WHAT CHEMISTRY OWES TO X-RAYS*

Abstract

IN the nineteenth century, the table of atomic weights and the periodic classification of the elements were obtained by classical chemical methods. In the twentieth century, X-ray methods have added to these, atomic numbers and atomic radii. The twin sciences of X-ray spectroscopy and X-ray crystallography were founded in 1912 by the famous von Laue experiment. By measuring the X-ray spectra of all the known elements from aluminium to gold, Moseley, in 1913, was able to show that each element could be assigned a number (not always in the order of the known atomic weights) which was later identified with the charge on the nucleus in the Rutherford-Bohr atom, or the number of extra-nuclear electrons in the neutral atom. But while the ideas of integral nuclear charge and of the K, L, M9 etc., orbits of electrons were primarily due to investigations of X-ray spectra, the study of X-ray scattering by the rare gases has shown that the Bohr orbital model is inadequate, and has demonstrated the superiority of the Sommerfeld wave-mechanical model of the atom, with its statistical electron -density distribution and its range of energy states. Atomic structure is fundamental to chemistry.