The International Union of Pure and Applied Chemistry (IUPAC) defines an element to exist if its lifetime is longer than 10 seconds, which is the time it takes for the nucleus to form an electron cloud. However, a nuclide is generally considered to exist if its lifetime is longer than about 10 seconds, which is the time it takes for nuclear structure to form. Consequently, it is possible that some Z values can only be realised in nuclides and that the corresponding elements do not exist.