Observation of relativistic corrections to Moseley's law at high atomic number

Transitions between low-lying electron states in atoms of heavy elements lead to electromagnetic radiation with characteristic discrete energies between about 0.1 keV and 100 keV (x rays). Moseley's law is an empirical relation, described in 1914, that supported predictions of the Bohr model of the atom. It predicts that the energy of these x rays scales as Z^2 while also identifying the atomic number Z as the measure of nuclear charge. The foundational nature of Moseley's experiment has led to popularity in undergraduate advanced laboratory physics subjects. We report observations of deviations from Moseley's law in the K-alpha x-ray emission of 13 elements from Z = 29 to Z = 92. While the deviations follow the square-law predictions of the Bohr model fairly well at low Z, they become larger with increasing Z. We find that relativistic models of atomic structure are necessary to fit the full range of elements observed (p = 0.23 for the relativistic Bohr model). As has been argued by previous authors, measurements of relativistic deviations from Moseley's law are pedagogically valuable at the advanced lab level and accessible with modern but modest apparatus. Here, we show this value can be extended by using higher Z elements, where the effects are more dramatically observable.