A hypothetical element with atomic number 137. The significance of this number is that Richard Feynman argued that, in the case of a point nucleus, there would not be bound state solutions of the Dirac equation for atomic numbers higher than 137. It was subsequently argued that in the more realistic case of nuclei with finite volumes this difficulty does not occur until atomic numbers of about 150. It was also argued that, at atomic numbers greater than 173, the electric field would be so strong that pair production of electrons and positrons would occur.

If, as may very well be the case, it is impossible for nuclei with such high atomic numbers to form, then all these discussions would be hypothetical. However, it is not known what the maximum possible atomic number of a nucleus is.