434 Dr. H. Stanley Allen on Optical Rotation, 



electrons held in common between two atoms is proof that 

 the electrons in the pair are bound into a kind of unit, in 

 which the relationship between the two electrons is very 

 different from that between two adjacent electrons in the 

 neon atom .... In practically all the compounds of carbon, 

 there are eight electrons arranged around the kernel of the 

 carbon atom, but these electrons are gathered into four pairs, 

 each pair constituting a unit or bond between the carbon 

 atom and atoms surrounding it. The nature of this unit is 

 evidently closely related to that of the pair of electrons 

 in the helium atom and hydrogen molecule, and my guess is 

 that such a pair consists of two electrons revolving about a 

 line connecting the kernels of the adjacent atoms, whereas 

 the electrons in the neon atom are revolving about eight 

 positions located at tlie corners of a cube. We are thus led 

 to a conception of the carbon atom which is practically 

 identical with that of the organic chemist, namely : that the 

 carbon atom exhibits four valence bonds arranged in space 

 in a symmetrical way, corresponding to the corners of a 

 tetrahedron/' 



According to the present theory, optical activity arises 

 from a difference effect, and can be manifested only when 

 there is lack of compensation amongst the electrons associated 

 with the various parts of the molecule. If the chemical bond 

 is to be attributed to a pair of electrons arranged side by side, 

 it is easy to understand how such compensation can be 

 brought about in the great majority of chemical compounds. 

 In the case of a single asymmetric carbon atom, the sym- 

 metrical arrangement of each of the four electron pairs is 

 disturbed by the presence of the adjacent groups, resulting 

 in only partial compensation. Thus, in the compound 

 Cabcd, the pair of electrons associated with group a is 

 under the influence of the unlike groups c and d, and the 

 condition of symmetry is absent. But if c and d are made 

 alike, the whole molecule will have a plane of symmetry 

 indicated by the broken line in the left half of fig. 3, Thus 

 the molecule will be inactive through ''internal compensation" 

 with respect to the electrons which form the outer shell of 

 the carbon atom. 



This arrangement of the electron pair side by side gives 

 readily an explanation of the fact that most chemical com- 

 pounds show diamagnetic properties, for such a pair would 

 produce a very weak external magnetic field, and this 

 configuration has in fact been utilized by Oxley * to account 

 for the diamagnetism of the hydrogen molecule. 

 * ' Oxley, < Nature,' vol. cv. p. 327 (1920). 



