Electron Theory of Chemistry to Solids. Vox 



an atom and the nearest electron in the solid must not 

 exceed c, the distance between the core of the atom and its 

 electron when in the gaseous state, by more than a small 

 fraction of this distance. 



The value of d can easily be calculated, for if N is the 

 number of atoms in unit volume of the substance, M 

 the mass of an atom, and A the mass of unit volume of the 

 solid, 



Nx(2a) s = l; M = A; 

 hence 



8d 3 =M/A. 



With this arrangement of electrons 1*72 x d, where d is deter- 

 mined by the preceding equation, should be a close approxi- 

 mation to the distance of the electron from the centre in a 

 gaseous atom. 



The highest frequency vibration of the electrons is when 

 the electrons are not displaced relatively to one another but 

 only with respect to the atoms, in this case e 1 = € 2 = e 3 =l,. 

 %€ 1 € 2 e 3 ~Bp qr =0, and 



mp 2 ='3S4,ce 2 l ld i 



= -384x8e 2 (A/M)c/rf (5) 



As a vibrating system of electrons cannot transmit light of 

 a higher frequency than the maximum free frequency of the 

 system, this equation would give the value of the frequency 

 of the shortest waves which could get through the crystalline 

 medium. 



This type of vibration is the one that would be excited by 

 waves such as those of visible or ultra-violet light whose 

 wave-length is large compared with the distance between the 

 atoms in the solid. We might therefore expect evidence of 

 it in the behaviour of monovalent solids when acted upon 

 by light ; the effect produced by light on such solids would 

 be greatest when the frequency of the incident light was that 

 given by equation (5) . An interesting case when the action 

 of light on a solid is a maximum for light of a particular 

 wave-length is that known as the selective photo-electric 

 effect (Hughes, ' Photo-Electricity,' chap. 5). This has been 

 measured by Pohl and Pringsheim (Verh. d. Deutsch. Phys. 

 Ges. xiii. p. 474 (1911)), and in the following table I give 

 the comparison of the wave-lengths X for which the effect is 

 a maximum for the monovalent metals, sodium, potassium, 

 rubidium, as determined by Pohl and Pringsheim, with the 



