no. ii structure of the atom — parson ii 



§3. Stereochemical Evidence 



The rapid orbital motion of the valence electrons, together with 

 the other electrons in the atom, which is a feature common and 

 essential to most theories of atomic structure, makes it hard to 

 see how these latter can ever furnish an extended explanation of 

 chemical phenomena. 



The difficulty here is twofold. In the first place, it is known that 

 the action of a single electron is the predominating feature of any- 

 chemical bond that undergoes electrolytic dissociation ; and the 

 general regularities of the Periodic Scheme make it highly probable 

 that the same is true of bonds that do not, such as those in hydro- 

 carbon molecules ; besides, there is a fine gradation between these 

 two extreme types. This, together with the stereochemical evidence 

 for a definite spatial arrangement of the groups attached to a Carbon 

 or other atom, makes it very unlikely that the valence electrons can 

 be taking part in the rapid orbital motion of a system of electrons in 

 rings. It is indeed conceivable that in a molecule of the type XH n , 

 where all the bonds are ionizable, the nuclear X atom may take into 

 its own system of rings the electrons it has extracted from the H 

 atoms, while the positively charged H residues arrange themselves 

 symmetrically around it; but this could not apply to the bonds in 

 which no actual transfer of an electron takes place, such as those 

 probably are which do not ionize or leave charged groups when 

 broken. In such cases, at least, it appears that the electron associated 

 with a unit of combining action must remain near the point of contact 

 with the atom that is held by that action. 



The second objection, and for the Thomson model this merges 

 with the first, is that rings of electrons must usually all rotate about 

 the same axis, so that the symmetrical action in three dimensions 

 which seems to be a normal property of the atom could be exerted 

 by the Thomson atom only in the limited electrostatic sense 

 already described, and not at all by Rutherford's atom. Fully to 

 appreciate this difficulty one need only turn to that point in Dr. 

 Bohr's papers (loc. cit.) at which he comes to consider the " tetra- 

 hedral " Carbon atom. We see there that the theory comes to a 

 complete halt when confronted with the problems of " Chemistry in 

 Space." Nor is the tetrahedral Carbon atom an isolated problem: 

 the asymmetric compounds of other elements, such as Nitrogen and 

 Cobalt, are still further beyond the reach of such theories, not only 

 in their present form, but, it would appear, in any conceivable state 

 of development along the same lines. 



