28 CONTEMPORARY SCIENCE 



of sodium (N = 11). Ten electrons are needed for the 

 stable pair in the first shell and the octet in the 

 second shell, as in the neon atom. The sodium atom 

 has one more electron than is needed to give this 

 stable structure while the fluorine atom has one electron 

 too few. It is obvious then that the extra electron of the 

 sodium atom should pass over completely to the fluorine 

 atom. This leaves the sodium atom with a single positive 

 charge while the fluorine becomes negatively charged. If 

 the two charged atoms or ions 2 were alone in space they 

 would be drawn together by the electrostatic force and 

 would move as a unit and thus constitute a molecule. How- 

 ever, if other sodium and fluorine ions are brought into 

 contact with the "molecule" they will be attracted as well 

 as the first one was. There will result (at not too high 

 temperature) a space lattice consisting of alternate positive 

 and negative ions and the "molecule" of sodium fluoride 

 will have disappeared. Now this is just the structure 

 which we find experimentally for sodium fluoride by 

 Bragg's method of X-ray crystal analysis. There are no 

 bonds linking individual pairs of atoms together. The 

 salt is an electrolytic conductor only in so far as its ions 

 are free to move. In the molten condition or when dis- 

 solved in water, therefore, it becomes a good con- 

 ductor. 



The case of magnesium (N = 12) and oxygen (N = 8) 

 is similar except that two electrons are transferred from 

 the magnesium to the oxygen atom. The resulting ions 



2 It is convenient and it has been customary with many physi- 

 cists to speak of a charged atom or molecule as an ion, irre- 

 spective of whether or not the particle is able to wander under 

 the influence of an electric field. The writer has used the term 

 in this way in his recent publications. This practice is very dis- 

 tasteful to many physical chemists and is likely to be misunder- 

 stood by them. Nevertheless, it seems to me probable, especially 

 in view of the recent work of Milner and Ghosh, that it will be 

 desirable to abandon the physical chemists' definition of the ion 

 and to apply it to all charged atoms or molecules. The ion 

 which wanders may then be referred to as a "free ion." 



