Hesperium (also known as esperium; atomic symbol Es) was the name assigned to the element with atomic number 94, now known as plutonium. It was named in Italian Esperio after a Greek name of Italy, Hesperia, "the land of the West". The same team assigned the name Ausonium to element 93, after Ausonia, a poetic name of Italy. By comparison, Uranium has atomic number 92.
The discovery of the element, now discredited, was claimed by Enrico Fermi and a team of scientists at the University of Rome in 1934. Following the discovery of nuclear fission in 1938, it was realized that Fermi's hesperium was a mixture of barium, krypton and other elements. The actual element was discovered several years later, and named plutonium.
- Element name etymologies. Retrieved February 23, 2010.
- Enrico Fermi, Artificial radioactivity produced by neutron bombardment, Nobel Lecture, December 12, 1938.
- Sime, Ruth Lewin (2000). "The Search for Transuranium Elements and the Discovery of Nuclear Fission". Physics in Perspective 2 (1): 48–62. DOI:10.1007/s000160050036.