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 projects aimed at learning more about x rays themselves and about their effects on matter. Charles Grover Barkla, a physicist at the Universities of London and Cambridge, initiated one line of x-ray research. Beginning in 1903, he analyzed the way in which x rays were scattered by gasses, in general, and by elements, in particular. He found that the higher an element was located in the periodic table, the more penetrating the rays it produced. He concluded that the x-ray pattern he observed for an element was associated with the number of electrons in the atoms of that element.
Barkla's work was brought to fruition only a few years later by the English physicist H. G. J. Moseley. In 1913, Moseley found that the x-ray spectra for the elements changed in a simple and regular way as one moved up the periodic table. Moseley, like Barkla, attributed this change to the number of electrons in the atoms of each element and, thus, to the total positive charge on the nucleus of each atom. (Since atoms are electrically neutral, the total number of positive charges on the nucleus must be equal to the total number of negatively charged electrons.)
Moseley invented the concept of atomic number to describe his findings. He assigned atomic numbers to the elements in such a way as to reflect the regular, integral, linear relationship of their x-ray spectra. It soon came to be understood that the atomic number of an atom is equal to the number of protons in the atom's nucleus.
Moseley's discovery was a very important contribution to the understanding of Mendeleev's periodic law. Mendeleev's law was a purely empirical discovery. It was based on properties that could be observed in a laboratory. Moseley's discovery provided a theoretical basis for the law. It showed that chemical properties were related to atomic structure (number of electrons and nuclear charge) in a regular and predictable way.
Arranging the periodic table by means of atomic number also resolved some of the problems remaining from Mendeleev's original work. For example, elements that appeared to be out of place when arranged according to their atomic weights appeared in their correct order when arranged according to their atomic numbers.
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