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 (blue). 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.
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