Bond Energies of Some Alkali Fluorides

Abstract

THE attenuation of microwaves by flames containing traces of alkali metals may be used to determine the electron concentration, arising from the ionization of the alkali, in these flames^1,2. If a halogen is added to such a flame, the electron concentration is reduced directly by the formation of negative ions, and indirectly by the formation of the alkali halide^3,4. Flame photometric studies show that the halogens also reduce the excess hydrogen atom concentration in the flame⁵; but it has been observed that fluorine has the least effect⁶. This reduction makes it difficult to estimate the bond energies of the alkali halides from microwave studies⁷, because of the indirect nature of the arguments used in the estimations⁸. In the case of fluorine, however, the argument is simplified by the unimportance of the negative ion effect, since the electron affinity of fluorine is low relative to the bond energies of the alkali fluorides, and by the much smaller disturbance produced in the hydrogen atom concentration. In addition, the observations are made at high temperatures where the hydrogen atom concentration approaches equilibrium⁵ and it is possible to obtain reliable and consistent results, which it is of interest to place on record.