the element with the highest atomic mass is Oganesson (Og) with an atomic number of 118. Oganesson is a synthetic element that was first synthesized in 2002 and is highly unstable. It belongs to the group of superheavy elements and has a very short half-life. Its atomic mass is estimated to be around 294 atomic mass units (u).
However, please note that scientific research and discoveries are ongoing, and new elements may be discovered or the properties of known elements may be updated. It's always a good idea to consult the latest scientific sources for the most up-to-date information.