Fermium

Atomic number: 100

Symbol: Fm

Category: actinide 

Density at 25°C: solid: density: n/a

This element is the last of the synthetic elements that can be made by slamming neutrons in another lighter element. This seems to suggest that it is hard to get neutrons to stick into the nucleus.  It was first discovered in debris from the first hydrogen bomb explosion.

Named after Enrico Fermi.