PAPERS PRESENTED AT GENEK.vr. SESSIONS 47 



a plus charge after it. The neutral atom then must have 

 as many electrons around the nucleus as it has positive 

 charges on the nucleus. It should be obser^'ed that ex- 

 cept in the cases of substances like sodium chloride 

 where the atoms are alternately positively and negatively 

 charged. the atoms must be on the average neutral. In 

 our representation of the electrons, we have apparently 

 arbitrarily placed two of the electrons close to the nu- 

 cleus and the remaining electrons are arranged at some 

 distance away from the nucleus. The following consid- 

 eration, however, shows the justification for doing this. 

 When lithium combines with fluorine to form lithium- 

 fluoride, we get a salt which resembles sodium chloride. 

 The lithium atom becomes positively charged and the 

 fluorine atom negatively charged. This can be brought 

 about only by an electron passing from the lithium atom 

 to the fluorine atom. The lithium atom, however, never 

 loses more than one electron. This is a fact which has 

 been recognized for a long time. Even before the electron 

 was discovered, it was thoroughly understood that the 

 lithium atom could acquire only one positive charge while 

 the glucinum atom combines Avith two fluorine atoms and 

 acquires two positive charges. That is to say, the gluci- 

 num atom may lose two electrons. Since it has four elec- 

 trons, however, apparently there are two electrons in it 

 just as in the lithium atom which may not be lost. We 

 find the same considerations to apply throughout the rest 

 of this row of elements, and we assume that these elec- 

 trons are closer to the nucleus than those electrons which 

 take part in a chemical reaction Those electrons which 

 take part in a chemical reaction we, somewhat arbitrarily 

 perhaps, assume to be at a greater distance from the nu- 

 cleus and clcsigTiate them as valence electrons. A valence 

 electron, then, is an electron in the periphery of the atom. 

 Lithium has one, glucinum two, boron three, and so on 

 up to neon which has eight. Lithium combines with one 

 fluorine atom, beryllium with two fluorine atoms, boron 

 with three and carbon with four. Lithium fluoride and 

 glucinum fluoride are salt-like in their character, that is, 

 they resemble sodium chloride, and the valence bonds 



