CONTEMPORARY ADVANCES IN PHYSICS 59 



Sooner or later, in expounding almost any topic in physics, one 

 arrives at a place where the introduction of an atom-model greatly 

 simplifies what remains to be said. In the present article, this is the 

 place. 



Physicists commonly employ an atom-model in which a certain 

 number of electrons are arranged around a nucleus bearing a charge 

 equal in magnitude and opposite in sign to the sum of their charges. 

 For any particular element the number of electrons assigned to its 

 atom-model is equal to its atomic number, which can be obtained from 

 any modern chart of the Periodic Table. In such a chart the elements 

 are arranged in the order of their atomic numbers from 1 to 92, 

 composing what I shall call the procession of the elements — a procession 

 from which only two are now missing. In dealing with an element of 

 high atomic number — all of the radioactive elements are of this char- 

 acter, ranging in atomic number from 81 upwards'* — the electrons are 

 assigned to various locations, some being close to the nucleus and 

 others intermediate and others at the periphery of the atom-model. 

 In fitting the various regions and divisions of the atom-model to the 

 various properties of the element which it represents, the outermost 

 electrons are assigned to the task of accounting for those properties 

 which vary exceedingly with the state of chemical combination and 

 with the other circumstances of the element ; for being at the surface of 

 the atom they should be most exposed to outer influences. The inner 

 electrons, being partly shielded, are used to account for such properties 

 as the X-ray frequencies, which depend so little upon the circumstances 

 of the element that their variations are scarcely perceptible or not at 

 all. The nucleus is the best shielded of all, and it receives for its 

 quota the two properties which within the accuracy of experiment are 

 immune from change — radioactivity and mass. 



There are additional reasons for assigning mass and radioactivity 

 to the nucleus. As for the mass: since the sum of the masses of the 

 electrons constituting an atom-model never attains 1/1800 of the known 

 mass of the atom, the balance which the nucleus must take is prac- 

 tically the whole of it. Again, there are experiments which show that 

 a single chemical element may have several kinds of atoms differing 

 in mass and yet quite alike in chemical properties, in their line-spectra, 

 in their X-ray spectra; since these similarities require that the same 

 nucleus-charge and the same number and arrangement of electrons be 

 imposed upon all these atoms, the outstanding diff'erence in their 

 masses must be ascribed to their nuclei.^ Again, there are slight 



* Except potassium and rubidium (compare footnote 13). 

 ^ Introduction, pp. 29-39, 65-66. 



