7°4 



NA TURE 



[November 25, 1922 



sources, that substances could exist which, though 

 chemically identical, had different atomic weights. 

 These substances Soddy called " isotopes " as they 

 occupy the same place in the periodic table of the 

 elements. 



The first experimental comparison of the weights of 

 individual atoms was made by Sir J. J. Thomson in 

 his analysis of positive rays by the " parabola " method. 

 Subjected to this test most of the lighter elements 

 appeared to follow Dalton's rule, but the results with 

 the rare gas neon suggested the possibility of the atoms 

 of this element being of two different weights, roughly 

 20 and 22 respectively. In other words the parabolas 

 of neon indicated that it might be a mixture of isotopes, 

 but the accuracy of measurement by this method was 

 not sufficient to settle the point with certainty. 



The requisite accuracy has been obtained by an 

 instrument for the analysis of positive rays called the 

 " mass-spectrograph." By this device, the weights 

 of atoms can be compared to an accuracy of one- 

 tenth per cent., and it has been demonstrated not 

 only that neon (20-2) is a mixture of atoms of weights 

 exactly 20 and 22, but also that chlorine (35-46) is a 

 mixture of isotopic atoms of weights 35 and 37. 

 Furthermore, about half the elements investigated turn 

 out to be mixtures, some of the heavier ones consisting 

 of six or more different constituents. Most important 

 of all is the fact that every element investigated, with 

 the exception of hydrogen, consists of atoms the 

 weights of which are expressible as whole numbers on 

 the oxygen scale used by chemists. 



This remarkable generalisation called the " whole 

 number rule " has removed the last obstacle in the 

 way of the unitary theory of matter. We now have 

 no hesitation in affirming that Nature uses the same 

 standard bricks in the construction of the atoms of 

 all elements, and that these standard bricks are the 

 primordial atoms of positive and negative electricity, 

 protons and electrons. 



These are the natural unit charges of electricity, 

 equal but of opposite sign. Of the shape of these 

 particles we know next to nothing, but the wonderful 

 advances of modern physics, in particular those of 

 radioactivity, enable us to speak of their weights and 

 dimensions with some assurance. The weight of the 

 proton is very nearly the weight of a hydrogen atom, 

 the electron is nearly two thousand times lighter, so 

 that the atomic weight of an element (not consisting 

 of isotopes) will be roughly equal to the number of 

 protons in its atoms. The dimensions of the electron 

 are about one hundred thousand times less than those 

 of the atoms as illustrated above, and the proton is 

 probably nearly two thousand times smaller still. 



We now know of what atoms are constructed, and 

 may go on to consider the evidence as to how their 

 constituent parts are arranged. In the foregoing 

 diagrams the atoms are represented as spheres, and 

 in respect to the small forces and velocities which 

 occur in the collisions between the atoms of gases at 

 ordinary temperatures they do behave very exactly 

 as smooth elastic spheres. But unfortunately the idea 

 of a sphere carries the suggestion of a portion of space 

 full of something ; that is, the atom as a sort of spherical 

 bag packed full of electric charges. Nothing could 

 be further from the actuality, for from the figures 



NO. 2769, VOL. I IO] 



already given, it can be seen at once that even in the 

 heaviest atom known the constituent charges fail to 

 fill even the million millionth part of its whole volume. 

 To convey any direct idea of these numerical relations 

 by diagrams is practically hopeless, and were we to 

 construct a scale model of the atom as big as the dome 

 of St. Paul's we should have some difficulty in seeing 

 the electrons, which would be little larger than pin 

 heads, while the protons would escape notice altogether 

 as dust particles invisible to the unaided eye. Ex- 

 perimental evidence leaves us no escape from the 

 astounding conclusion that the atom of matter, as a 

 structure, is empty, empty as the solar system, and 

 what we measure as its spherical boundary really 

 only represents the limiting orbits of its outermost 

 electrons. 



The hypothesis which has led to the greatest advances 

 in our knowledge of the inner construction of atoms 

 is Rutherford's theory of the " nucleus atom " put 

 forward in 191 1. This is supported by so many 

 results of direct experiment that it is now universally 

 accepted and must be substantially correct. It pos- 

 tulates that all of the positive and about half of the 

 negative electricity, that is, practically the entire 

 weight, of the atom is concentrated at its centre, 

 forming a very small body called the nucleus. In 

 other words, all the protons and about half the electrons 

 in the atom are packed together, forming a sort of sun 

 round which revolve the remaining electrons as planets. 

 The number of protons in excess of electrons in the 

 nucleus will clearly be its net positive charge, and since 

 this will not depend on the gross numbers of protons 

 and electrons but only on their difference, we can have 

 elements the atoms of which have nuclei of different 

 weights but the same net charge. These are isotopes, 

 for the chemical properties of an atom are determined 

 by the charge on its nucleus. 



The nucleus is extremely small compared with the 

 whole atom. Thus, if in the atom of helium atomic 

 weight 4 atomic number 2 we take the nucleus, con- 

 sisting of 4 protons and 2 electrons, as represented by 

 a rather large pea, its planetary electrons may be 

 represented on the same scale as two rather smaller 

 peas revolving round it at a distance of a quarter of 

 a mile. The dislodgement of one of its planetary 

 electrons from an atom requires comparatively little 

 energy and is the well-known process called ionisation. 

 This change is only a temporary one, as the atom takes 

 the first opportunity of attracting it or any other 

 stray electron back into its orbit and becoming neutral 

 again. It is by a sort of continual exchange of such 

 loose electrons that electricity is conducted along 

 metallic wires. Disruption of the nucleus, on the 

 other hand, needs enormous energy, but once performed 

 must give rise to the atom of a new element. This 

 process of transmutation has been achieved by Sir 

 Ernest Rutherford, in the case of some of the lighter 

 elements, by bombarding their atoms with alpha rays, 

 which are charged helium nuclei expelled at enormous 

 speeds from radioactive atoms during their natural 

 process of disintegration. From the tiny dimensions 

 of the nucleus compared with those of the atom it 

 is obvious that the chance of getting a direct hit on 

 the nucleus is only one in many millions, but the 

 experiments show that when this does take place 



