Actinium is in Group 3 of the periodic table. Its chemical properties are similar to those of lanthanum and of members of the actinide series, of which it is usually considered the first member. It reacts with water to form an insoluble hydroxide; with halides to form a trifluoride, trichloride, bromide, or iodide; with oxalic acid to form the oxalate; with oxygen or sulfur to form the sesquioxide or sesquisulfide.
Actinium was first recognized in 1899 by André Debierne in uranium residues from pitchblende after the radium was extracted by Pierre and Marie Curie. It was later found to be identical with an element discovered in 1902 by Fritz Giesel and which he called emanium.
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