Periodic table › Actinium
1. Identity and classification
Dense, silvery, radioactive f-block metal that tarnishes rapidly in air
- Name and symbol
- Actinium (Actinium) · Ac
- Atomic number
- 89
- Relative atomic weight / mass number
- [227]
- Group · period · block
- 3 · 7 · f
- Category
- Actinoid
- State near room temperature
- Solid
- Stable isotopes
- No stable isotopes
- CAS number
- 7440-34-8
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 89 · 89
- Neutrons in reference isotope
- 138 (227Ac)
- Full electron configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s²- Abbreviated configuration
[Rn]7s² 6d¹- Electrons per shell
- 2, 8, 18, 32, 18, 9, 2
- Listed oxidation states
- +3
- Electronegativity
- 1.1 (Pauling)
- First ionization energy
- 498.829 kJ/mol · 5.17 eV
- Electron affinity
- Not established
- Covalent radius
- 215 pm
- van der Waals radius
- 260 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 10.07 g/cm³
- Melting point
- 1324 K · 1050.85 °C · 1923.53 °F
- Boiling point
- 3471 K · 3197.85 °C · 5788.13 °F
- Crystal structure
- FCC
- Specific heat
- 0.12 J/(g·K)
- Thermal conductivity
- 12 W/(m·K)
- Enthalpy of fusion
- 14 kJ/mol
- Enthalpy of vaporization
- 400 kJ/mol
- Magnetic ordering
- Paramagnetic
- Mohs hardness
- Not established
- Speed of sound
- Not established
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
The +3 state is common in this series, with important element-specific exceptions.
Selected compounds: AcO / Ac₂O₃ (Oxide) · AcClₙ (Chloride) · AcFₙ (Fluoride)
Reaction reference
Many members oxidize in air; behavior depends on the element and oxidation state.
Water reactivity varies across this series and with surface conditions.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 227Ac | Radioactive | — | 21.772 Y |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- 1899 CE
- Scientists associated with discovery
- Friedrich Oskar Giesel & André-Louis Debierne
- Location
- Germany / France
- Origin
- Trace Natural / Synthetic
- Listed crustal abundance
- Not established
Minerals and naming reference
Trace in Uranium ores; neutron irradiation of ²²⁶Ra
Named from scientific/historical roots associated with Actinium (Ac), discovered or characterized by Friedrich Oskar Giesel & André-Louis Debierne.
7. Applications
Actinium-225 (²²⁵Ac) targeted alpha-particle cancer therapy (TAT), neutron-source generators (Ac-Be)
Detailed applications by field
- everyday
- Actinium-225 (²²⁵Ac) targeted alpha-particle cancer therapy (TAT), neutron-source generators (Ac-Be)
- industry
- Used in actinoid processing, specialized alloys, and chemical manufacturing (Actinium-225 (²²⁵Ac) targeted alpha-particle cancer therapy (TAT), neutron-source generators (Ac-Be)).
- electronics
- Applied in electronic components, semiconductors, or functional coatings where Actinium's electronic structure ([Rn] 6d¹ 7s²) provides target conductivity.
- medicine
- Diagnostic imaging, biomedical instrumentation, or pharmaceutical synthesis research.
- energy
- Utilized in power generation, battery materials, catalysts, or nuclear technology involving Actinium.
- aerospace
- High-performance materials, structural alloys, or space instrumentation utilizing Actinium.
- agriculture
- Not used in agriculture due to radioactivity or heavy-metal toxicity.
- science
- Spectroscopic standard, crystallography, and quantum chemistry studies of Period 7 f-block elements.
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Classified as highly radioactive / synthetic in human and biological systems.