B.— CHEMISTRY. 29 



the structure of those compounds with which the former theory breaks 

 down : it can account for their composition, their properties, their 

 isomerism, and even their stereoisomerism. There thus appear to be 

 two different modes of chemical combination, each holding within its own 

 sphere, but neither applicable to the whole of chemistry. This was 

 obviously a most unsatisfactory position, and one which could only be 

 temporary. It was clear that the true theory of molecular structure 

 when it was discovered must be one which would apply to all compounds, 

 both organic and inorganic, and that the two rival theories, that of 

 structural chemistry and that of co-ordination, must ultimately prove 

 to be two partial aspects of the same general phenomenon. 



The final solution of the problem was scarcely to be expected until a 

 more definite idea had been reached of the physical mechanism of atomic 

 linkage, and this could only be attained when more was known of the 

 structure of the atom. The discovery by Sir Joseph Thomson and others 

 of the electron as a universal constituent of all forms of matter had 

 suggested that it was in this that the mechanism of valency was to be 

 sought ; but a further development of our knowledge of the electronic 

 arrangement was necessary before it could be applied in detail to answer 

 the questions asked by the chemist. This development was reached, in 

 the years from 1911 onwards, mainly through the work of Rutherford, 

 Bohr, and Moseley. Through their researches we learnt that the atom 

 consists of a positive nucleus surrounded by groups of electrons, and that 

 each successive element in the periodic table contains one more unit of 

 positive charge on its nucleus than the one before it, and one more 

 planetary electron : the atomic number being at once the ordinal number 

 of the element in the periodic table, the number of units of positive 

 charge on the nucleus, and the number of surrounding electrons. The 

 conceptions of the nuclear atom and of atomic number may be said to 

 give us the empirical formula of the atom. The next stage, the deter- 

 mination of the structural formula, of the way in which the surrounding 

 atoms are arranged, although it is not yet complete, has been so far 

 developed by means of the Bohr theory and its subsequent modifications, 

 that we are now in a position to apply the physical results to the solution 

 of the purely chemical problems of valency and molecular structure. 



It is evident that the cause of chemical combination is the striving 

 of atoms to attain more stable arrangements of their planetary electrons 

 by some kind of redistribution. The inert gases, since they do not enter 

 into chemical combination, must already possess an arrangement too 

 stable to be capable of improvement ; their atomic numbers therefore 

 give us the sizes of a series of completed or stable groups, and it may be 

 expected that when other atoms combine to form a molecule, they thereby 

 attain these numbers of electrons, or something like them. 



The application of these ideas in detail to the explanation of valency 

 was primarily due to Kossel and G. N. Lewis, who published their views 

 almost simultaneously in 1916. Kossel dealt with ionised links, and 

 showed that their structure could be explained by supposing that they 

 were due to the migration of one or more electrons from an atom which 

 had a few more than a stable (inert gas) number, to another which had 

 a few less ; hence the valency in ionised compounds was usually equal to 



