September 8, 1921] 



NATURE 



53 



little opportunity, as yet, of extending it to any 

 considerable number of the metallic elements. 

 These, as will be obvious from the nature of the 

 methods employed, present special difficulties. It 

 is, however, highly probable that mercury is a 

 mixed element consisting of many isotopes. These 

 have been partially separated by Bronsted and 

 Hevesy by fractional distillation at very low pres- 

 sures, and have been shown to vary very slightly 

 in density. Lithium is found to consist of two iso- 

 topes, 6 and 7. Sodium is simple, potassium and 

 rubidium are complex, each of the two latter ele- 

 ments consisting, apparently, of two isotopes. The 

 accepted atomic weight of caesium, I32"8i, would 

 indicate complexity, but the mass spectrum shows 

 only one line at 133. Should this be confirmed, 

 caesium would afford an excellent test case. The 

 accepted value foV the atomic weight is sufficiently 

 far removed from a whole number to render 

 further investigation desirable. 



This imperfect summary of Dr. Aston 's work is 

 mainly based upon the account he recently gave to 

 the Chemical Society. At the close of his lecture 

 he pointed out the significance of the results in 

 relation to the periodic law. It is clear that the 

 order of the chemical or " mean " atomic weights 

 in the periodic table has no practical significance ; 

 anomalous cases, such as argon and potassium, 

 are simply due ta the relative proportions of their 

 heavier and lighter isotopes. This does not neces- 

 sarily invalidate or even weaken the periodic law, 

 which still remains the expression of a great 

 natural truth. That the expression as Mendeleeff- 

 left it is imperfect has long been recognised. The 

 new light we have now gained has gone far to 

 clear up much that was anomalous, especially 

 Moseley's discovery that the real sequence is the 

 atomic number, not the atomic weight. This is 

 one more illustration of the fact that science ad- 

 vances by additions to its beliefs rather than by 

 fundamental or revolutionary changes in them. 



The bearing of the electronic theorv- of matter, 

 too, on Front's discarded hypothesis that the 

 atoms of all elements were themselves built up of a 

 primordial atom — his protyle, which he regarded 

 as probably identical with hydrogen^is too ob- 

 vious to need pointing out. In a sense, Front's 

 hypothesis may be said to be now re-established, 

 but with this essential modification — the primor- 

 dial atoms he imagined are complex and are of two 

 kinds — atoms of positive and negative electricity 

 — respectively known as protons and electrons. 

 These, in Dr. Aston 's words, are the standard 

 bricks that Nature employs in her operations of 

 element building. 



The true value of any theory consists in its com- 

 prehensiveness and sufficiency. As applied to 

 chemistrj^-, this theory of " the inner mechanism 

 of the atom " must explain all its phenomena. We 

 owe to Sir J. J. Thomson its extension to the 

 explanation of the periodic law, the atomic num- 

 ber of an element, and of that var\'ing power of 

 chemical combination in an element we term 



lency. This explanation I give substantially in 

 XO. 2706. VOL. 108] 



his own words. The number of electrons in an 

 atom of the different elements has now been deter- 

 mined, and has been found to be equal to the 

 atomic number of the element, that is to the posi- 

 tion which the element occupies in the series when 

 the elements are arranged in the order of their 

 atomic weights. We know now tlie nature and 

 quantity of the materials of which the atoms are 

 made up. The properties of the atom will depend 

 not only upon these factors, but also upon the way 

 in which the electrons are arranged in the atom. 

 This arrangement will depend on the forces be- 

 tween the electrons themselves and also on those 

 between the electrons and the positive charges or 

 protons. One arrangement which naturally sug- 

 gested itself is that the positive charges should be 

 at the centre with the negative electrons around it 

 on the surface of a sphere. Mathematical investi- 

 gation shows that this is a possible arrangement 

 if the electrons on the sphere are not too crowded. 

 The mutual repulsion of the electrons resents ov'er- 

 crowding, and Sir J. J. Thomson has shown that 

 when there are more than a certain number of 

 electrons on the sphere, the attraction of a posi- 

 tive charge, limited as in the case of the atom in 

 magnitude to the sum of the charges on the elec- 

 trons, is not able to keep the electrons in stable 

 equilibrium on the sphere, the layer of electrons 

 explodes, and a new arrangement is formed. The 

 number of electrons which can be accommodated 

 on the outer layer will depend upon the law of 

 force between the positive charge and the elec- 

 trons. Sir J. J. Thomson has shown that this num- 

 ber will be eight with a law of force of a simple 

 type. 



To show^ the bearing of this result as afi"ording 

 an explanation of the periodic law, let us, to 

 begin with, take the case of the atom of lithium, 

 which is supposed to have one electron in the 

 outer layer. As each element has one more free 

 electron in its atom than its predecessor, glucinum, 

 the element next in succession to lithium, will 

 have two electrons in the outer layer of its atom, 

 boron will have three, carbon four, nitrogen five, 

 oxygen six, fluorine seven, and neon eight. As 

 there cannot be more than eight electrons in the 

 outer layer, the additional electron in the atom of 

 the next element, sodium, cannot find room in the 

 same layer as the other electrons, but wiU go out- 

 side, and thus the atom of sodium, like that of 

 lithium, will have one electron in its outer layer. 

 The additional electron, in the atom of the next 

 element, magnesium, will join this, and the atom 

 of magnesium, like that of glucinum, will have two 

 electrons in the outer layer.. Again, aluminium, 

 like boron, will have three ; silicon, like carbon, 

 four; phosphorus, like nitrogen, five; sulphur, like 

 oxygen, six ; chlorine, Hke fluorine, seven ; and 

 argon, like neon, eight. The sequence will then 

 begin again. Thus the number of electrons, one, 

 two, three, up to eight, in the outer layer of the 

 atom, will recur periodically as we proceed from 

 one element to another In the order of their atomic 

 weights, so that any property of an element which 



