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
Am being an order of magnitude longer lived than ²⁴¹
Am, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.

Eighteen radioisotopes of americium, ranging from ²²⁹Am to ²⁴⁷Am with the exception of ²³¹Am, have been characterized; another isotope, ²²³Am, has also been reported but is unconfirmed. The most stable isotopes are 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 (t_1/2 = 141 years). This isomer is unusual in that its half-life is far longer than that of the ground state of the same isotope.