Fermium is a
synthetic element with symbol
Fm and
atomic number 100. It is a member of the
actinide series. It is the heaviest element that can be formed by
neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although pure fermium metal has not yet been prepared. A total of 19 isotopes are known, with
257Fm being the longest-lived with a half-life of 100.5 days.