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.