518 Sir J. J. Thomson on the Structure of 



force we are considering no element could have more than 

 8 electrons in the outside layer. 



I shall suppose (as in my paper on the "Forces between 

 Atoms and Chemical Affinity," (Phil. Mag. ser. 6, vol. xxvii. 

 p. 757, 1914), that those atoms which have one electron in the 

 outer layer form the first Mendeleefian group, those with two 

 in this layer the second group, and so on. Thus the hydrogen . 

 and lithium atoms are supposed to have one electron in the 

 outer layer, the beryllium atom 2, the boron atom 3, the 

 carbon atom 4, the nitrogen atom 5, the oxygen atom 6, the 

 fluorine atom 7, and the neon atom 8. 



Let us now consider if any light is thrown on this view by 

 the evidence afforded by positive rays. It eight is the 

 maximum number of electrons which can exist in ihe outer 

 layer, then the atom of neon already possesses that number 

 and cannot accommodate another electron, and so cannot 

 receive a negative charge. On the other hand, the atoms with 

 a smaller number of electrons in the outer layer have, as a 

 reference to the table 2 shows, a superfluity of stability, and 

 so could accommodate another electron and thus acquire a 

 negative charge ; the superfluity is however not great enough 

 for them to accommodate two so that we should not expect to 

 find any with a double negative charge. 



In experiments with positive rays neon has never been 

 observed with a negative charge, while negative charges are 

 common on hydrogen, chlorine, oxygen, and carbon ; we have 

 no information about boron and beryllium : thus far the 

 evidence is in favour of the view. On the other hand, no 

 negative charge has as yet been observed on the atom of 

 nitrogen : this is remarkable, as the atoms of the elements on 

 either side of it, carbon and oxygen, readily acquire a negative 

 charge. It must be remembered, however, that the number 

 of atoms with negative charges A'aries verv much with the 

 conditions of the discharge and the gases in the discharge- 

 tube. To get a negative charge a neutral atom has to drag 

 an electron from another atom or molecule, and unless an 

 atom of nitrogen came in contact with an atom of some sub- 

 stance which held its electrons more loosely than the nitrogen, 

 it might not be able to capture the electron necessary to <uve 

 it the negative charge. It will be seen that all the atoms 

 except hydrogen, lithium, sodium, potassium, could lose more 

 than one electron and thus have double or treble positive 

 charges : this is in accordance with the evidence afforded by 

 the positive rays. 



We have, however, to explain why an atom with 7 electrons 



