feynmanium

Feynman +‎ -ium.
Named after Richard Feynman, who used a back-of-the-envelope calculation using classical physics to determine that element 137 is the last stably neutrally charged element, as larger elements would need electrons traveling faster than light to become neutral. (This problem goes away when relativity is considered.)

feynmanium (uncountable)

1. The hypothetical chemical element with atomic number 137.

This is not (yet) an official designation, but is a nickname used by some members of the nuclear physics community. The element will not have an official name until it is actually discovered.

• (element 137): Fy (symbol), untriseptium (systematic element name), Uts (symbol)