Actinium gets its name from the Greek word aktinos which means "ray" or "beam." This is because actinium is a radioactive metal. Actinium was first discovered in 1899 by French scientist André Debierne and later rediscovered by German chemist Friedrich Otto Giesel in 1902.

Actinium has 26 known isotopes, the most common of which is ²²⁷Ac. This isotope of actinium forms during the decay process of ²³⁵U, and has a half-life of about 21.6 years. Regarding the Periodic Table of Elements, actinium has a unique aspect that is only shared with the element lanthanum. Certain elements following actinium are called the actinides, and include the elements from actinium to lawrencium. These elements are grouped together because each of them add electrons to an inner orbital shell, the second shell from the outer valence shell, and each has similar chemical and physical properties. Another name for the actinides is the Second Inner Transition Series.

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