Ultimium

Ultimium is the provisional non-systematic name of a theoretical element with the Ul and  173. The element name comes from the Latin word ultimus, meaning "last," implying that this is the last possible element. In the scientific literature, this element is referred as the unsepttrium (Ust), dvi-Francium, or simply element 173. On the periodic table it is the only element in the second row of and is the second element of the Saturnium family, located in the periodic table coordinate 6g$1$.

Atomic
Ultimium is the last possible element before electrons in 1s orbital need to go faster than the speed of light, which is physically impossible. So any atom heavier than ultimium would only exist as a positively charged ion and therefore would not be an element. Ultimium atom acquires the first electron in 6g orbital when going from the previous element gustavon. It is also the heaviest possible atom for the most stable isotope of an element, weighing 512.