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 travelling faster than light to become neutral.

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.

• (element 137): Fy (symbol), untriseptium (designation using default convention for unnamed elements), Uts (symbol)