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Periodic Table

The elements with IUPAC/CIAAW standard atomic weights — browse by position or list.

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Element: Actinium
Atomic number: 89
Standard atomic weight: Not assigned
Period: 7
Group: — (Actinoid)

No CIAAW standard atomic weight is assigned: this radioactive element has no stable or characteristic terrestrial isotopic composition. Isotope mass numbers are deliberately not shown in its place.

Groups 1–2 Groups 3–12 Groups 13–18 Lanthanoids (57–71) & actinoids (89–103), shown outside the 18 groups

Scroll the table horizontally to view all 18 groups.

Standard atomic weights: IUPAC/CIAAW abridged standard atomic weights, 2024 table (elements shown with “–”/“Not assigned” have no standard atomic weight). Layout — periods and groups 1–18 — follows the IUPAC periodic-table convention; lanthanoids (57–71) and actinoids (89–103) are shown outside the 18 groups. Need a formula weight? Use the Molecular Weight Calculator.

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