8 HYDROGEN ION CONCENTRATION 



neutral elementary corpuscles of electricity, each of which in turn 

 consisted of one negatively and one positively charged particles. 

 These particles were supposed to serve as an inexhaustible source 

 for electrical charges arising in dissociation. Thus in the dissocia- 

 tion of water an H-atom removes from this reservoir of electricity 

 a positive charge and the OH-residue removes a negative charge. 

 This assumption has become superfluous with the introduction of 

 the Rutherford-Bohr atomic model. The reservoir of these free 

 charges is in the atoms themselves. Each atom consists of a posi- 

 tively charged nucleus, the charge consisting of a definite number 

 of elementary charges. The nucleus is surrounded by negatively 

 charged electrons. In an electrically neutral atom the number of 

 electrons is equal to the number of positive charges upon the nu- 

 cleus. Thus in the dissociation of an electrically neutral molecule 

 of H2O into H+ and 0H~ what happens is that the dissociated OH 

 radical carries along with it one electron in excess of its electro- 

 neutral state, this extra electron having been derived from the 

 H-atom which now becomes the positively charged H-ion, or H+. 

 The electrons which determine the state of an atom are held by 

 the latter with varying degrees of firmness. If one of the electrons 

 happens to be held in the atom by a loose bond, then it may be 

 easily thrown off, as it were, and, as a result of this loss of a nega- 

 tive charge, the rest of the atom becomes a positively charged ion. 

 This electron is designated as the valence-electron. If two such are 

 present in an atom, it forms bivalent positive ions, etc. On the 

 other hand, there are atoms in which the saturation with their 

 full quota of electrons is not complete, when given in their usual 

 electroneutral state. In these the positively charged atomic nu- 

 cleus is capable of combining with one or more electrons, and thus 

 to become a uni- or polyvalent negative ion. According to their 

 capacity to give up or to take on electrons the positive and nega- 

 tive elements are differentiated. The Berzelius theory is thus, in 

 a modified form, revived again. When a "positive" (Na) and 

 "negative" (CI) atom are found together, a competition arises 

 between the two for the labile electron, A nmnber of these pairs 

 of atoms arrange themselves so that the electron leaves the sphere 

 of attraction of the positive atom and is bound by the negative atom; 

 another part becomes so arranged that the valence-electron becomes 

 attached to the negative atom, but without becoming completely de- 



