520 Sir J. J. Thomson on the Structure of 



reached. Thus, if A were a 6 electron atom it could form 

 the unsaturated compound AB and the saturated one AB 2y 

 in which again there is a cell of 8 electrons round the more 

 heavily charged atom ; A could not Combine with more 

 than 2 of the B atoms. Tims, A with its 6 electrons would 

 act like a divalent atom, and in this respect could represent 

 oxygen. We see, too, that if A contained 5 electrons it 

 could form the unsaturated compounds AB, AB 2 , and the 

 saturated one AB 3 , but not any containing more than 3 B 

 atoms. The 5 electron atom would thus act like a trivalent 

 atom, and could thus in this respect represent nitrogen, the 

 electrons in the molecule forming a cell of 8 round the 

 nitrogen atom. 



We see too that since the attachment of a 1 one-electron 

 atom like that of hydrogen requires two electrons on the 

 layer round the atom with which it is combined, we cannot 

 have more than four hydrogen atoms attached to a single 

 atom; this is in accordance with experience. 



We can illustrate by meaus of the 5 electron atom the 

 fact that, on the view of the constitution of the atom we have 

 assumed, an atom may possess two valencies, the sum of these 

 two being always equal to eight. Let us take for example 

 the nitrogen atom, then, if it enters into combination with 

 hydrogen, we have seen that 3 hydrogen atoms will saturate 

 it as they will bring the number of electrons in the layer- 

 round the nitrogen nucleus up to 8, the limiting number. 

 But suppose that, instead of combining with a hydrogen 

 atom, it combines with chlorine whose atom we assume to 

 have 7 electrons in the outer layer. One of the electrons of 

 the nitrogen atom may join the layer round the chlorine 

 atom, bringing the number of electrons up to 8, the limiting 

 number, and leaving 4 electrons in the layer round the 

 nitrogen nucleus. These 4 electrons can link up with four 

 atoms of hydrogen, this will bring the number of electrons in 

 the layer round the nitrogen nucleus up to 8, the limiting- 

 number, giving the compound NH 4 C1 7 the arrangement of 

 the electrons being as represented in fig, 3, where the ring 

 denotes a cell of 8 electrons. 



Fig. 3. 



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As the chlorine nucleus has a positive charge of 7 and is 

 surrounded by a layer containing 8 electrons, the system © 



