Actinium (Ac)

Actinides · Period 7 · Solid

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Element identity

Atomic number (Z): 89
Symbol: Ac
Atomic weight / isotope mass number: [227]
Category: Actinides
Physical state (reference): Solid
Period: 7
Group (reference): 3
Block: f

Physical properties

Appearance: Silvery-white, glowing with an eerie blue light, sometimes with a golden cast
Density: 10 g·cm⁻³
Melting point: 1500 K / 1227 °C / 2240 °F
Boiling point: 3500±300 K / 3200±300 °C / 5800±500 °F
Sublimation point: Not available

Atomic and material properties

Electron configuration: [Rn] 6d¹ 7s²
Electrons per shell: K2 L8 M18 N32 O18 P9 Q2
Oxidation states: 3, 2
Electronegativity (Pauling): 1.1
Atomic radius: Not available
Covalent radius: 215 pm
Molar ionization energies: 1st: 499 kJ·mol⁻¹ 2nd: 1170 kJ·mol⁻¹ 3rd: 1900 kJ·mol⁻¹ 4th: 4700 kJ·mol⁻¹
Thermal conductivity: 12 W·m⁻¹·K⁻¹
Molar heat capacity: 27.2 J·mol⁻¹·K⁻¹
Crystal structure: Face-centered cubic
CAS number: 7440-34-8

Property source: Wikipedia — Actinium, adapted from the Nuclides App snapshot. Quoted conditions and qualifiers are retained.

Sources and interpretation

Snapshot exported 24 October 2026 from the element assets used by Nuclides. Properties are a reference compilation, not a new evaluation. Missing values are shown as “Not available”. Density and phase depend on temperature, pressure and allotrope; retained conditions are part of the quoted property.

Atomic weight is dimensionless. Parentheses give uncertainty in the last quoted digits. A single integer in square brackets is an isotope mass number, not a standard atomic weight. The snapshot can differ from current evaluations; consult CIAAW standard atomic weights and the IUPAC periodic table for current values and conventions.

Element properties in the App snapshot are adapted from Wikipedia. Each element page links to its source article; text adaptations are available under CC BY-SA 4.0. No third-party images are included.

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