62 PHENOMENA, ATOMS, AND MOLECULES 



lattice of tungsten atoms. The forces between the caesium adatoms art 

 typical dipole repulsions. 



Oxygen on Tungsten or Carbon. The attractive forces are probably 

 typical valence forces. That they are not ordinary Coulomb forces, such as 

 those effective in caesium adsorption, is suggested by a consideration of the 

 relation of the contact potential to the heat of evaporation of the adatoms. 

 A film of caesium which has the maximum effect on the electron emission 

 (0 = 0.67) has a contact potential of + 3-0 volts against tungsten, while 

 the heat of evaporation of the adatoms corresponds to 1.9 electron volts. 

 With an oxygen film produced when a tungsten filament at 1,600° is in 

 contact with oxygen at low pressure, the contact potential against tungsten 

 at 1,600° is — 1.6 volts, but the heat of evaporation is about 7 volts. 



The adsorption of oxygen, hydrogen and carbon monoxide on platinum 

 (36) affords other examples of adsorption dependent on primary valence 

 forces. 



Oil Films on Water. In these cases the attractive forces that cause the 

 active heads to spread over the surface are principally dipole forces 

 between the heads and the water molecules with some contribution from 

 VAN DER Waals' forces. Above the heads there is a hydrocarbon phase in 

 which VAN DER Waals' forces and the repulsive forces between completed 

 shells predominate. 



Non-Polar Gases on Glass. The adsorption of such gases as nitrogen, 

 argon, etc. on glass or mica at low temperatures is determined by typical 

 van DER Waals' forces (24). 



Activated Adsorption. 



Because of the various types of adsorption that exist, a single gas may 

 be adsorbed by a given surface in several different ways. At sufficiently low 

 temperatures van der Waals' forces alone would be sufficient to cause 

 adsorption (van der Waals' adsorption). At higher temperatures mole- 

 cules on the surface may undergo chemical change and thus be held by 

 valence forces. The heat of adsorption is then far greater than corresponds 

 to the VAN DER Waals' adsorption. The only reason that such adsorption 

 does not occur at low temperatures is that the reaction velocity of the 

 chemical change may be too low. Since there is an activation energy as- 

 sociated with each such reaction, H. S. Taylor (34) has proposed the term 

 activated adsorption for the adsorption which involves such chemical 

 changes. 



In some cases, such as the adsorption of caesium atoms on tungsten, the 

 adsorption involves only the transfer of one electron and can thus occur at 

 very low temperatures so that activation is not required, and only one type 

 of adsorption is observed. 



The distinction between van der Waals' adsorption and activated 



