Actinium

Actinide · atomic number 89

89227AcActiniumSolidActinide

Actinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.

Properties of Actinium

Element category	Actinide
Atomic number	89
Atomic mass	227 u
Phase at STP	Solid
Group	3
Period	7
Block	f
Electron configuration	[Rn] 6d1 7s2
Electrons per shell	2, 8, 18, 32, 18, 9, 2
Electronegativity (Pauling)	1.1
Melting point	1500 K (1227 °C)
Boiling point	3500 K (3227 °C)
Density	10 g/cm3
Discovered by	Friedrich Oskar Giesel

← Radium · Periodic table · Thorium → Th

About the actinide group

Mostly radioactive, heavy metals — several made only in reactors — whose fission powers nuclear energy and weapons.

Data: Periodic-Table-JSON (CC BY-SA 3.0) · stored statically on our server. https://en.wikipedia.org/wiki/Actinium

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11 elements: Elements that readily gain or share electrons — the building blocks of water, air, living tissue and most organic chemistry.

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6 elements: Full outer electron shells make these gases almost inert: they barely react, glow in discharge lamps and are used to create unreactive atmospheres.

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15 elements: The rare-earth “inner transition” metals that give strong permanent magnets, screen phosphors and catalyst polish.

🧪 Actinide

15 elements: Mostly radioactive, heavy metals — several made only in reactors — whose fission powers nuclear energy and weapons.

🧪 Unknown properties

8 elements: Superheavy synthetic elements made atom-by-atom, existing for fractions of a second; their true chemistry is still unmeasured.