April 29, 1920] 



NATURE 



26] 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond wtth 

 the writers of. rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Theories of Atomic Structure. 



l.N a letter to Nature (March ii, p. 41) S. C. Brad- 

 ford stated: "The great objection to Langmuir's 

 theory of atomic structure is the difficulty of accept- 

 ing his hypothesis of stationary electrons." The cases 

 cited are all discussed in G. N. Lewis's paper, "The 

 Atom and the Molecule " (Journ. Amer. Chem. Soc, 

 xxxviii., p. 762, April, 1916), so it is scarcely fair to 

 Lewis to refer to the theory as "Langmuir's theory." 



Although Lewis frankly implied that the electrons 

 in atoms are stationary, his theory of valency did not 

 depend upon such an assumption. The chemical data 

 give information in regard to the geometry of atoms, 

 and, in particular, tell us of the kinds of symmetry 

 which they possess. From the chemical point of view 

 it is at present a matter of comparative indifference 

 what the motions of the electrons may'^e so long as 

 they conform to the required conditions of symmetry. 

 For this reason I was careful to state in my first 

 paper (Journ. Franklin Inst., clxxxvii., p. 359, March, 

 1919, and Journ. Amer. Chem. Soc, xli., p. 932, 1919) 

 that " the electrons in atoms are either stationary or 

 rotate, revolve, or oscillate about definite positions in 

 the atom,^^ It was, perhaps, not sufficiently em- 

 phasised that the positions of the electrons shown in 

 the diagrams may be regarded as the centres of their 

 orbits. 



It is sometimes thought that the success of Bohr's 

 theory furnishes reason for believing that all the 

 electrons in atoms are rotating in coplanar orbits 

 about the nucleus. There is little justification for this 

 opinion. The remarkable results yielded by Bohr's 

 theory, particularly in the hands of Sommerfeld, for 

 the case of the hydrogen atom and the helium ion 

 seem to prove beyond question that in an atom con- 

 taining only one electron this electron actually revolves 

 in a circular or elliptical orbit about the nucleus. 

 Although Bohr's theory has had some applications to 

 other atoms, these are, for the most part, of a very 

 general nature, such as those which relate to the com- 

 bination principle. The theory does not give a satis- 

 factory model even for such simple structures as the 

 hydrogen molecult^ or helium atom (see, for example, 

 Sommerfeld's recent book, " Atombau und Spectral- 

 linien "). 



From the chemical point of view Bohr's theory is 

 wholly unsatisfactory when applied to atoms contain- 

 ing more than one electron. Thus, according to Bohr's 

 calculations {Phil. Mag., xxvi,, p. 492, 1913), a lithium 

 nucleus surrounded by three equidistant electrons 

 should have less potential energy (and, therefore, 

 greater stability) than one in which one electron is 

 further from the nucleus than the other two. Bohr's 

 theory thus gives no reason for the contrast between 

 the properties of lithium and helium. 



The two theories are not mutually incompatible if 

 we consider that, in general, the electrons do not 

 revolve about the nucleus, but about definite positions 

 symmetrically distributed in three dimensions with 

 respect to the nucleus. It is interesting to note that 

 Born and Land^ {Verh. deut. physik. Ges., xx., 

 pp. 210, 230, 1918), starting out from Bohr's theory 

 and without knowledge of Lewis's work, were led to 

 the theory of the cubical atom by a study of the com- 

 pressibilities of the alkali halides. They conclude that 

 the electron orbits do not lie in a plane, but are 

 arranged in space with cubic symmetry. Sommerfeld 



NO. 2635, VOL. 105] 



in his book suggests that this conception may-h^lp -to 

 solve some of the outstanding difficulties, and evidently 

 does not consider it inconsistent with Bohr's theory. 



In the case of atoms which do not share electi-ons 

 with other atoms, it is logical to assume that each- 

 electron in the outer shell has its own orbit. Thus 

 the atoms Ne, Na+, Mg+ + , F-, and- the S atom ih- 

 Sl% should have cubic symmetry, the eight outer elec 

 trons revolving about positions located at the corners 

 of a cube. But where a pair of electrons is held in 

 common between two atoms, the chemical evidence' 

 indicates that the pair acts as a unit. When an atom 

 shares four pairs of electrons with its neighbours, it- 

 thus has tetrahedral rather than cubic symmetry. So 

 far as the chemical evidence is concerned, it would be 

 satisfactory to adopt Bohr's model for the hvdrogen 

 molecule to represent the pair of electrons which con- 

 stitutes the chemical bond. We may thus picture the 

 chemical bond as a pair of electrons revolvirig in a 

 single orbit about the line connecting the centres of 

 the two atoms. 



Bohr in his 1913 paper {Phil. Mag., xxvi., p. 874) 

 states : "The configuration suggested by the theory for 

 a molecule of CH., is of the ordinary tetrahedron type; 

 the carbon nucleus surrounded by a very small ring of 

 two electrons being situated in the centre, and a 

 hydrogen nucleus in every corner. The chemical bonds 

 are represented by four rings of two electrons each 

 rotating round the lines connecting the centre and 

 the corners." This structure is quite consistent with 

 the octet theory. Bohr did not, in general, identify a 

 pair of electrons with a valency bond. 



When we consider, however, that Bohr's theorv in 

 its present form does not furnish an explanation of the 

 stability of the pair of electrons in the helium atom 

 and in the bond between atoms, it is evident that the 

 model described above can scarcely be regarded as 

 satisfactory. It seems as though some factor of vital 

 importance is still missing in Bohr's theory. The 

 chemical data suggest that the ultimate theory will be 

 extremely simple, but perhaps more radical than any- 

 thing yet proposed. 



I am in full agreement with the views put forward 

 by Dr. H. S. Allen in Nature for March 18, p. 71. 



Irving Langmuir. 



Research Laboratory, General Electric Co., 

 Schenectady, New York, April 12. 



Decimal Coinage. 



In Nature of April i, p. 145, reference is made to 

 Ihe unfavourable report of the Royal Commission 

 appointed to inquire into the above subject. It would 

 appear from a close study of the findings of the 

 Commission that the failure to solve this century-old 

 problem was due more to differences between the 

 advocates than to opposition to the principle. 



Although fifteen of the twenty Commissioners would 

 prefer to decimalise the existing £ sterling rather 

 than to create a new monetary unit equal to 100 half- 

 pence, it is significant that only four of them could 

 agree that the advantages to be secured by the 

 decimalisation of the £ would outweigh the incon- 

 venience arising from the change. This is tantamount 

 to an admission that the method of dealing with the 

 penny difficulty as proposed in Lord Southwark's Bill 

 (;^ — Vnil) was unduly complicated. (No exact equiva- 

 lent of the penny was provided, the choice of a 4-mil 

 and 5-mil piece being alternatively offered.) 



Retaining the £ as the unit, there are three possible 

 values for the penny, viz. : 



4 mils = the present penny less 4 per cent. 



5 mils = the present penny plus 20 per cent. 

 si mils = the present penny exactly. 



The claims of these denominations may be summed 

 up as follows : 



