Optical Isomerism, and the Ring-Electron, 

 Part II. 



431 



Optical Isomerism. 



In the theory of atomic structure first put forward by 

 Gr. N. Lewis and afterwards developed by Langmuir, the 

 electrons, instead of rotating in rings as in Bohr's theory, are 

 supposed to occupy, or to oscillate about, positions which are 

 fixed in space with reference to the atomic nucleus. This 

 fixity of the electrons is a characteristic feature of the 

 magneton theory of the atom advanced by Parson — a theory 

 which has not met with the recognition it merited, partly 

 because it is based on the notion of a positive sphere, partly 

 because Parson did not accept the atomic numbers of 

 Moseley, which are now regarded as being determined not 

 merely in a relative but also in an absolute sense. The 

 magneton, or ring-electron, makes it possible to have 

 stationary electrons. The most stable groupings of electrons, 

 according to Lewis and Langmuir, are (1) the pair, as 

 illustrated in the helium atom, (2) the octet, or group of 

 eight electrons arranged approximately at the corners of a 

 cube. Parson showed that such a group of eight magnetons 

 formed a system possessing very low magnetic energy and 

 producing a very weak external field. This assumption is in 

 agreement with the " rule of eight/' to which I have drawn 

 attention in connexion with atomic and molecular numbers*. 

 The number of unit electric charges in the atomic nuclei of 

 related atoms or molecules frequently differs by 8 or a 

 multiple of 8. 



In the formation of chemical compounds only an even 

 number of electrons can be held in common. " Two octets 

 may hold 1,2 or sometimes even 3 pairs of electrons in 

 common. A stable pair and an octet may hold a pair of 

 electrons in common. An octet may share an even number 

 of its electrons with 1, 2, 3 or 4 other octets. No electrons 



Fig. 1. 



can form parts of more than two octets" (Langmuir, Postu- 

 late 11). Thus the single bond commonly used in graphical 

 formula? involves two electrons held in common by two 

 atoms (fig. 1) ; the double bond implies that four electrons 

 * H. S. Allen, Trans. Chem. Soc. vol. cxiii. p. 389 (1918). 



