Fermium

Fermium was found alongside einsteinium in the debris of Ivy Mike, the 1952 American hydrogen bomb test, by a team led by Albert Ghiorso. Its discovery was kept secret until 1955. It was named after the physicist Enrico Fermi, who built the first nuclear reactor.

Fermium marks a limit. It is the heaviest element that can be made by soaking lighter elements in neutrons, as happens in reactors and bomb blasts. Beyond it, the next isotopes split apart almost instantly, so heavier elements must be built by smashing nuclei together. Only tiny traces have ever been made, far too little to see. Its longest-lived form, fermium-257, has a half-life of about 100 days.