Actinium symbol Ac is a radioactive metallic element found in all uranium ores. The atomic number of actinium is 89; the element is in the actinide series of the periodic table.

Actinium: Atomic Information
Actinium was discovered in 1899 by the French chemist André Louis Debierne.
The element is found in uranium ores to the extent of 2 parts to every 10 billion parts of uranium.
Two naturally occurring isotopes of actinium are known:

                       1. Actinium-227 is a member of the actinium series, called the actinium decay series, resulting from
                           the radioactive decay of uranium-235. It has a half-life of 21.8 years.
                       2. Actinium-228, is a member of the thorium series resulting from the decay of thorium-232. This isotope,
                          known also as a mesothorium-2, has a half-life of 6.13 hours.

Isotopes ranging in mass number from 209 to 234 are known.
Actinium melts at about 1050° C (about 1922° F), boils at about 3200° C (about 5792° F), and has a specific gravity of about 10.