82 SCIENCE PROGRESS 



It will be noticed that some of the places in the table have 

 been left blank. As the result of Mosely's investigation of the 

 characteristic X-ray spectra of the elements, it can now be said 

 that there are only ninety-two elements between hydrogen and 

 uranium, both inclusive, and as eighty-six are already known 

 there are still six left to be discovered. These will fill the blank 

 spaces. 



With this knowledge of the Periodic Table, to which we 

 shall have to refer again in a moment, let us go back to the 

 experiments on the a and /3 particles. In the course of the 

 investigations it was observed that whilst most of the a par- 

 ticles went straight through the atom there were a few, of the 

 order of one in many thousands, that were thrown entirely 

 off their line of motion, much as are the yS particles. Now 

 the a particle is so massive compared with the electrons that 

 a considerable force is required to deflect it in this waj^, and this 

 presupposes another body of large mass and charge. On this 

 as basis there has been built the theory of the structure of 

 the atom, the most important features of which are now well 

 established. 



At the centre of the atom is a core or nucleus, which may 

 contain charges of both positive and negative electricity, but 

 on which the positive ones always predominate in such a way 

 that the nucleus is positively charged to the extent of about one 

 unit for every two units of atomic mass. Around it, and spaced 

 out to occupy the rest of the atom, are the electrons, one for 

 each surplus positive charge on the nucleus, so that the atom 

 as a whole is electrically neutral. The electrons may revolve 

 round the nucleus much as do the planets of the solar system 

 round the sun, but whereas in the solar system the sun is a very 

 great deal larger than any of the planets, in the atom the 

 nucleus is no bigger, but probably smaller, than the electrons, 

 which are themselves exceedingly minute compared with the 

 diameter of the atom, something of the order of about one- 

 hundred-thousandth part of it, a somewhat similar proportion 

 to that of the earth compared with the whole of the solar 

 system. Yet practically the whole of the mass of the atom is 

 concentrated on this tiny nucleus I 



We shall see in a moment that there can never be more 

 than ninety-two electrons spaced out around the central core 

 of the atom, and since they are all so very small there is brought 

 about the strange result that practically the whole of the atom 

 is empty space. 



Dr. A. S. Russell has given an illustration which will help 

 us to realise what this means. Consider an atom so magnified 

 as to appear superficially as large as Ireland. On such a scale 

 the nucleus, situated at Athlone, the centre of Ireland, would 



