www.jlab.org/

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License note: Please see citation and linking information: https://education.jlab.org/faq/index.html

6 Los Alamos National Laboratory, U.S. Department of Energy

Actinium

The periodic table at the LANL (Los Alamos National Laboratory) contains basic element information together with the history, source, properties, use, handling and more. The provenance data may be found from the link under the source name.

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7 NIST Physical Measurement Laboratory

Actinium

The periodic table contains NIST's critically-evaluated data on atomic properties of the elements. The provenance data that include data for atomic spectroscopy, X-ray and gamma ray, radiation dosimetry, nuclear physics, and condensed matter physics may be found from the link under the source name. Ref: https://www.nist.gov/pml/atomic-spectra-database

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8 PubChem Elements

Actinium

This section provides all form of data related to element Actinium.

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9 PubChem Elements

Actinium

The element property data was retrieved from publications.

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Last updated: May 1, 2026

Data verified: May 12, 2026

Content is reviewed against latest scientific data.