Soddy and the Classification of the Elements

Abstract

The periodic system of the chemical elements was discovered during the 1860s on the basis of experiment rather than theory. In 1913 its theoretical confirmation as a classification system appeared from several directions by a replacement of the concept of atomic weight by that of atomic number, a modification to which Soddy contributed greatly. In the second part of his book The Chemistry of the Radio-Elements, subtitled Radio-Elements and the Periodic Law 1914), he published a three–dimensional helical representation of the periodic law, which successfully showed the central place of the noble gases and their function in linking the alkali metals with the halogens. Even earlier, with his concept of isotopy, Soddy clearly realized that most of the newly discovered products of the several radioactive disintegration series were not new elements which had to be positioned within the periodic system, and he defended such a classification against those who would discredit the periodic system altogether. With his displacement law of the elements (1913), the classification of all radioactive elements, including the end products of the radioactive series, received a practical and theoretical explanation as significant as Bohr’s atomic theory of the same year.