Absorption Spectra of Saturated Chlorides of Multivalent Elements.

FRANCK and his co-workers have shown that the vapours of alkali halides show a continuous absorption spectrum beginning from a long waverlength limit and extending towards the ultra-violet. He interpreted the absorption spectrum as follows : if we take a substance like sodium chloride the valency electron of Na in the normal state may be supposed to pass over completely to Cl and the combination consists of Na+Cl-. Such a combination is called ionic. When light falls on sodium chloride vapour, the electron passes over from Cl-to Na+, the molecule thus splitting into Na and Cl. Franck obtains the heat of formation Q of NaCl from atomic Na and atomic Cl with the aid of a Born cycle and finds that Q =N hν where ν = frequency of the beginning of continuous absorption.