Actinium (Ac)
Element 89 · Actinide · atomic mass [227] u · f-block, period 7
Electron shells: 2, 8, 18, 32, 18, 9, 2
Actinium is about 150 times more radioactive than radium and glows in the dark from ionising the air around it. It heads the actinide series.
Actinium glows pale blue in the dark — it is ionising the air itself.
- Atomic number
- 89
- Atomic mass
- [227] u
- Electron configuration
- [Rn] 7s² 6d¹
- Group · period · block
- f-block · 7 · f
- Oxidation states
- +3
- Electronegativity
- 1.1 (Pauling)
- Melting point
- 1,051 °C
- Boiling point
- 3,198 °C
- Density
- 10.07 g/cm³
- State at 20 °C
- Solid
- Appearance
- Silvery metal, glows pale blue
- Discovered
- André-Louis Debierne, 1899
89 protons, and 89 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Shown in the strip below the table
What Actinium is used for
- Targeted alpha therapy for cancer
- Neutron source for well logging
Atomic structure of Actinium
A neutral actinium atom has 89 protons and 89 electrons. Its most common isotope carries about 138 neutrons, which is where the mass number 227 comes from. Change the neutron count and you have a different isotope of actinium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 18, 9, 2. In orbital notation that is [Rn] 7s² 6d¹. As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Actinium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Actinium sits in the periodic table
Discovery and naming
Actinium was discovered by André-Louis Debierne in 1899. Its name comes from Greek aktis, "ray".
Every isotope of actinium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Actinium?
Actinium has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 227 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Actinium have?
Actinium has 89 protons, because the atomic number is the proton count and that is what makes an atom Actinium at all. A neutral Actinium atom has the same number of electrons, 89. The neutron count varies by isotope; the most common one has about 138, since 227 − 89 = 138.
What is the electron configuration of Actinium?
The electron configuration of Actinium is [Rn] 7s² 6d¹. Its electrons are arranged 2, 8, 18, 32, 18, 9, 2 across 7 shells.
Is Actinium a metal?
Yes. Actinium is classified as a actinide, on the metallic side of the table.
What group and period is Actinium in?
Actinium sits in the f-block strip below the main table, in period 7, among the actinides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Actinium?
Actinium most commonly forms a 3+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound.
What is the melting point of Actinium?
Actinium melts at 1,051 °C and boils at 3,198 °C. That puts it in the solids at everyday temperatures.
What is Actinium used for?
Targeted alpha therapy for cancer, Neutron source for well logging. Actinium is about 150 times more radioactive than radium and glows in the dark from ionising the air around it.
Who discovered Actinium, and where does its name come from?
Actinium (Ac) was discovered by André-Louis Debierne in 1899. The name comes from Greek aktis, "ray".
Is Actinium radioactive?
Yes. Every isotope of Actinium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.