 A more accurate calculation must take into account the small, but nonzero, size of the nucleus, which is predicted to push the limit further to "Z" ≈ 173.The Bohr model exhibits difficulty for atoms with atomic number greater than 137, for the speed of an electron in a 1s electron orbital, "v", is given by

where "Z" is the atomic number, and "α" is the fine structure constant, a measure of the strength of electromagnetic interactions.Under this approximation, any element with an atomic number of greater than 137 would require 1s electrons to be traveling faster than "c", the speed of light.