The chemistry of the radio-elements / by Frederick Soddy.

  • Soddy, Frederick, 1877-1956.
Date:
1914
Catalogue details

Licence: In copyright

Credit: The chemistry of the radio-elements / by Frederick Soddy. Source: Wellcome Collection.

    40/64 (page 28)
    NATURE OF THE END PRODUCTS. ATOMIC WEIGHT OF LEAD The question thus arises whether common lead is a homogeneous element of atomic weight 207.1, as expressed in the International List, or a mixture of the end products of the three series in various proportions. There has long been an unexplained discrepancy between the atomic weight of lead and that calculated for the end product of the radium series. For the atomic weight of radium is now known with an accuracy at least as great as that of lead, owing to the splendid work of Honigschmid with over one gram of pure radium chloride.-® From his figure, 225.95, it is clear that the atomic weight may be taken without serious error as 226, so that the atomic weight of the end product, after the expulsion of 5 a-particles of mass four, would be 206, or i.i units below the value for lead. But the calculated value for both the end products of the thorium series is 232.4 minus 24, or 208.4. The end products of actinium would only be present in relatively small proportion, and may be neglected. If actinium has the same atomic weight as radium, both end products would have the atomic weight 210, which is the same as that of the end product of the minor branch of the radium series, formed in altogether negligible proportion. Hence it is clear that, whatever be the actual case, the atomic weight of lead agrees with what is to be expected for a mixture of the end pro- ducts of the uranium and thorium series in similar proportion. The difficulty at the present time is to be sure that what experimentally appear as the end products in all cases are really completely stable elements. It is quite possible that some of these products may be changing too slowly to give a detectable activity, and yet too rapidly to survive over geological epochs of time. Indeed, Fajans has suggested that the bismuth and thallium found in radioactive minerals may be the result of further changes of what have been regarded hitherto as the end products. These, for the thorium series, have the calculated atomic weight about 208,