Chemical element · Atomic number 89
Actinium
Actinium in the periodic table: atomic number 89, electron configuration, atomic mass, physical data, oxidation states, media credit and visible sources.
Actinide
solid
[227.02775] u
Documented element sampleActinium 225 in a vial at Oak Ridge National Laboratory; the blue glow arises because alpha particles ionise the air.
Image credit: Oak Ridge National Laboratory
Auto-oriented, limited to 1600 × 1200 pixels and re-encoded as WebP; the subject was not altered.
Atomic classification
Shell occupancy
Actinium in the Bohr shell model
This shows the electron distribution of the neutral atom in a simplified shell model.
- K · n=1
- 2 electrons
- L · n=2
- 8 electrons
- M · n=3
- 18 electrons
- N · n=4
- 32 electrons
- O · n=5
- 18 electrons
- P · n=6
- 9 electrons
- Q · n=7
- 2 electrons
- Electron configuration
- [Rn]7s2 6d1
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 9 · 2
- Group
- not reported
- Period
- 7
- Block
- F
- Element category
- Actinide
Physical and chemical properties
- Atomic mass
- [227.02775] u
- Standard state
- solid
- Density
- 10.07 g/cm³
- Melting point
- 1,324 K
- Boiling point
- 3,471 K
- Electronegativity
- 1.1 (Pauling)
- First ionisation energy
- 5.17 eV
- Oxidation states
- +3
- Discovery
- 1899
Safety and periodic classification
Safety
Safe handling cannot be inferred from Actinium's position in the periodic table alone. Laboratory, classroom and disposal decisions must follow the documentation for the exact material and its safety data sheet.
Position and comparison
Actinium is in period 7 and the F block. Its direct neighbours by atomic number are Radium and Thorium. The recorded Pauling electronegativity is 1.1. Periodic trends are compared only through the separately sourced neighbouring values.
Sources and scope
PubChem attributes element data to sources including IUPAC, NIST and IAEA. Quanta stores the referenced snapshot locally and leaves unknown values unavailable.
Where Actinium sits among the 118 elements
The table compares the reported values for Actinium with every other element that carries the same value in the snapshot. A rank is not a judgement, it is orientation: it says how unusual this value is across the periodic table.
| Value | Actinium | Position in the data set |
|---|---|---|
| Melting point | 1,324 K | Rank 45 of 103 elements with a reported value. Highest: Carbon. |
| Boiling point | 3,471 K | Rank 30 of 93 elements with a reported value. Highest: Rhenium. |
| Density | 10.07 g/cm³ | Rank 26 of 96 elements with a reported value. Highest: Osmium. |
| Electronegativity (Pauling) | 1.1 | Rank 85 of 95 elements with a reported value. Highest: Fluorine. |
| Ionisation energy | 5.17 eV | Rank 97 of 102 elements with a reported value. Highest: Helium. |
All values and ranks come from the PubChem Periodic Table snapshot, retrieved on 2026-07-15. Elements without a reported value are excluded, so the population differs per row.
The neighbours of Actinium in the periodic table
- Place in the table
- No group number, period 7, F block. Classified as actinide, solid at standard conditions.
- Direct neighbours
- Atomic number 89. Preceded by Radium, followed by Thorium.
- Discovery
- Recorded in the snapshot as: 1899. Who, where and under what circumstances is not part of that record, so this page carries the sourced year and no discovery narrative.
- Oxidation states
- +3. They follow from the electron configuration [Rn]7s2 6d1 and indicate which compounds to expect.
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