Actinium (Ac). Diagram of the nuclear composition and electron configuration of an atom of actinium-227 (atomic number: 89), the most stable isotope of this element. The nucleus consists of 89 protons (red) and 138 neutrons (yellow). 89 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Actinium is an actinide in period 7, and the f-block of the periodic table. Actinides are soft radioactive metals. Actinium-227, isolated by Debierne from uranium ores in 1899, has a half-life of nearly 22 years. Its unstable nucleus decays by emitting a beta or alpha particle to form thorium-227 or francium-223.