Atomic number is defined as the number of protons in the nucleus of an atom. This concept was historically important because it provided a theoretical basis for the periodic law. Dmitri Mendeleev 's discovery of the periodic law in the late 1860s was a remarkable accomplishment. It provided a key organizing concept for the chemical sciences. One problem that remained in Mendeleev's final analysis was the inversion of certain elements in his periodic table. In three places, elements arranged according to their chemical properties, as dictated by Mendeleev's law, are out of sequence on the basis of their atomic weights.

The solution to this problem did not appear for nearly half a century. Then, it evolved out of research with x ray s, discovered in 1895 by Wilhelm Röntgen. Roentgen's discovery of this new form of electromagnetic radiation had inspired a spate of new research...