July 26, 19 17] 



NATURE 



43; 



emerging could be questioned, as it was questioned 

 effectively by Rutherford, with regard to what was 

 inside. Sir J. J. Thomson, using the electron as the 

 messenger, had obtained valuable information as to 

 the number of electrons in the atom, but the massive 

 material a particle alone can disclose the material 

 atom. It was found that though the vast majority of 

 a particles re-emerge from their encounters with . the 

 atoms practically in the same direction as they started, 

 suffering only slight hither and thither scattering due 

 to their collisions with the electrons in the atom, a 

 minute proportion of them suffer very large and abrupt 

 changes of direction. Some are swung round, emerg- 

 ing in the opposite to their original direction. The 

 vast majority, that get through all but undeflected, 

 have met nothing in their passage save electrons, 

 8000 times lighter than themselves. The few that are 

 violently swung out of their course must have been 

 in collision with an exceedingly massive nucleus in 

 the atom, occupying only an insignificant fraction of 

 its total volume. The atomic volume is the total 

 volume swept out by systems of electrons in orbits of 

 revolution round the nucleus, and beyond these rings 

 or shells guarding the nucleus it is ordinqxily impossible 

 to penetrate. The nucleus is regarded by Rutherford 

 as carrying a single concentrated positive charge, 

 equal and opposite to that of the sum of the electrons. 



Chemical phenomena deal almost certainly with the 

 outermost system of detachable or valency electrons 

 alone, the loss or gain of which conditions chemical 

 combining power. Light spectra originate probably 

 in the same region, though possibly more systems of 

 electrons than the outermost may contribute, while 

 the X-rays and 7 rays seem to take their rise in a 

 deep-seated ring or shell around the nucleus. But 

 mass phenomena, all but an insignificant fraction, 

 originate in the nucleus. 



In the original electrical theory of matter the 

 whole mass of the atom was attributed to electrons, 

 of which there would have been required nearly 2000 

 times the atomic weight in terms of hydrogen as 

 unity. With the more definite determination of this 

 number and the realisation that there were only about 

 half as many as the number representing the atomic 

 weight, it was clear that all but an insignificant frac- 

 tion of the mass of the atom was accounted for. In 

 the nuclear hypothesis this mass is concentrated in the 

 exceedingly minate nucleus. The electromagnetic 

 theory of inertia accounts -for the greater mass if the 

 positive charges that make up the nucleus are very 

 much more concentrated than the negative charges 

 which constitute the separate electrons. The experi- 

 ments on scattering clearly indicated the existence of 

 such a concentrated central positive charge, or nucleus. 



The mathematical consideration of the results of 

 o-ray scattering, obtained for a large number of 

 different elements, and for different velocities of a ray, 

 gave further evidence that the number of electrons, 

 and therefore the + charge on the nucleus, is about 

 half the number representing the atomic weight. But 

 van der Broek, reviving an isolated suggestion from 

 a former paper full of suggestions on the periodic 

 law, which were, I think, in every other respect at 

 fault, pointed out that closer agreement with the theory 

 would be obtained if the number of electrons in the 

 atom, or the nuclear charge, was the number of the 

 place the element occupied in the periodic table. This 

 is now called the atomic number, that of hydrogen 

 being taken as i, helium 2, lithium 3, and so 

 wi to the end of the table, uranium 92, as we now 

 know. For the light elements it is practically half 

 the atomic weight, for the heavy elements rather less 

 than half. 



I pointed out that this accorded well with the law of 

 radio-active change that had been established to hold 

 NO. 2491, VOL. 99] 



over the last thirteen places in the periodic table. 

 This law might be expressed as follows : — ^The expul- 

 sion of the a particle carrving two positive charges 

 lowers the atomic number by two, while the expul- 

 sion of the /3 particle, carrying a single negative 

 charge, increases it by one. In ignorance of van der 

 Brock's original suggestion, I had, in representing 

 the generalisation, shown the last thirteen places as 

 differing uijit by unit in the number of electrons 

 in the atom. 



Then followed Moseley's all-embracing advance, 

 showing how from the wave-lengths of the X-rays, 

 characteristic of the elements, this conception ex- 

 plained the whole periodic table. The square roots 

 of the frequency of the characteristic X-rays are pro- 

 portional to the atomic numbers. The total number 

 of elements existing between uranium and hydrogen 

 could thus be determined, and it was found to be ninety- 

 two, only five of the places being vacant. The "excep- 

 tions " to the periodic law, such as argon and potass- 

 ium, nickel and cobalt, tellurium and iodine, in which 

 an element with higher atomic weight precedes in- 

 stead of succeeding one with lower, were confirmed by 

 the determination of the atomic numbers in every 

 case. From now on, this number, which represents 

 the ■¥ charge on the nucleus rather than the atomic 

 weight, becomes the natural constant which deter- 

 mines chemical character, light, and X-ray spectra, 

 and, in fact, all the properties of matter except those 

 that depend directly on the nucleus — mass and weight 

 on one hand, and radio-active properties on the other. 

 What, theji, were the isotopes on this scheme? 

 Obviously they were elements with the same atomic 

 number, the same net charge on the nucleus, but 

 with a differently constituted nucleus. Take the very 

 ordinary sequence in the disintegration series, one 

 o and two j8 rays being successively expelled in any 

 order. Two + and two — charges have been ex- 

 pelled, the net charge of the nucleus remains the 

 same, the chemical character and spectrum the same 

 as those of the first parent, but the mass is reduced 

 four units because a helium atom, or rather nucleus, has 

 been expelled as an a particle. The mass depends on 

 the gross number of -f charges in the nucleus, chem- 

 ical properties on the difference between the gross 

 numbers of ~ and — charges. But the radio-active 

 properties depend not only on the gross number of 

 charges, but on the constitution of the nucleus. We 

 can have isotopes with identity of atomic weight, as 

 well as of chemical character, which are different in 

 their stability and mode of breaking up. Hence we 

 can infer that this finer degree of isotopy may also 

 exist among the stable elements, in which case it 

 would be completelv beyond our present means to 

 detect. But when transmutation becomes possible 

 such a difference would be at once revealed. 



The case is not one entirely of academic interest, 

 because it is probable that the reconciliation of the 

 conflicting views of the geologists and chemists who 

 concluded that lead was not the ultimate product of 

 thorium, and those who by atomic weight determina- 

 tions on the lead have shown that it is, depends 

 probably on this point. 



As has loner been known. thorium-C, an isotope of 

 bismuth, disintegrates dually. For 35 per cent, of 

 the atoms disintegrating, an o ray is expelled, fol- 

 lowed by a /3 ray. For the remaining 65 per cent, 

 the ^ ray is first expelled, and is followed by the a ray. 

 The two products are both isotopes of lead, and 

 both have the same atomic weight, but they are not 

 the same. More energy is expelled in the changes 

 of the 65 per cent, fraction than in those of the 

 35 per cent. Unless they are both completely stable 

 a difference of period of change is to be anticipated. 

 The same thing is true for radium-C, though here all 



