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[ ? ] Transition metal:

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 is considered a member of the "Transition metal" class of elements. At a pressure of 101.325 kPa, it has a boiling point of 3,198 °C and a melting point of 1,051 °C.


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.[1]


  1. William M.Haynes (2010). Handbook of Chemistry and Physics, 91st Edition. CRC Press. ISBN 1-4398-2077-5.