Unsepttrium

Unsepttrium (), also known as dvi-francium or element 173, is a possible chemical element which has not been observed to occur naturally, nor has it yet been made. Due to instabilities, it is not known if this element is possible, as the instabilities may hint that the periodic table ends soon after the island of stability at unbihexium; however, if possible, it is likely the heaviest possible neutral element. Its atomic number is 173 and its atomic symbol is Ust.
The name "unsepttrium" is a temporary IUPAC systematic element name.
Significance.
Although Richard Feynman noted that a simplistic interpretation of the relativistic Dirac equation runs into problems with electron orbitals at "Z" &gt; 1/α = 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.