the Molecule and Chemical Combination. 517 



the order of the number of electrons they contain, there will 

 be a periodicity in the number of electrons in the outer layer ; 

 it will increase from one to eight, then drop again to one, 

 increase again to eight, drop to one, and so on. Thus as far 

 as properties depending on the outer layer are concerned the 

 element lvill exhibit a periodicity similar to that expressed by 

 ' Mendeleef's periodic law in Chemistry. 



It maybe pointed out, however, that the number of elements 

 included within a period may possibly for the elements of 

 large atomic weight be greater than eight. This might 

 arise in the following way. Suppose that for a particular 

 value of N a central charge N can hold n electrons in stable 

 equilibrium, while N + l can hold n + l. Then the atom of 

 the element whose atomic number is N would have a layer of 

 n electrons next the centre and other layers outside, the atom 

 of the element next in order would have a central charge of 

 N + l units and contain (N + l) electrons ; but as a charge 

 N + 1 can hold n + l electrons in stable equilibrium the 

 innermost layer might now contain n + l electrons, and thus 

 the additional electron might be used up in the inner layer 

 and not affect the number in the surface layer. Thus the 

 atoms of the elements whose atomic numbers are N and N + l 

 will have different atomic weights and different central 

 charges, while the number of electrons in the surface layer 

 will be the same : hence, if there are any properties which 

 depend exclusively on the number of electrons in the outer 

 layer, these two elements will have these properties in 

 common. If we are right in supposing that the valency of 

 the element is a property of this kind, then the two elements 

 will have the same valency. 



When there are a great number of electrons in the atom 

 arranged in many layers, it may require the addition of 

 several electrons before a new electron rinds its way to the 

 outer layer, and thus there might be a considerable number of 

 elements with different atomic weights but with very similar 

 chemical properties. There are groups of elements such as 

 the iron, nickel, and cobalt group, the rhodium group, and the 

 crowd of elements known as the rare earths which fulfil this 

 condition. 



Unless the atom is electrified the total charge inside the 

 outer layer of electrons must always be equal to the charge 

 on those electrons. Thus, however many electrons there may 

 be in the atom, the electrons in the outer layer cannot be 

 under the influence of an effective charge greater than the 

 sum of the charges on these electrons, so that with the law oi' 



Phil. Mag. S. 6. Vol. 41. No. 243. March 1921, 2 M 



