RADIOACTIVITY AND THE COMPLETION OF THE PERIODIC SYSTEM*

Abstract

ONE of the tributes paid by the Government and men of science of the U. S. S. R. to the memory of Mendeléeff on the occasion of the centenary of his birth¹ was the publication of his “Collected Works”. It contains all the different tables of the Periodic System designed by Mendeléeff at various times, and so offers an excellent opportunity to study at what pace, and up to what point, the completion of the system proceeded during his life. In the first tables, which appeared around the year 1870, empty spaces were left for the elements eka-boron, eka-aluminium and eka-silicon ; for eka-, dwi-, tri-, and chatur-manganese ; for the rare earth elements between barium and tantalum (many of which were known but could not be assigned to definite places) ; for several elements between bismuth and uranium ; and, to complete the last horizontal row, for a few trans-uranium elements. The atomic weight of tellurium was assumed to be 125, instead of the experimental value 128, in order to make it smaller than that of the following element iodine, 127. All these statements were equivalent to predictions. Which of them did the author see fulfilled?