The Relative Abundance of the Isotopes of Lithium, Potassium and Rubidium

Abstract

The abundance ratios of the isotopes of lithium, potassium and rubidium have been measured with a Dempster type mass spectrograph. Artificial aluminum silicates of these alkalis were used as the source of ions. An abundance ratio of 12.14±0.4 was obtained for ${\mathrm{Li}}^7$/${\mathrm{Li}}^6$. On the basis of this ratio the masses of the two isotopes are 7.016±0.002 and 6.016±0.002 and the packing fractions are 23±3 and 26±3, respectively. The observed abundance ratio for potassium is ${\mathrm{K}}^{39}$/${\mathrm{K}}^{41}$=13.88±0.4. This value gives 38.96±0.003 and 40.96±0.003 for the masses of the two isotopes, and -9.85±0.9 and -9.37±0.9 for their respective packing fractions. A ratio of ${\mathrm{Rb}}^{85}$/${\mathrm{Rb}}^{87}$=2.59±0.04 was obtained for rubidium. On the basis of 85.44 for the atomic weight the masses of the isotopes come out to be 84.88 and 86.88 while the corresponding packing fractions are -14.1±0.7 and -13.8±0.7. The values for the packing fractions given above fall fairly closely on the packing fraction vs. mass number curve given by Aston for lithium and potassium but are about 70 percent low for rubidium.