ActinideElement 89 · d-block

Actinium (Ac)

Actinium (Ac) is an actinide with atomic number 89; its most stable known isotope has mass number 227. At room temperature it is a solid that melts at 1,050 °C and boils at 3,200 °C.

89AcActinium[227]

Atomic number

protons

Atomic weight

[227]

most stable isotope

Melting point

1,050 °C

1,323.15 K

Boiling point

3,200 °C

3,473.15 K

Density

10 g/cm³

near room temperature

Electronegativity

1.1

Pauling scale

State at 25 °C

Solid

Discovered

1899

France / Germany

Actinium at a glance

Identity

Symbol	Ac
Atomic number (Z)	89
Atomic weight	[227] mass number of the most stable isotope
Category	Actinide
Group	— (f-block: lanthanide series)
Period	7
Block	d-block
CAS number	7440-34-8

Atomic properties

Electron configuration	[Rn] 6d¹ 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s²
Electrons per shell	2, 8, 18, 32, 18, 9, 2
Oxidation states	+3
Electronegativity (Pauling)	1.1
First ionisation energy	5.38 eV (519.1 kJ/mol) 2nd: 11.75 eV · 3rd: 17.436 eV
Electron affinity	0.35 eV
Atomic radius (empirical)	195 pm
Covalent radius	186 pm
Van der Waals radius	247 pm

Physical properties

State at 25 °C	Solid
Melting point	1,050 °C · 1,922 °F · 1,323.15 K
Boiling point	3,200 °C · 5,792 °F · 3,473.15 K
Density	10 g/cm³
Specific heat capacity	0.12 J/(g·K)
Crystal structure	Face-centred cubic

Isotopes & abundance

Natural occurrence	From radioactive decay
Radioactive	Yes
Most stable isotope	²²⁷Ac — half-life 21.772 years
Known isotopes	33
Abundance in Earth's crust	0 mg/kg

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Actinium electron configuration

Actinium has 89 electrons. Written in short form, its ground-state configuration is [Rn] 6d¹ 7s² — the electrons of the noble gas radon, plus 6d¹ 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s².

An exception to the filling rule. The simple aufbau (Madelung) order predicts 5f¹, 6d⁰, but actinium actually has 5f⁰, 6d¹. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.

6d↑

7s↑↓

Discovery and name

Discovered

1899 — André-Louis Debierne; independently Friedrich Oskar Giesel (1902) · France / Germany.

Name

Greek aktis, ray.

Known for

Glows blue in the dark from its radioactivity. Actinium-225 is used in targeted alpha therapy for cancer.

Isotopes of actinium

Actinium has no stable isotopes. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²²⁷Ac	89	138	21.772 years

33 isotopes of actinium are known in our data set (half-lives as published in nuclear data tables).

Actinium and its neighbours in the actinide series

Neighbouring elements in the same f-block series:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Actinium	89	[227]	1,050 °C	3,200 °C	10 g/cm³	1.1
Thorium	90	232.04	1,750 °C	4,785 °C	11.7 g/cm³	1.3
Protactinium	91	231.04	1,572 °C	—	15.4 g/cm³	1.5
Uranium	92	238.03	1,135 °C	4,131 °C	19.1 g/cm³	1.38

Frequently asked questions

6 answers

What is the atomic mass of actinium?

Actinium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [227], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does actinium have?

A neutral actinium atom has 89 protons and 89 electrons. Its most stable isotope, actinium-227, has 138 neutrons.

What is the electron configuration of actinium?

[Rn] 6d¹ 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s²).

What are the melting and boiling points of actinium?

Actinium melts at 1,050 °C (1,922 °F) and boils at 3,200 °C (5,792 °F).

Is actinium a metal?

Actinium is a metal. It is in the f-block, period 7, and is a solid at room temperature.

Who discovered actinium?

1899 — André-Louis Debierne; independently Friedrich Oskar Giesel (1902) (France / Germany).