Actinium
[Rn] 6d¹ 7s² · 2 · 8 · 18 · 32 · 18 · 9 · 2
The first actinide and the element that named the family. Actinium is half again as radioactive as radium and glows pale blue in the dark by ionising the air around it.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 89
- Atomic mass
- 227 а.е.м. ≈
- Electron configuration
- [Rn] 6d¹ 7s²
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 9 · 2
- Block
- d-block
- Group
- no data
- Period
- 7
- Electronegativity (Pauling)
- 1.1
- Electronegativity (Allen)
- 1
- Atomic radius
- 260 pm
- Covalent radius
- 215 pm
- Van der Waals radius
- 260 pm
- Ionisation energy 1
- 499 kJ/mol
- Ionisation energy 2
- 1 170 kJ/mol
- Electron affinity
- 33.8 kJ/mol
- Common oxidation states
- +3
- Oxidation states
- +2, +3
Physical properties
- State at 25 °C
- solid
- Density
- 10.07 g/cm³
- Melting point
- 1 500 K · 1 226.9 °C ≈
- Boiling point
- 3 500 K · 3 226.9 °C ≈
- Speed of sound
- no data
Thermal properties
- Heat of fusion
- 14 kJ/mol ≈
- Heat of vaporisation
- 400 kJ/mol ≈
- Specific heat capacity
- 0.12 J/(g·K)
- Thermal conductivity
- 12 W/(m·K)
Mechanical properties
- Young's modulus
- no data
- Shear modulus
- no data
- Bulk modulus
- no data
- Poisson ratio
- no data
- Mohs hardness
- no data
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- 220 nΩ·m
- Magnetic ordering
- non-magnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- face-centred cubic (fcc)
- Lattice constants
- a = 567 pm
Abundance
- In the crust
- 5.5·10⁻¹⁰ mg/kg
- In the ocean
- no data
- In the universe
- no data
- In the human body
- no data
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 225Ac | — | 9.92 d | α |
| 227Ac | — | 21.77 yr | β− |
| 228Ac | — | 6.15 h | β− |
Actinium-227, half-life 21.8 years, is the main natural isotope. Actinium-225, half-life 10 days, is the key alpha therapy isotope, and its scarcity is the field's main bottleneck today.
Discovery
- Year of discovery
- 1899
- Discovered by
- André-Louis Debierne
- Where
- France
- Origin of the name
- from Greek aktis (ray)
History
André Debierne isolated actinium in 1899 from pitchblende processing residues in the Curie laboratory. The name is from the Greek for ray.
Where it occurs
Around 5·10⁻¹⁰ mg/kg of the crust: traces in uranium ores, where actinium is made and destroyed continuously.
How it is produced
By neutron irradiation of radium-226 in a reactor, or extraction from thorium-229 stocks. It is made in milligrams.
Role in living things
No biological role. Like radium, actinium accumulates in bone.
Safety
Extremely radiotoxic: on ingestion it exceeds plutonium in hazard.
Uses
- Actinium-225 in targeted alpha therapy — one of the most promising directions in oncology
- A neutron source when paired with beryllium
- Thermoelectric power sources (experimentally)
Curiosities
- Actinium glows pale blue in the dark, like radium, from ionising the air.
- World production of actinium-225 is under two grams a year, and demand vastly exceeds it.
- The name means ray, and it named the entire actinide family.
- A single actinium-225 atom throws four alpha particles in succession as it decays — which is exactly why it is so effective against tumours.