516 - Sir J. J. Thomson on the Structure of 



more than 8 on the surface of the sphere, the positive charge 

 at the centre will have to be greater than the charge on the 

 electrons. I believe that it is this fact which governs the 

 structure both of the atom and the molecule and determines 

 the qualities which the chemists group under the term 

 valency. 



For the chemical atom is electrically neutral, the positive 

 charge is equal to but not greater than the" sum of the charges 

 on the negative electrons, hence no arrangement of electrons 

 is possible which requires for its stability a central charge 

 greater than the sum of the charges on the electrons. 



Very simple considerations will show the consequences 

 which follow from this fact. I will suppose that the electrons 

 are arranged symmetrically on the surface of a sphere whose 

 centre is at the centre of the positive charges. This arrange- 

 ment, as we have seen, is possible if there are not more than 



8 electrons, and we can have atoms with from one to eight 

 electrons on the surface of a sphere surrounding a positive 

 charge equal to the sum of the charges on the electrons. 



Now let us consider the case of an atom containing 



9 electrons. The symmetrical distribution of 9 electrons over 

 the surface of a sphere requires a central charge of more 

 than 9 units to keep it in stable equilibrium, but when there 

 are 9 electrons the central charge is 9 and hence is insuffi- 

 cient for this distribution. A new distribution will be 

 required which will be of the following kind. The central 

 charge 9 can hold 8 electrons in equilibrium on the surface 

 of a concentric sphere, so that 8 of the electrons will group 

 themselves round the central charge and there will be one 

 over, this will go outside the shell of '8 and find a position of 

 stable equilibrium at a greater distance from the centre. 

 Thus the external layer of this atom will contain only one 

 electron and in this respect will resemble the atom with one 

 electron and unit positive charge. Now suppose we have an 

 atom with 10 electrons, eight of these will form the inner 

 shell and two will be left over to form the outer ; thus we 

 shall have an atom resembling as far as the outer ring is 

 concerned the 2 electron atom with the double positive 

 charge. If we increase the number of electrons still further 

 we shall get outer rings with 3-4 ---8, but there will never 

 be more than 8 in the outer shell. When the electrons have 

 increased so much that 8 on the outer shell is not sufficient 

 to accommodate them, a third shell will be formed, and when 

 this is filled up with 8 electrons a fourth will be formed, and 

 when this is filled up a fifth and so on. Thus, if we confine 

 our attention to the outer layer and arrange the elements in 



