528 Sir J. J. Thomson on the Structure of 



the number of electrons concerned in binding the atoms 

 together ; as was shown in that paper and will be seen 

 in this, there is much more involved than a mere change in 

 notation. 



The union of two bi-electron atoms may be either of the 

 type fig. 6 or 6 a. When only one pair of electrons is used 



Figs. 6 & 6 a. 



ci! a- 



P:' 



H 



■8 



up in uniting the atoms, this type would be represented by 

 — A — A — ; or both the electrons in each atom might be used 

 for coupling the atoms, the four electrons being at the corners 

 of a square in a plane bisecting AA at right angles, fig. 6 a. 

 This system of four electrons between the positive charges 

 may be regarded as a double bond between the atoms. 

 Inasmuch as the equilibrium of four electrons in one plane 

 when the displacements are not confined to the plane re- 

 quires very strong restoring forces to make it stable, we 

 should not expect the double bond to be permanent when 

 the positive charges which exert these forces are as small as 

 in this case when their sum is only equal to the charges on 

 the four electrons. 



If there are two-trielectron atoms in the molecule, if all 

 the electrons were coupling up the atoms there would be a 

 hexagonal ring of electrons in a plane bisecting AA at 

 right angles ; as this ring requires a central positive charge 

 greater than 6 to keep it in stable equilibrium, it would be 

 unstable, and the more probable arrangement of electrons is 



Pig. 7. 



the octahedral one shown in fig. 7, and represented sym- 

 bolically by — A = A — . With two four-electron atoms in 

 the molecule we have 8 electrons to dispose of, the maximum 



