Frequency and Atomic Number. 493 



potential determined by <?y E = 7ij/ E . The value o£ V E found 

 from this equation is 



6-558 xl0- 27 x 3-289 x 10 15 



4-774x10" 



= 0-04518 E.S.U.. 



or 13*55 volts. This potential is the same as that required 



according to Bohr's theory for the ionization of the hydrogen 



atom. For in this theory the work done in moving the 



single electron from its orbit round the central nucleus to a 



2ir 2 me 4: 

 position of rest at infinity is W = — ^ — , and the fundamental 



27r 2 me* 

 electronic frequency is determined by pa= — ^ — . Hence 



W, which is eY lh is equal to /u> E . Thus V H , the ionization 

 potential for atomic hydrogen, is equal to hv E /e or V E . The 

 value of the ionization potential determined experimentally 

 for hydrogen gas, that is for the hydrogen molecule, is of 

 the same order of magnitude, being 11 volts instead of 

 13-55 volts. 



The following Table (V.) gives the value of the ionization 

 potential, V , measured by Franck and Hertz *, and also the 

 value of the product NVo. The table includes all gases 

 for which direct experimental determinations are available. 

 The results in the last column show that in the case of the 

 monatomic gases there is remarkable agreement with the 

 relation NV = nV E . 



Table V. 



Element. N. 



y (voits). 



NV . 



Hydrogen 



1 

 2 



7 



8 



10 



18 



80 



11-0 lxll'O 

 20-5 .3 y 1 3-7 



Nitrogen 



7-5 



90 



160 



120 



4-9 



4X131 



5x144 



12x133 



16x13-5 



30x13-1 



Neon 





Mercury 



In the case of the diatomic gases, hydrogen and oxygen, 

 the agreement is not so good; nitrogen, however, falls in line 

 with the monatomic elements. 



The experimental values of Franck and Hertz for the 

 ionization potentials have been confirmed for certain gases 



* Franck and Hertz, Verh. Deutsch. Phys. Gesell. vol. xv. p. 34 (1913) ; 

 vol. xvi. pp. 457, 512 (1914). 



Phil. Mag. S. 6. Vol. 34. No. 204. Dec. 1917. 2 N 



