The chemical element with the longest name is Ununoctium, which has the symbol Uuo.

While it has a long name, it is actually a temporary name used until a permanent name is agreed upon by the International Union of Pure and Applied Chemistry (IUPAC). This name is derived from the number of protons in the atom (118), using the Latin roots "un" for 1, "bi" for 2, and "oct" for 8.