178 PROTOPLASM 



block are satisfied, but the surface atoms have exposed valences 

 which may become satisfied through the adsorption of a foreign 

 substance. 



In the case of carbon, the unsatisfied surface forces are primary 

 valence bonds; forces of such strength, involving, as they do, 

 the sharing of an electron, are not frequently left unsatisfied 

 and therefore are not usually free to take part in surface phe- 

 nomena. More numerous are those looser bonds which go under 

 the name of coordination numbers. These latter are significant 

 in adsorption. Every atom in a crystal of sodium chloride is 

 surrounded by six of the opposite kind. All atoms at the surface 

 of the crystal lack one of their six mates; those at the edges lack 

 two, and those at the corners lack three (Fig. 143). While 

 sodium is united by its primary valence of one to only one 

 chlorine atom, its coordination number of six unites it to six 

 chlorine atoms. Were we arbitrarily to recognize but one valence 

 bond between a sodium and a chlorine atom in the sodium 

 chloride crystal, it would be impossible to say between which 

 two atoms this bond exists. We are, therefore, forced to admit 

 that each of the six bonds represented by coordination numbers 

 is equal, and where one or more are unsatisfied, as at the surface, 

 they are left free to unite with foreign substances. Bonds for 

 which coordination numbers stand are responsible for adsorption 

 phenomena rather than primary valence. The situation existing 

 in sodium chloride is true for all crystalline substances and 

 probably for most others; therefore, every soUd has unsatisfied 

 bonds at its surface, and upon these does adsorption depend. 



The nature of the bonds expressed by coordination numbers 

 is not fully known, but they, and therefore also adsorption bonds, 

 may be interpreted as follows: any molecule considered at a 

 distance is an electrically neutral body, but at close range the 

 individual atoms, considered as ions (as they are in crystals, 

 for example), exercise an individual electric effect which is not 

 counterbalanced by the other atoms or ions of the molecule. 

 For example, the water molecule is, as a whole, electrically neu- 

 tral. The two negative charges of the oxygen atom are balanced 

 by the two positive charges of the hydrogen atoms (Fig. 138). 

 At close range, however, these negative and positive charges will 

 exercise an influence independent of each other and thus attract 

 and hold a body of opposite charge. This is true of all polar 



