Ultimium is the provisional non-systematic name of a theoretical element with the symbolUl and atomic number 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 systematic nameunsepttrium (Ust) or simply element 173. On the periodic table it is the only element in the second row of g-block and is the second element of the lavoisiumfamily, located in the periodic table coordinate 6g1.
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 ramson. It is also the heaviest possible atom for the most stable isotope of an element, weighing 5121⁄6daltons.