Density of Sodium Chloride The Atomic Weight of Fluorine by Combination of Crystal Density and X-Ray Data

The densities of carefully purified crystals of sodium chloride have been determined by the method of "crystal flotation" in pure ethylene dibromide. The results yield d_27.634^∘C=2.16165±0.00002g/ml, which reduces to d_20^∘=2.16366±0.00003g/ml.It is found that six successive precipitations of NaCl in the manner employed by Richards and Wells in their determination of the atomic weights of sodium and of chlorine are required to effect purification to constant density (±{} about 4×10^-6 g/ml), and also that exposure to air produces surface contamination sufficient to cause erratic changes in apparent density that may amount to as much as 5×10^-4 g/ml within a few minutes. Combination of our value for dNaCl with that of C. A. Hutchison and H. L. Johnston for dLiF and with Straumanis, Ievins, and Karlsons' value for the lattice constant of LiF relative to the Siegbahn value for NaCl yields 0.443640±{}0.000025 for the ratio of the molecular weights of LiF and NaCl, respectively. With the adoption of 22.997 (International Atomic Weight Committee) for the atomic weight of sodium, this ratio yields 18.994±{}0.001 for the atomic weight of F. With the adoption of 22.994 (Birge) for sodium, the F atomic weight comes out 18.992. Either of these figures is in reasonable agreement with the value 22.995±{}0.002, based on densities and lattice constants of fluorite and calcite, and with the gas density determinations for compounds of F, but are somewhat lower than the mass spectrograph value of 18.999±{}0.001 for F¹⁹. It appears that the determination of relative molecular weights by combination of x-ray and density data are as reliable, in favorable cases, as by other standard atomic weight methods. In calcite, fluorite, and rock salt crystals, used to obtain the data underlying these computations, there is no evidence of any appreciable influence of "crystal mosaic" patterns which Zwicky thought might influence crystal densities by as much as 1 percent.