Actinium

Actinium is a soft, silvery radioactive metal and the first member of the actinide series, the row of 15 elements from actinium to lawrencium. It has the configuration 6d1 7s2, with no f electrons, and like lanthanum above it in group 3, it almost always forms the colourless Ac3+ ion. Its chemistry is so similar to lanthanum's that the two are hard to separate. Actinium glows with a pale blue light in the dark, because its intense radioactivity ionises the surrounding air.

All actinium isotopes are radioactive. Actinium-227, with a half-life of 21.8 years, occurs naturally in the uranium-235 decay chain, but only in tiny traces. Actinium-225, with a 10-day half-life, has become important in medicine: attached to molecules that seek out cancer cells, it releases four alpha particles as it decays through its short-lived daughters, destroying tumour cells while sparing more distant tissue. Supplying enough Ac-225 for clinical trials is now a major effort at national laboratories.

Discovery

André-Louis Debierne, a colleague of the Curies, reported actinium in 1899 from pitchblende residues and named it from the Greek aktis (aktinos), meaning 'ray' or 'beam'. In 1902 Friedrich Oskar Giesel independently found the same element, which he called emanium. Debierne's early samples were impure, and some historians credit Giesel with the first clear identification, but Debierne's name was kept.