Actinium
The first actinide, intensely radioactive and faintly glowing.
Inside the Actinium atom
Switch between Bohr and Quantum Cloud modes to compare a simple teaching model with a more realistic probability-based view, and follow the guided tour to explore the Actinium atom step by step.
Electron configuration
[Rn] 6d1 7s2
A neutral Actinium atom has 89 electrons (equal to its proton count). Choosing a different isotope above changes only the neutron count.
Shell distribution
Electrons fill inner shells before outer ones; the outermost (valence) shell drives the element's chemistry.
Physical & atomic properties
- State (room temp)
- Solid
- Melting point
- 1323 K (1050 °C)
- Boiling point
- 3471 K (3198 °C)
- Density
- 10.07 g/cm³
- Electronegativity
- 1.1 Pauling
- Atomic radius
- 195 pm
- 1st ionization energy
- 499 kJ/mol
- Category
- Actinides
Discovery & naming
- Discovered
- 1899
- Discovered by
- André-Louis Debierne
- Origin of name
- Greek 'aktinos', meaning ray.
Notable uses
Neutron sources and targeted cancer therapy.
Where Actinium comes from
Neutron star mergers
Occurs on Earth only as a fleeting decay product of uranium and thorium, so it inherits their r-process origin.
Simplified origin map — many elements form through more than one astrophysical pathway.
Summary
- Atomic number
- 89
- Atomic mass
- [227]
- Category
- Actinides
- Group · Period
- — · 7
- Block
- f-block
- Shells
- 2 · 8 · 18 · 32 · 18 · 9 · 2