August 5, 1922] 



NA TURE 



to suppose that the mass of the electron is entirely 

 electrical in origin, and no advantage is gained by 

 supposing that any other type of mass exists. If the 

 atom is a purely electrical structure, the mass of the 

 atom itself must be due to the resultant of the electric 

 mass of the charged particles which make up its 

 structure. We shall see that only a small fraction 

 of the mass of an atom can be ascribed to the negative 

 electrons contained in it, but the main part is to be 

 ascribed to the positively charged units of its structure. 

 One of the main difficulties in our attack on the question 

 of atomic constitution has lain in the uncertainty of 

 the nature of positive electricity. Without entering 

 upon the changes in point of view on this important 

 question, it may suffice to say that the evidence as a 

 whole supports the idea that the nucleus of the hydrogen 

 atom, i.e. a positively charged atom of hydrogen, is 

 the positive electron. No evidence has been obtained 

 of the existence of a positively charged unit of mass 

 less than that of the hydrogen nucleus, either in 

 vacuum tubes or in the transformation of the radio- 

 active atoms, where the processes occurring are very 

 fundamental in character. 



It might a priori have been anticipated that the 

 positive electron should be the counterpart of the 

 negative electron and have the same small mass. 

 There is, however, not the slightest evidence of the 

 existence of such a counterpart. On the views out- 

 lined, the positive and negative electrons both consist 

 of the fundamental unit of charge, but the mass of 

 the positive is about 1800 times that of the negative. 

 This difference in the mass of the two electrons seems 

 a fundamental fact of nature and, indeed, is essential 

 for the existence of atoms as we know them. The 

 unsymmetrical distribution of positive and negative 

 electricity that is characteristic for all atoms is a 

 consequence of this wide difference in the mass of the 

 ultimate electrons which compose their structure. No 

 explanation can be offered at the moment why such 

 a difference should exist between positive and negative 

 electricity. 



Since it may be argued that a positive unit of elec- 

 tricity associated with a much smaller mass than the 

 hydrogen nucleus may yet be discovered, it may be 

 desirable not to prejudge the question by calling the 

 hydrogen nucleus the positive electron. For this 

 reason, and also for brevity, it has been proposed that 

 the name " proton " should be given to the unit of 

 positive electricity associated in the free state with a 

 mass about that of the hydrogen nucleus, namely, 

 about 1-007 in terms of = i6. A name for this unit 

 will be found very convenient in discussing the inner 

 structure of atoms. In the following, the term " elec- 

 tron " will be applied only to the well-known negative 

 unit of electricity of small mass. 



On the classical electrical theory, the mass of the 

 electron can be accounted for by supposing that the 

 negative electricity is distributed on a spherical surface 

 of radius about ixio" 1! cm. This is merely an 

 estimate, but probably gives the right order of magni- 

 tude of the dimensions, though it should be pointed 

 out that in some recent theories of Compton and others 

 it has been supposed that the electron behaves like a 

 flexible ring, the dimensions of which are about 10^ u 

 cm., or about 100 times the original estimate. Without 



NO. 2753, VOL. I IO] 



going into these difficult questions, what little experi- 

 mental evidence there is seems to me to support the 

 older estimate of size. Taking the view based on 

 the older theory, the greater mass of the proton is to 

 be explained by supposing that the distribution of 

 electricity is much more concentrated for the proton 

 than for the electron. Supposing the shape spherical, 

 the radius of the proton should be only , J„ lT of that 

 of the electron. If this be so, the proton has the 

 smallest dimensions of any particle known to us. It 

 is admittedly very difficult to give any convincing 

 proof in support of this contention, but at the same 

 time there is no evidence against it. From the point 

 of view of simplicity of explanation, it is natural to 

 make the assumption that the proton and the electron 

 are the fundamental units of which all atoms are built. 



It would take too long to consider in any detail the 

 gradual development in the last twenty years of our 

 ideas on the structure of atoms. Progress has depended 

 mainly on a clearer understanding of the relative part 

 played by positive and negative electricity in atomic 

 structure. It is now generally accepted that the 

 atom is an electrical system and that the atoms of 

 all the elements have a similar type of structure. 



The nuclear theory of atomic constitution has been 

 found to be of extraordinary value in offering an 

 explanation of the fundamental facts that have come 

 to light, and is now generally employed in all detailed 

 theories of atomic constitution. At the centre of each 

 atom is a massive positively charged nucleus of dimen- 

 sions minute compared with the diameter of the atom. 

 This nucleus is surrounded by'a distribution of negative 

 electrons which extend to a distance, and occupy rather 

 than fill a region of diameter about 2 x io" 8 cm. Apart 

 from the mass of the atom, which resides mainly in the 

 nucleus, the number and distribution of the outer 

 electrons, on which the ordinary physical and chemical 

 properties of the atom depend, are controlled by the 

 magnitude of the nuclear charge. The position and 

 motions of the external electrons are only slightly 

 affected by the mass of the nucleus. According to 

 this view of the atom, the problem of its constitution 

 naturally falls into two parts — first, the distribution 

 and mode of motion of the outer electrons, and secondly, 

 the structure of the nucleus and the magnitude of the 

 resultant positive charge carried by it. In a neutral 

 atom the number of external electrons is obviously 

 equal in number to the units of positive (resultant) 

 charge on the nucleus. 



The general conception of the nuclear atom arose 

 from the need of explanation of the very large deflec- 

 tions experienced by swift a- and ^-particles in passing 

 through the atoms of matter. A study of the number 

 of a-particles scattered through different angles showed 

 that there must be a very intense electric field within 

 the atom, and gave us a method of estimating the 

 magnitude of the charge on the nucleus. Similarly 

 the scattering of N-rays by the outer electrons provided 

 us with an estimate of the number of these electrons 

 in the atom, and the two methods gave concordant 

 values. The next great advance we owe to the experi- 

 ments of Moseley on the N-ray spectra of the elements. 

 He showed that his experiments received a simple 

 explanation if the nuclear charge varied by one unit 

 in passing from one atom to the next. In addition, 



