Americium is a chemical element, having the chemical symbol Am. Its atomic number (the number of protons) is 95. It has a standard atomic weight of 243 g•mol −1 and it is a solid in its elemental form.
Americium was the fourth transuranic element to be identified. It was discovered by Glenn T. Seeborg at the wartime (World War II) metallurgical laboratory of the University of Chicago in 1944. The plutonium isotopes Pu-241 and Pu-243 undergo β− decay, transforming into isotopes of americium.