436 FOOTE AND MOHIyER: IONIZATION 



Alkali halides. — Compounds of the alkali metals and the 

 halogens probably show a similar behavior, the ionization of 

 the vapor consisting in the production of positively charged 

 metal ions and negatively charged halogen ions. Thus the 

 ionization of NaCl into Na"*" and Cl~ would give rise to no spec- 

 tra characteristic of NaCl (except possibly the single frequency 

 above mentioned), but rather to the spectrum of sodium pro- 

 duced by recombination of the sodium ions and free electrons. 

 If positive salt ions were formed, on recombination with elec- 

 trons spectra of the salt would appear. No emission spectra 

 characteristic of the alkali halides have been observed.^ Further- 

 more, the flame emission spectrum characteristic of the metals 

 is suppressed by the presence of an excess of the halogen in the 

 flame. Kaiser and Runge^ concluded from this that an undis- 

 covered spectrum of the salt must exist, but the theory that NaCl 

 ionizes into Na"*" and Cl~ and hence has no spectrum except a 

 single line is a more probable explanation. The presence of 

 an excess of halogen gas in the flame simply reduces the pro- 

 portion of free sodium ions, which combine with chlorine ions in- 

 stead of electrons. The sodium spectrum is accordingly sup- 

 pressed. 



This type of ionization of the vapor is precisely that obtained 

 in the electrolytic dissociation of the fused salt, thus suggesting 

 that there may be a much closer relation between electrolytic 

 conduction and gaseous conduction than is ordinarily supposed. 



If a material in the vapor state ionizes by dissociation it is 

 sometimes possible to compute from chemical and physical data 

 the value of the ionization potential. As an example, we shall 

 illustrate the method of computing the work necessary to ionize 

 a gram mol of sodium chloride vapor. 



Let 



[ ] denote solid phase or crystalline state. 



( ) denote gaseous phase. 



D = heat of dissociation of V2 gram mol halogen gas into 

 monatomic gas. 



' Kaiser and RungE- Handbuch der Spektroskopie. 

 * Loc. cit. 6: 127. 



