RADIOACTIVITY— RUTHERFORD 167 



of the nuclear structure of atoms, the conception of the isotopic con- 

 stitution of the elements, and the proof of an extraordinarily simple 

 relation between the chemical properties of the radio-elements known 

 under the name of the "displacement law." 



The discoveiy of the electron in 1897 and the proof that it was a 

 constituent of all atoms gave a great impetus to the belief that atoms 

 were electrical structures. In 1904 Sir J. J. Thomson had proposed his 

 well-known model atom and devised methods for estimating the num- 

 ber of electrons contained in each atom. On account of its mass and 

 great energy of motion, the a-particle offered great advjjntages as a 

 projectile to investigate the inner structure of atoms. It was known 

 that it traveled through matter in nearly a straight line and must pene- 

 trate freely the structure of the atoms in its path. In addition, the 

 scintillation method provided a delicate means of counting individual 

 a-particles. The proof that the a-particle occasionally suffered a de- 

 flection through a large angle as the result of a single collision provided 

 clear evidence that enormous deflecting forces existed within the atom. 

 From these observations, I was led in 1911 to the idea that the atom 

 was a very open electronic structure containing at its center a very 

 minute charged nucleus in which most of the mass of the atom was con- 

 centrated. The properties of the atom were defined by an integer rep- 

 resenting the number of units of resultant charge carried by the nu- 

 cleus. The fine experiments of Geiger and Marsden gave convincing 

 evidence of the accuracy of the laws of scattering of a-particles calcu- 

 lated on this hypothesis and also gave us approximate estimates of the 

 nuclear charge of the elements. As you know, this conception that the 

 properties of the atom are defined by an integral number was verified 

 and extended by the splendid experiments of Moseley on the X-ray 

 spectra of the elements. He showed that the properties of an atom 

 depended on its ordinal number, and identified this with the nuclear 

 charge — a result later substantiated by Chad wick by direct determi- 

 nation of the nuclear charge by scattering experiments. Moseley's 

 work was of far-reaching importance, for it fixed once for all the 

 number of the elements between hydrogen and uranium and gave the 

 atomic number and X-ray spectra of the missing elements, several of 

 which have since been discovered. 



The next two important discoveries were a direct consequence of a 

 very careful study of the chemical properties of the radio-elements. 

 Several observers had noted that it was impossible, chemically, to sepa- 

 rate certain radioactive elements when mixed together; for example, 

 thorium and ionium, radium-D and lead, radium and mesothorium, 

 although these elements showed quite distinctive radioactive proper- 

 ties and were believed to be of different atomic weights. Soddy con- 

 cluded that these elements of identical chemical properties must occupy 



