Actinium (Ac)
Actinide · Radioactive
Actinium gives its name to the actinide series. This rare, intensely radioactive metal glows faint blue in the dark and is now precious in medicine: Ac-225 powers some of the most promising targeted cancer therapies.
Position on the periodic table
Atomic properties
| Atomic number | 89 |
|---|---|
| Atomic mass | 227 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1 |
| Noble gas shorthand | [Rn] 6d1 7s2 |
| Electrons per shell | 2, 8, 18, 32, 18, 9, 2 |
| Valence electrons (outer shell) | 2 |
| Common oxidation states | +3 |
| Electronegativity (Pauling) | 1.1 |
| Covalent radius (approx.) | 215 pm |
| First ionization energy | 499 kJ/mol |
| Electron affinity | 33.8 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 10 g/cm³ |
| Melting point | 1226.8 °C (1500 K) |
| Boiling point | 3226.8 °C (3500 K) |
| Appearance | Not available |
| Radioactive | Yes |
| Origin | Occurs naturally |
Electron configuration of actinium
Actinium's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1, usually shortened to [Rn] 6d1 7s2. Its electrons occupy 7 shells (2, 8, 18, 32, 18, 9, 2), placing it in period 7 of the f-block. Explore it interactively in the electron configuration calculator.
Uses of actinium
- Targeted alpha therapy for cancer (Ac-225)
- Neutron sources for research
Biological role: None.
Occurrence: Trace amounts in uranium ores; medical supplies are reactor- or accelerator-made.
Common compounds of actinium
Ac2O3
Actinium(III) oxide
History and discovery
Discovered: 1899 — Friedrich Oskar Giesel. Name origin: From Greek aktis, “ray” or “beam”.
André-Louis Debierne, a friend of the Curies, identified actinium in pitchblende residues in 1899.
Safety notes
Severely radiotoxic; specialist facilities only.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Actinium
1. What is the chemical symbol of Actinium?
2. What is the atomic number of Actinium?
3. Which category does Actinium belong to?
4. What is the state of Actinium at room temperature?
5. Which period of the periodic table is Actinium in?
Actinium FAQs
What is the atomic number of actinium?
Actinium's atomic number is 89 — every actinium atom has 89 protons in its nucleus.
What is the symbol for actinium?
Ac. From Greek aktis, “ray” or “beam”.
Is actinium a metal, nonmetal, or metalloid?
Actinium is classified as a actinide.
What state is actinium at room temperature?
At about 25 °C, actinium is a solid. It melts at 1226.8 °C.
What is actinium used for?
Key uses include: targeted alpha therapy for cancer (ac-225); neutron sources for research.
Related elements
Keep working with this element
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