MODIFIED ATOMIC VOLUME PLOTS THERALD MOELLER University of Illinois, Urbana, Illinois
W
HILE the periodicity exhibited by the elements is admirably illustrated by the original Lothar Meyer atomic volume curve, i t is often difficult for the student to visualize the relationship between this plot and the Mendeleeff type of periodic table. A more direct correlation may be obtained by superimposing the atomic volume curve on the long fonn of the periodic table. The advantages of this modification of the periodic system have already been pointed out ( I ) , and the arguments advanced in its favor are substantiated by combining i t with the original example of periodicity. Such a plot is given in Figure 1where the values have been calculated from the 1940 International Atomic Weights and the most recent density data (2, 3, 4, 5)
and usually refer to the solid elements a t 20°C. Hydrogen and helium are purposely omitted due to their rather anomalous atomic volumes, and dotted line interpolations are used where data are lacking. The value for the region 57 to 71, inclusive, is an arithmetical average for thirteen of the rare earth elements and is included to show that no discontinuity exists in the sixth period. While this plot, like all similar ones, is in error by not always giving data for the same physical state, i t is, nevertheless, of value in showing the similarity between any two periods and in bringing to the student's attention the fact that the elements within a given family occupy corresponding positions on the various curves.
the data of Klemm and Bommer (5) is given. Included in this plot are lanthanum and lutecium, although these two elements are sometimes referred to as merely transition elements rather than rare earths. While periodicity is less striking in this plot, there is definite evidence of its existence, and the graph serves as a convenient supplement to Figure 1. It is of especial interest that not only do europium and ytterbium, the elements showing the pronounced maxima, differ from the others in crystal structure (these two being cubic, the rest hexagonal), but also they are the ones most readily convertible to the divalent condition. LITERATURE CITED
111 I. The ~ -FOSTER. - ,~ - - - "Whv not modernize the textbooks also?
periodic syGem," J. CHEM.EDUC., 16, 409-12 (1939). "Handbook of chemistry," Third Edition, Handbwk (2) LANCE, Publishers, Inc., Sandusky, Ohio, 1939, pp. 54-9. 131 - von Tulius Sorinper. - \-, VAN -- ARKEL. "Reine Metalle." Verlae Berlin, 1939, pp. 550-52. (4) HODGMAN (Ed.), "Handbook of chemistry and physics. Chemical tables," Chemical Rubber Company, Cleveland, THE RARE EARTH METALS
The existence of periodicity within the rare earth group has already been demonstrated (6, 7). In Figure 2, a modified atomic volume plot constructed from
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tenen Erden," Z . enorg. allgem. Chem., 231, 138-7
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(7) PEARCEAND SELWOOD, "~ndmalous&Scnces of the rare earths," J. CHEM.EDUC., 1 3 , 2 2 4 3 0 (1936).
