Ac Actinium

Quick answerActinium (Ac) is an actinide with atomic number 89 and a mass number of 227 (the most stable isotope), in period 7. It was discovered by André-Louis Debierne in 1899. It is used for cancer therapy research and neutron sources. Its electron configuration is [Rn] 6d1 7s2.

Actinium is a rare radioactive metal that glows faintly blue in the dark and is being tested against cancer.

Key facts

Symbol	Ac
Atomic number	89
Atomic mass	[227] — mass number of the most stable isotope
Category	actinide
Group	— (f-block element, no group number)
Period	7
Block	f-block
Electron configuration	[Rn] 6d1 7s2
Discovered	André-Louis Debierne, 1899
Uses	cancer therapy research, neutron sources, research

Quick quiz

Three questions from the facts on this page. No sign-up, no score is saved.

1. What is the chemical symbol of Actinium?

Pa Th Ac U

2. What is the atomic number of Actinium?

89 91 88 90

3. Which category does Actinium belong to?

actinide halogen alkali metal noble gas

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Keep exploring

• Periodic table — all 118 elements
• Thorium (Th) — another actinide, atomic number 90
• Protactinium (Pa) — another actinide, atomic number 91
• Uranium (U) — another actinide, atomic number 92
• Neptunium (Np) — another actinide, atomic number 93
• Plutonium (Pu) — another actinide, atomic number 94
• Americium (Am) — another actinide, atomic number 95
• Curium (Cm) — another actinide, atomic number 96
• Berkelium (Bk) — another actinide, atomic number 97
• Californium (Cf) — another actinide, atomic number 98
• Einsteinium (Es) — another actinide, atomic number 99
• All actinide elements — on the periodic table

Sources & last updated

• CIAAW — Standard Atomic Weights
• PubChem Periodic Table
• Wikipedia — Timeline of chemical element discoveries

Last updated: 2026-11-12. Figures are shown with their year; where sources disagree, the value is left out rather than estimated.