[ 721 ] 



LXXXVII. Application of the Electron Theory of Chemistry 

 to Solids. By Sir J. J. Thomson, O.M., F.R.S* 



IN" discussions on the structure of solids and crystals atten- 

 tion is usually confined to the distribution of the atoms 

 and little or no consideration given to that of the electrons. 

 On the view I discussed in the Phil. Mag. Mar. 1921 

 the electrons play a very important part in determining 

 the arrangement of the atoms and the properties of the 

 substance. 



Each kind oF atom has associated with it a definite number 

 of electrons which form its outer layer when it is in a free 

 state : it is by the rearrangement of these electrons that it 

 is able to hold other atoms, whether of the same or different 

 kinds, in chemical combination. When these atoms aggre- 

 gate and form a solid there will be in each unit volume of 

 the solid a definite number of these electrons, and the 

 problem is to distribute the electrons so that they will form 

 with the atoms a system in stable equilibrium. 



We shall begin with the simplest case when the atoms are 

 all of the same kind, i. e. when the solid contains only one 

 chemical element. We suppose that the electrons are 

 arranged as a series of cells which fill space and that each 

 cell surrounds an atom ; the number of cells is equal to the 

 number of atoms. If the atom is monovalent the number 

 of electrons is equal to the number of atoms, if divalent to 

 twice that number, if trivalent to thrice that number, and 

 so on. This condition will determine the shape of the cell. 

 If the cells h.ive to be similar and equal and to fill up space 

 without leaving gaps, they must be of a limited number of 

 types. These are as follows : — 



(1) Parallelepipeda : if the atoms are of the same kind 

 these may be expected to be cubes. (2) Hexagonal Prisms. 

 (3) Rhombic Dodecahedra. (4) Cubo-octahedra. 



Let us consider these in order. 



Parallelepipeda. 



If there is an electron at each of the eight corners of the 

 parallelepipedon, then since each corner is common to eight 

 parallelepipeda, the number of electrons is equal to the 

 number of cells. Thus this is a possible arrangement for 

 monovalent elements. If all the atoms are of the same kind 

 the parallelepipeda will be cubes and the atoms themselves 

 will be on the simple space lattice formed by their centres. 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 13. No. 256. April 1922. 3 A 



