Element 89 · Actinide
Actinium (Ac) — Element 89
Greek aktis — ray
Actinium (Ac) is an actinide in Period 7 (f-block) of the periodic table with atomic number 89, standard atomic weight [227] u, and ground-state electron configuration [Rn] 6d¹ 7s². At standard temperature and pressure it is classified as a solid with 3 valence electrons and common oxidation states +3.
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What are the atomic properties and physical constants of Actinium?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 89
- Standard atomic mass
- [227] u
- Chemical family
- Actinide
- Ground-state configuration
- [Rn] 6d¹ 7s²
- Electrons per shell
- 2, 8, 18, 32, 18, 9, 2
- Valence electrons
- 3
- Common ion charges / oxidation states
- +3
- Phase at STP
- Solid
- Pauling electronegativity
- 1.10 Pauling
- First ionization energy
- 499 kJ/mol
- Atomic radius
- —
- Melting point
- 1,050 °C
- Boiling point
- 3,200 °C
- Discovery year
- 1899
- Table coordinate
- Period 7, f-block
- Radioactivity
- Radioactive (no stable isotopes)
How are electrons arranged across Actinium’s shells?
A neutral Actinium atom contains 89 electrons distributed across 7 occupied principal energy shells ([Rn] 6d¹ 7s²). The table below compares Actinium’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Ac | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 18 e⁻ | 18 e⁻ |
| n = 4 | N | 32 e⁻ | 32 e⁻ |
| n = 5 | O | 18 e⁻ | 50 e⁻ |
| n = 6 | P | 9 e⁻ | 72 e⁻ |
| n = 7 | Q | 2 e⁻ | 98 e⁻ |
Frequently Asked Questions about Actinium (Ac)
What is the atomic mass and atomic number of Actinium (Ac)?
Actinium (Ac) has atomic number Z = 89 (89 protons in its nucleus) and an IUPAC standard atomic weight of [227] u. It sits in Period 7 in the f-block and is classified as an actinide.
What is the ground-state electron configuration of Actinium?
The neutral ground-state electron configuration of Actinium (Z = 89) is [Rn] 6d¹ 7s². Across its 7 occupied principal shells (n = 1 to n = 7), the electrons per shell are 2, 8, 18, 32, 18, 9, 2, with 3 valence electrons participating in bonding.
What are the common ion charges and electronegativity of Actinium?
Actinium commonly forms +3 oxidation states or ion charges in compounds. Its Pauling electronegativity is 1.10 and its first ionization energy is 499 kJ/mol.
When was Actinium discovered and what is it used for?
Actinium was discovered in 1899 (Greek aktis — ray). Cancer therapy research
How does Actinium compare to neighboring elements?
Other actinide elements: