Actinium

**Definition:** A trivalent radioactive element and the prototypical member of the actinide family. It has the atomic symbol Ac, and atomic number 89. Its principal isotope is 227 and it decays primarily by beta-emission.

**Tree numbers:** - D01.268.271.100.033 - D01.268.556.025 - D01.496.749.305.100.033 - D01.552.020.042 - D01.552.544.025