48 ILLINOIS STATE ACADEMY OF SCIENCE 



which hold them together may be ascribed to the electro- 

 static forces between the atoms when an electron passes 

 from one atom to another leaving the one atom positively 

 charged and causing the other atom to be negatively 

 charged. Compounds of this type are called polar com- 

 pounds because the molecule consists of a positive and a 

 negative atom or atoms resembling a magnet with a north 

 and a south pole. The reason that the electron passes so 

 readily from the lithium atom to the fluorine atom ap- 

 pears to be because the positiv^e charge upon the nucleus 

 of the fluorine atom is so much greater than that upon 

 the lithium atom. The recipe for making a polar com- 

 pound, that is, one of salt-like character, then, is to take 

 two atoms with a considerable difference in the positive 

 charge upon their nuclei. This rule is subject to a modi- 

 fication which will be obvious a little farther on. When 

 that modification is introduced, it is found to be verified 

 absolute^ by the facts of chemistry. When we consider 

 boron trifluoride, we find that the difference in positive 

 charges upon the nuclei of the two atoms in question is 

 not so great. Therefore, the tendency for an electron to 

 leave the boron atom and go to the fluorine atom will be 

 very much less. As a matter of fact, boron trifluoride 

 does not appear to be a polar substance, that is, of salt- 

 like character, and when we consider carbon tetrafluoride, 

 where the fluorine has only three positive charges more 

 than the carbon, we have a compound which has no re- 

 semblance whatsoever to a salt. There is no reason to 

 believe that the electron has left the carbon atom and 

 gone to the fluorine atom, and we are forced to the con- 

 clusion that the valence bonds here must be quite differ- 

 ent in character from those in lithium fluoride. G. N. 

 Lewis has proposed the theory that the valence bond in 

 a compound of this non-polar, type consists of two elec- 

 trons occupying a position somewhere intermediate be- 

 tween the two atoms. We do not have time to go into the 

 detailed considerations which led Lewis to the idea that 

 a non-polar bond consisted of two electrons. We can only 

 try out the arrangement of a number of compounds to 

 see how it will work out and, of course, that is in the last 

 analysis the only consideration of importance in regard 



