60 



SCIENCE 



[N. S. Vol. LIV. No. 1386 



regard to tlie structure of atoms, and these 

 are consistent with those previously pro- 

 posed. 



Postulate 1. — The electrons in atoms tend 

 to surround the nucleus in successive layers 

 containing 2, 8, 8, 18, 18 and 32 electrons 

 respectively. 



The word atom is used in the broader sense 

 which includes charged atoms (ions). If the 

 number of electrons in an atom is such that 

 they can not all form into complete layers in 

 accord with Postulate 1, the extra 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 out- 

 side layer of such an atom is nearly com- 

 plete, the tendency expressed by Postulate 

 1 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 distinction 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 defbiition will make 

 this clearer. The sheath of any atom {or 

 atomic ion) consists of all the electrons in 

 the outside layer provided that this layer is 

 incomplete when the atom is electrically neu- 

 tral. Thus atoms of the inert gases (neon, 

 argon, etc.) and ions such as ISTa*, 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 fluo- 

 rine atom has an incomplete sheath of Y elec- 

 trons. 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 com- 

 plete layers in accordance with Postulate 1. 

 In all other atoms, the tendency expressed by 



this postulate can only be satisfied by an inter- 

 action between atoms involving a rearrange- 

 ment 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. 



Wlien as the result of such rearrangement 

 of electrons, the sheath of an atom has become 

 complete, we may speak of the atom as a com- 

 plete atom. Similarly if the interaction be- 

 tween atoms leads to complete satisfaction of 

 the tendency of Postulate 1, 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 1. In such 

 cases incomplete atoms and compounds may 

 result. 



According to Postulate 1, 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 prox- 

 imity to one or more positive charges. We 

 may now state the second postulate. 



Postulate 2. — Two atoms may he coupled to- 

 gether hy one or more duplets held in common 

 hy the completed sheaths of the atoms. 



Let us now analyze the conditions that must 

 be fulfilled if the interaction between atoms 

 is to result in the formation of a complete 

 compound. 



A given group of neutral atoms may interact 

 to complete their sheaths in two ways: 



1. By transfer of electrons. 



a. Atoms having sheaths containing 

 only a few electrons may give up 

 these extra electrons to other 

 atoms. 



6. Atoms having nearly complete 

 sheaths may take up electrons from 

 other atoms. 



2. By sharing duplets. 



Atoms may share duplets with other 

 atoms (Postulate 2) and thus com- 

 plete their sheaths with fewer elec- 



