Periodic Classification of Elements

Abstract

To the three requirements of a modern Periodic Table, as stated by A. A. Clifford¹, must be added a fourth, namely, that it should be in as close accord as possible with chemical facts, and above all with the most fundamental of these, valency. It is only in Group IV that there is any strong chemical support for the sub-group arrangement. In other groups the only real justification is that the members of both sub-groups have the same maximum valency and some have the same characteristic valency. In Clifford's table even this slender justification is destroyed by the inclusion of Pr with Pa, Nd with U, Sm with Pu and Eu with Am, not merely in the same group but also in the same sub-group c. Advocates of ‘long’ forms of table are unlikely to be converted in this way.