Actinium

The first of the actinides, the row it names, and so radioactive that it glows pale blue in the dark.

At a glance

Symbol: Ac
Atomic number: 89
Mass number of its longest-lived isotope: [227]
Group: none; it sits in the actinide row below the table
Period: 7
Family: actinide
Electron configuration: [Rn] 6d¹ 7s²
Electronegativity: 1.10
Common ions: Ac³⁺
Isotopes: none are stable; ²²⁷Ac, with a half-life of about 22 years, is the natural one

On the table

Period 7, in the actinide row set below the main table. The other actinides are tinted, and every cell opens its own article.

Electrons and bonding

Radioactive, like every actinide. It gives up electrons to form positive ions.

Properties

State at 25 °C: solid
What it looks like: soft, silvery-white radioactive metal that glows pale blue in the dark
Melting point: 1051 °C
Boiling point: 3198 °C
Density: 10.1 g/cm³

Where it is found

Present in uranium ores in tiny traces. Most is made by bombarding radium with neutrons in reactors.

Discovery and name

André-Louis Debierne reported it in 1899 in residues from pitchblende, and Friedrich Oskar Giesel found it independently in 1902. The name comes from the Greek aktis, ray.

What it is used for

Actinium-225 is being developed for targeted alpha therapy against cancer, and mixtures of actinium and beryllium have served as neutron sources.

In compounds

Ac₂O₃actinium oxideAcCl₃actinium chloride

Handle with care

Intensely radioactive, handled only in specialized facilities.