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Actinium

Element 89 · Actinide

Specimen

Atomic number
89
Atomic mass
[227]
State at room temperature
Solid
Melting point
1,051 °C
Boiling point
3,198 °C
Density
10.07 g/cm³
Discovered
1899

Mass number of the longest-lived isotope, from IUPAC.1 Other physical data from PubChem.2

Can I eat it?

In food and water
Radioactive. All actinium is radioactive. Uranium ores contain traces of it. Its radiation is the main danger. 3

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The atom

Ac
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 18
4 (N) 32
5 (O) 18
6 (P) 9
7 (Q) outermost 2
Total89

Configuration [Rn] 6d¹ 7s²

6d ↑
7s ↑ ↓
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 89 protons. The outermost shell is shown in red. Electrons do not travel on rings: they fill orbitals, regions where an electron is likely to be found. The boxes are those orbitals for the electrons outside the Rn core, with one arrow per electron.

Related elements

Actinides
Th Thorium Pa Protactinium U Uranium Np Neptunium Pu Plutonium Am Americium Cm Curium Bk Berkelium Cf Californium Es Einsteinium Fm Fermium Md Mendelevium No Nobelium Lr Lawrencium

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Sources

  1. 1

    IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024. Radioactive Elements .

    Mass number and half-life of the longest-lived isotope of the 34 elements with no standard atomic weight.

  2. 2

    National Center for Biotechnology Information (PubChem), 2026. Periodic Table of Elements .

    Electron configuration, melting and boiling points, density and discovery year for all 118 elements.

  3. 3

    Royal Society of Chemistry, Periodic Table, 2026. Actinium - Element information, properties and uses .

    Reference page; its Biological role and Natural abundance sections are quoted.

    • Actinium has no known biological role. It is toxic due to its radioactivity. Natural abundance Actinium used for research purposes is made by the neutron bombardment of radium-226. Actinium also occurs naturally in uranium ores.