Although Richard Feynman noted that a simplistic interpretation of the relativistic Dirac equation runs into problems with electron orbitals at "Z" &gt; 1/� = 137, suggesting that neutral atoms cannot exist beyond untriseptium, and that a periodic table of elements based on electron orbitals breaks down at this point, a more thorough analysis calculates the limit to be "Z" ≈ 173, meaning that neutral atoms most likely cannot exist with atomic number greater than 173. This makes unsepttrium theoretically the heaviest neutral element possible.