Actinium (Ac)

Actinium glows pale blue in the dark, its own radiation exciting the surrounding air — about 150 times more radioactive than radium, weight for weight. Actinium-225 has become one of the most sought-after isotopes in medicine, because a single alpha particle deposits its energy within a few cell diameters and will kill a targeted tumour cell without touching its neighbours; global supply is measured in a few grams a year. Its configuration is anomalous, [Rn] 7s² 6d¹ with no 5f electron at all, exactly as lanthanum has no 4f electron.

Properties

Atomic number

89

Symbol

Ac

Mass number of most stable isotope

[227]

Category

Actinide

Period

7

Block

d-block

Electron configuration

[Rn] 7s² 6d¹

Electrons per shell

2, 8, 18, 32, 18, 9, 2

Melting point

1,324 K — 1050.8 °C — 1923.5 °F

Boiling point

3,471 K — 3197.8 °C — 5788.1 °F

Density

10.07 g/cm³

Electronegativity

1.1 (Pauling)

Oxidation states

+3

Discovered

1899 by André-Louis Debierne

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d¹

Noble gas shorthand

[Rn] 7s² 6d¹

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 9 · Q: 2

What it is used for

• Actinium-225 for targeted alpha therapy against prostate and other cancers
• Actinium-beryllium neutron sources

Hazards

• Intensely radioactive alpha emitter; actinium-227 has a 21.8-year half-life

Anomalous electron configuration: [Rn] 7s² 6d¹ rather than the predicted 7s² 5f¹. No stable isotope; the mass given is the mass number of actinium-227, the longest-lived at 21.8 years.

Neighbours in the table