89
Ac
Actinium
227
Actinium (Ac)
ActinidePeriod 7f-block-blockRadioactive
Actinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.
Latin name: Actinium
Atomic Number
89
Atomic Mass
227 g/mol
Phase (STP)
Solid
Electron Config.
[Rn] 6d¹ 7s²
General
SymbolAc
Atomic Number89
Atomic Mass227 g/mol
CategoryActinide
Period7
Blockf-block
Electron Configuration[Rn] 6d¹ 7s²
Valence Electrons3
Coloursilvery metallic
CAS NumberCAS7440-34-6
Physical Properties
Phase (STP)Solid
Density10.07 g/cm³
Melting Point1226.85°C · 2240.33°F · 1500.00 K
Boiling Point2926.85°C · 5300.33°F · 3200.00 K
Atomic Properties
Atomic Radius260 pm
Electronegativity1.1 Pauling
Ionization Energy499 kJ/mol
Oxidation States+3
Ion ChargeAc3+
Thermodynamic
Molar Heat27.2 J/(mol·K)
Crystal Structure
Crystal Structureface-centered cubic
Nuclear & Hazards
RadioactiveYes
Half-life21.77 years (Ac-227)
Abundance
In Universe0.000000000001%
In Earth's Crust0.000000000001%
Discovery & History
Discovery Year1899
Discovered ByAndré-Louis Debierne
Country of DiscoveryFrance
Isotopes of Actinium
| Isotope | Abundance | Half-life | Decay |
|---|---|---|---|
| Ac-225 | — | 10.0 days | α |
| Ac-227 | — | 21.772 years | β⁻, α |
| Ac-228 | — | 6.15 hours | β⁻ |