Isotopes of americium

Americium (95Am) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was 241Am in 1944. The artificial element decays by ejecting alpha particles. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom). Despite 243
being an order of magnitude longer lived than 241
, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.

Main isotopes of americium (95Am)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
241Am syn 432.2 y SF
α 237Np
242m1Am syn 141 y IT 242Am
α 238Np
243Am syn 7370 y SF
α 239Np

Nineteen radioisotopes of americium—223Am, 229Am, 230Am, and those ranging from 232Am to 247Am—have been characterized, with the most stable being 243Am with a half-life of 7,370 years, and 241Am with a half-life of 432.2 years. All of the remaining radioactive isotopes have half-lives that are less than 51 hours, and the majority of these have half-lives that are less than 100 minutes. This element also has 8 meta states, with the most stable being 242m1Am (t1/2 = 141 years). This isomer is unusual in that its half life is far longer than that of the ground state of the same isotope.

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