ElementsWhat is actinium?
Actinium is a soft, silvery, radioactive metal that glows pale blue in the dark. It is the first of the actinides.
What is actinium used for?
Research, and a radioactive form that is being developed for cancer treatment.
Where the name comes from
From the Greek word aktis, ray or beam.
Key facts
- Symbol
- Ac
- Atomic number
- 89
- Atomic mass
- 227.02775 u
- Family
- Actinide
- Period
- 7
- Group
- none (f-block)
- Block
- f
- State at room temperature
- Solid
- Melting point
- 1,051 °C (1,324 K, 1,924 °F)
- Boiling point
- 3,198 °C (3,471 K, 5,788 °F)
- Density
- 10.07 g/cm³
- Electrons per shell
- 2, 8, 18, 32, 18, 9, 2
- Electron configuration
- [Rn]7s2 6d1
- Common oxidation states
- +3
- Electronegativity (Pauling)
- 1.1
- First ionisation energy
- 5.17 eV
- Electron affinity
- no value
- Atomic radius (van der Waals)
- 260 pm
- Discovered
- 1899
Place in the periodic table
Actinium is element number 89. It sits in period 7 (row 7) of the periodic table, in the separate row of actinides drawn below the main table, and belongs to the f-block. Just before it comes radium (88), and just after it comes thorium (90).
Family: Actinides.
Physical properties
At room temperature actinium is a solid. It melts at 1,051 °C and boils at 3,198 °C, so it stays liquid over a range of 2,147 degrees. Its density is 10.07 grams per cubic centimetre, so a one-litre block would weigh about 10.07 kilograms. By density it ranks 26th among the 96 elements with a measured value.
Inside the atom
An atom of actinium has 89 protons in its nucleus and, when neutral, 89 electrons around it. The electrons are arranged in 7 shells: 2, 8, 18, 32, 18, 9, 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is [Rn]7s2 6d1.
Its electronegativity on the Pauling scale is 1.1. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 5.17 electronvolts (about 499 kilojoules per mole). One mole of actinium weighs 227.02775 grams, so a single gram contains about 2.65 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains actinium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.