The atomic weight of protactinium

Up to the present time the atomic weight of not a single member of the actinium series of radioactive elements has been determined by orthodox chemical methods. An opportunity for such a determination was given for the first time in 1927 when protactinium, the longest-lived isotope of element 91, was isolated and obtained in the state of pure compounds. Unfortunately the quantities of the element then available were extremely small. In the following years the political and financial difficulties in obtaining raw material and working it up for larger quantities of protactinium have been overcome and recently over 0.1 gm of pure Pa2 O5 were isolated and made available for an atomic weight determination. In the meantime Aston succeeded in determining with his mass-spectrograph the isotopic mass of AcD, the end product of the actinium series, in the mass spectra of leads from different sources. His value of 207 for AcD leads by simple additions to 231 as the atomic weight of protactinium. On the basis of Aston’s data Rutherford estimated the half-period of actino-uranium, the parent of the series, to equal 4·0 × 108 years, a figure which has been fully confirmed by subsequent research.