Fermium (Fm) is a radioactive, metallic element of the actinide series; atomic number 100. It was first identified by G. R. Choppin and coworkers in 1952 as a decay product of uranium-255 after it was produced in the first large hydrogen bomb explosion; it is named after the physicist Enrico Fermi. Sixteen isotopes have been identified, the most stable of which is ²⁵⁷Fm (half-life 80 days).