584 



NATURE 



[May 6, 1922 



Artificial Disintegration of the Elements. 

 By Sir Ernest Rutherford, F.R.S. 



SINCE the development of the atomic theory on 

 an experimental foundation by Dalton, the 

 progress of chemistry has been based on the central 

 idea of the permanency and indivisibility of the atoms 

 of the elements. The whole experience of chemistry 

 for nearly a century had shown clearly that it was 

 impossible to break up the atoms of the elements by 

 the application of ordinary chemical and physical 

 processes. This idea has had to be modified to some 

 extent by the rapid growth of our knowledge during 

 the last twenty years of the inner constitution of the 

 atoms. 



It is now generally accepted that the atoms of the 

 different elements have all the same general type of 

 structure. At the centre of the atom is a positively 

 charged nucleus of minute dimensions which is re- 

 sponsible for most of the mass of the atom. This is 

 surrounded by a distribution of electrons held in 

 equilibrium by the forces from the nucleus. The 

 electrons occupy rather than fill a region the diameter 

 of which is of the order of 2 x lo"^ cm. The nuclear 

 charge of the atoms follows a very simple rule first 

 clearly brought to light by Moseley. The resultant 

 nuclear charge of an atom is equal to its atomic or 

 ordinal number, and varies from i " atom " of elec- 

 tricity in the case of hydrogen to 92 in the case of 

 uranium. These ordinal numbers represent also the 

 number of " planetary " electrons, as they have been 

 called, which surround the nucleus of the atom. On 

 this view of the atom, its ordinary physical and chemical 

 properties, apart from its mass, are governed entirely 

 by the nuclear charge, for this controls the number and 

 arrangement of the external electrons on which these 

 combining properties mainly depend. The mass of 

 the atom is a property of the nucleus and exercises only 

 a second order effect on the distribution of the electrons 

 and so on the ordinary properties of the atom. 



This point of view offers at once a simple explanation 

 of isotopes, which consist of atoms of the same nuclear 

 charge but of different nuclear masses. By the action 

 of light and electrical discharges, we can readily remove 

 one or more of the external planetary electrons from 

 the atom, while by the action of X-rays and swift ^- 

 rays we may even eject one of the more strongly bound 

 electrons of the system. In this way, we can effect, in 

 a sense, a transformation of the atom, but it is merely 

 a temporary one, and a new electron is soon captured 

 from outside, and the atom is as before. The general 

 evidence indicates that, even if a number of the planet- 

 ary electrons were removed by suitable agencies, the 

 stability of the nucleus would not be disturbed and the 

 atom would in a short time regain its original structure. 

 In order to effect a permanent change in the atom, it 

 appears to be necessary to disrupt the nucleus itself. 

 When once a charged unit of the nuclear structure is 

 removed, the nuclear charge is altered permanently, 

 and there is no evidence that this process is reversible 

 under ordinary experimental conditions. 



The discovery of the instability of the radioactive 



» A Lecture delivered before the Chemical Society on February 9. 

 NO. 2740, VOL. 109] 



elements was the first severe shock to the idea of the 

 permanency of all atoms. This radiating property is, 

 however, confined mainly to the two heaviest elements, 

 uranium and thorium, and their long series of descend- 

 ants, and is shown only by two other elements, potas- 

 sium and rubidium, and then only to a minor extent. 

 Apart from these exceptions, the great majority of the 

 atoms appear to be exceedingly stable structures, and 

 to remain unaltered under ordinary conditions in this 

 earth for periods of probably thousands of miUions of 

 years. 



The property of radioactivity belongs to the nucleus, 

 and is shown generally by the emission of a swift 

 a-particle or helium nucleus and, occasionally, a swift 

 electron or /3-ray from the nucleus. The number and 

 velocity of emission of these particles appear to be 

 quite uninfluenced by the most powerful physical or 

 chemical agencies, and to be an inherent property re- 

 sulting from the instability of these very complex nuclei. 



These results show clearly that the nuclei of heavy 

 atoms contain both positively charged helium nuclei 

 and negative electrons, and lead to the general view 

 that the complex nuclei of all atoms are built up of 

 hydrogen and helium nuclei and electrons. It is also 

 generally supposed that a helium nucleus itself is a 

 secondary unit composed of four hydrogen nuclei and 

 two electrons. If this be the case, we may suppose 

 the nuclei of all atoms to be composed ultimately of 

 hydrogen nuclei, or " protons," as they have been 

 termed, with the addition of negative electrons. 



Radioactivity has thus not only provided us with 

 the key of the structure of the elements, but has at the 

 same time given us in the swift a- and /i-particles a 

 powerful method of probing the inner structure of the 

 atom. By firing a-particles into the atoms of matter, 

 we are able, by following the deflexions of the path of 

 the a-particle, to find out the magnitude and law of 

 the forces close to the nucleus and to form some idea 

 of the dimensions of the latter. The general results 

 suggest that the diameter of the nucleus of heavy atoms 

 is of the order of 4X io~i2 ^-^^ or about 1/5000 of the 

 diameter of the whole structure of the atom. The law 

 of the inverse square of repulsion between electric 

 charges is found to hold for a considerable region 

 surrounding the nucleus. No doubt the size of the 

 nuclei of light atoms is even smaller, and in the case 

 of helium appears to be of the order of 5 x lo"^^ cm. 

 It is thus clear that the nuclei of atoms, although of 

 very complex structure, are of exceedingly small 

 dimensions. 



It is probable that the forces which bind together the 

 components of the nucleus are exceedingly powerful, 

 and that consequently a large amount of energy will 

 be required to disrupt its structure. The swift a- 

 particle from radium or thorium, which is by far the 

 most concentrated source of energy known to us, seems 

 the agent most likely to succeed in an attack on the 

 strongly-bound nucleus. The a-particle is expelled 

 from radium with a velocity of about ten thousand 

 miles per second, and thus has a speed twenty thousand 



