TYPES OF VALENCE 389 



electrons remain in the outside layer as an incomplete layer which we may 

 designate as the sheath of the atom. Every electrically neutral atom must 

 contain a number of electrons equal to the atomic number of the nucleus. 

 If the outside layer of such an atom is nearly complete, the tendency ex- 

 pressed by Postulate i may cause a few additional electrons to be taken 

 up in order to complete the layer, thus forming a negatively charged atom 

 or ion. In such a case we may say that the sheath has been completed. 



In the following discussion it is important to keep in mind this dis- 

 tinction between sheath and outside layer. Every incomplete outside layer 

 which occurs normally is a sheath, but a complete outside layer may or may 

 not be a sheath. The following definition will make this clearer. The sheath 

 of any atom (or atomic ion) consists of all the electrons in the outside 

 layer provided tJiat this layer is incomplete when the atom is electrically 

 neutral. Thus atoms of the inert gases (neon, argon, etc.) and ions such as 

 Na"", Ca''", etc., have no sheaths for the outside layers of these atoms consist 

 of electrons which already form a complete layer in the neutral atom. The 

 sodium atom, however, has an incomplete sheath containing one electron, 

 while the fluorine atom has an incomplete sheath of 7 electrons. The fluorine 

 ion, on the other hand, has a complete sheath of 8 electrons. 



The inert gases are the only elements whose neutral atoms have no 

 sheaths, or in other words have all their electrons arranged in complete 

 layers in accordance with Postulate i. In all other atoms, the tendency ex- 

 pressed by this postulate can only be satisfied by an interaction between 

 atoms involving a rearrangement of the electrons. This is to be regarded 

 as the fundamental cause of chemical action and it is by such interaction 

 that chemical compounds are formed. 



When as the result of such rearrangement of electrons, the sheath of an 

 atom has become complete, we may speak of the atom as a complete atom. 

 Similarly if the interaction between atoms leads to complete satisfaction of 

 the tendency of Postulate i, so that all the atoms become complete, we may 

 say that a complete compound is formed. We shall see that there are some 

 factors which may oppose the formation of complete atoms and counter- 

 act the tendency of Postulate i. In such cases incomplete atoms and com- 

 pounds may result. 



According to Postulate i, the first complete layer in any atom consists 

 of two electrons close to the nucleus. Let us call this stable pair of electrons 

 a duplet and let us broaden the definition of duplet to include any pair of 

 electrons which is rendered stable by its proximity to one or more positive 

 charges. We may now state the second postulate. 



Postulate 2. — Two atoms may he coupled together by one or more 

 duplets held in common by the completed sheaths of the atoms. 



