Ununoctium also known as eka-radon or element 118, is the temporary IUPAC name for the transactinide element having the atomic number 118 and temporary element symbol Uuo. On the periodic table of the elements, it is a p-block element and the last one of the 7th period. Ununoctium is currently the only synthetic member of Group 18. It has the highest atomic number and highest atomic mass of all discovered elements.


The radioactive ununoctium atom is very unstable, and since 2002, only three atoms (possibly four) of the isotope 294Uuo have been detected. While this allowed for very little experimental characterization of its properties and possible compounds, theoretical calculations have resulted in many predictions, including some unexpected ones. For example, although ununoctium is a member of Group 18, it is probably not a noble gas, as are all the other Group 18 elements. It was formerly thought to be a gas but is now predicted to be a solid under normal conditions.