II.

Historical Development

As a result of discoveries that firmly established the atomic theory of matter in the first quarter of the 19th century, scientists could determine the relative weights of atoms of the then known elements. The development of electrochemistry during this period by the British chemists Sir Humphry Davy and Michael Faraday led to the discovery of many additional elements. By 1829 a sufficient number of elements had been discovered to permit the German chemist Johann Wolfgang Döbereiner to observe that certain elements with closely similar properties occur in triads, or groups of three, such as chlorine, bromine, and iodine; calcium, strontium, and barium; sulfur, selenium, and tellurium; and iron, cobalt, and manganese. Because of the limited number of known elements and the confusion that existed concerning the distinction between atomic weights and molecular weights, however, chemists were unable to grasp the significance of the Döbereiner triads.

The development of the spectroscope in 1859 by the German physicists Robert Wilhelm Bunsen and Gustav Robert Kirchhoff made possible the discovery of many more elements (see Spectrum). In 1860, at the first international chemical congress ever held, the Italian chemist Stanislao Cannizzaro clarified the fact that some of the elements—for example, oxygen—have molecules containing two atoms. This realization finally enabled chemists to achieve a self-consistent listing of the elements.

These developments gave new impetus to the attempt to reveal interrelationships among the properties of the elements. In 1864 the British chemist John A. R. Newlands listed the elements in the order of increasing atomic weights and noted that a given set of properties recurs at every eighth place. He named this periodic repetition the law of octaves, by analogy with the musical scales. Newlands's discovery failed to impress his contemporaries, probably because the observed periodicity was limited to only a small number of the known elements.