foote; and mohIvEr: ionization 437 



S = heat of sublimation at absolute zero of i gram atom 



metal or gram molecule of salt. 

 Q = heat of formation of the salt. 

 / = work necessary to ionize i gram mol salt or i gram atom 



metal. 

 E = electron affinity referred to i gram atom halogen gas. 

 The following thermochemical relations may be written: 

 [NaCl] + QNaci = [Na] + (V2CI2) 

 (V2CI2) + Z^ci = (CI) 

 [Na] + 5Na = (Na) 

 (NaCl) - 5Naci = [NaCl] 

 (Na) + /Na = (Na) + 

 (Cl) - Eci = (Cl)- 



Adding : 



(NaCl) + QNaCl + ^Cl + ^Na + ^Na " EqI - S^^ci = 



(Na)+ + (CI)- (i) 



Equation (i) accordingly gives the amount of energy /Naci 

 required to ionize i gram mol of sodium chloride vapor. Whence : 



/(NaCl) = 0NaCl + DqI + vS^a + ^Na ~ -Ecl ~ -JNaCl (2) 



A similar relation holds for any salt of compositions R X where 

 R is an alkali metal and X a halogen. For the greater portion 

 of these salts, all the terms on the right of equation (2), with the 

 exception of the heat of sublimation of the salt are known. 

 Hence a determination of the heat of sublimation would permit 

 the computation of the ionization potential of the vapor of the 

 salt. 



On the assumption that, in addition to the ordinary Coulomb 

 force of repulsion or attraction between the charges on the ions 

 forming the crystal structure of these salts, there exists between 

 two ions a repulsive force, the potential of which is inversely 

 proportional to the ninth power of the distance apart, Born^ 

 has computed the grating energy of the crystal, i. e., the work 

 U necessary to convert i mol of the crystal into free positive 

 .and negative ions — a purely electrostatic problem. The phys- 

 ical significance of the quantity U is apparent. It may be con- 

 * Born. Verb. d. Phys. Ges. 21 : 16. 1919. 



