Actinium is the first member of the Actinide series on the periodic table of elements. It is located on the second row below the periodic table. It is a silvery-white radioactive metal with a melting point of 1050 degrees Celsius. It was discovered in 1899 by Andre Debierne. It has a half-life of only 21.7 years. It is so radioactive that its radiation ionizes the air around it so that it glows blue.  

Actinium's uses are limited due to its radioactivity. However, this property makes it useful in radiotherapy.