

522 Sir J. J. Thomson on the Structure of 



regard to valency very similar to those advanced by Abegg 

 and BodUinder (Zeit. awry. Chemk, 1899, xx. p. 453, 1904, 

 xxxix. ]>. 330), who ascrilNed to each element two valencies 

 according as it was combine^vdik a more electro-negative 

 or more electro-positive element, the sum of these valencies 

 always being eight. Cohen (Organic Chemistry, vol. 2. 

 p. 3) says that the weak point of this scheme is that there 

 are no compounds in which the alkali metals possess the 

 valency 7. We should not expect, on the views given above, 

 that these compounds could exist, for on this view the two 

 valencies arise from saturation being arrived at in two 

 different ways, one, e.g., when nitrogen was acting like a 

 triad, through the layer round the atom having acquired the 

 maximum number of atoms consistent with stability ; the 

 other, e. g. n when nitrogen acts like a pentad, when all the 

 electrons associated with the atom have been used up in 

 binding other atoms to it. We should not expect those 

 atoms which possess only a small number of electrons to show 

 both kinds of valencies. For example, we should not expect 

 to find hydrogen acting like a septavalent element, for this 

 would mean that a layer of eight electrons w T as in stable 

 equilibrium round the hydrogen nucleus which has only unit 

 positive charge. Now a layer of eight electrons requires a 

 very considerable positive charge to keep it in stable equi- 

 librium, and it is improbable that a single positive charge 

 inside, even though it were assisted by seven positive charges 

 outside, would be able to do so. 



The freedom of motion of the electrons in an atom is of 

 importance in connexion with the attraction which the atom 

 is likely to experience from other atoms. We can illustrate 

 this point by considering^an atom containing one electron ct 

 which will be free to move in any direction provided its 

 distance from the centre of the atom does not vary. Suppose 

 that the atom is placed near a positively electrified body B,. 

 then whatever may have been the initial position of the elec- 

 tron a, it will swing round A until it gets as near as possible 

 to B ; the attraction of B on a will then be greater than the 

 repulsion between B and the positive nucleus A, so that 

 the atoms will always be attracted towards B. If a. had not 

 been free to adjust itself under the force exerted by B, the 

 atom would just as likely be repelled from B as attracted by 

 it. We see from this that a very rigid disposition of the- 

 electrons in an atom will diminish the likelihood of its being, 

 attracted by or attracting other atoms. 



