Actinium is a chemical element, having the chemical symbol Ac. Its atomic number (the number of protons) is 89. It has a standard atomic weight of 227 g•mol −1 and it is a solid in its elemental form.
Actinium has been discovered in 1899 by André-Louis Debierne, and independently by F. Giesel in 1902. This element was named from the Greek word aktis, aktinos (ακτίς, ακτίνος), meaning beam or ray.
- William M.Haynes (2010). Handbook of Chemistry and Physics, 91st Edition. CRC Press. ISBN 1-4398-2077-5.