research on the transuranium elements (although it had been proposed as early as 1892 by British chemist Henry Bassett).

Actinium reacts rapidly with oxygen and moisture in air, forming a white coating of actinium oxide that impedes further oxidation. As with most lanthanides and actinides, actinium exists in the oxidation state +3, and the Ac³⁺ ions are colorless in solutions. The oxidation state +3