The Strudicre of the Atom. ii 



speed of this nucleus are so great that for the most part it ploughs 

 its way right through any atoms which may lie in its path, keeping 

 its course a straight line. Occasionally, however, it suffers a 

 violent deflection from its course. This cannot be due to collision 

 with an electron, for this would result in the deflection of the 

 electron and not of the a-particle, which is seven thousand times 

 as heavy. The explanation is that the a-particle has approached 

 so near the comparatively heavy nucleus of an atom that the 

 repulsion due to the positive charges on both the nuclei has swung 

 the lighter body right off its path. On this supposition we can 

 calculate the distance between the colliding bodies at their nearest 

 approach to each other, and we find that the distance from the 

 centre of the a-particle to the centre of the nucleus of the atom 

 must be less than one ten-thousandth of the diameter of an atom, 

 taking this diameter as that of the outer electronic ring. 



This is the justification of Rutherford's claim that the central 

 nucleus must be extremely small compared with the atom in which 

 it resides. 



We see that atoms must be extremely empty regions. If we 

 imagine an atom of helium magnified to the size of the earth, the 

 central nucleus and its two attendant electrons would be represented 

 to scale by three modern airships. 



We are still extremely ignorant of the constitution of the 

 nucleus. It must be very complicated in the case of heavy atoms. 

 Radium, for example, during its series of changes, shoots out 

 five a-particles, that is, helium nuclei, and these particles must 

 all have been present in the original radium nucleus. 



The picture of the atom here presented, though of an 

 impressionist character in many details, is probably true to 

 Nature in its main outlines. Much attention is being concentrated 

 on the subject by research workers, and the attack on the problem 

 is being carried out by more and more varied methods. The 

 work is extremely fascinating, partly on account of the fundamental 

 nature of the investigations, partly from the mental pleasure 

 experienced in piecing together, and removing from the darkness 

 which envelops it, the delicate fabric of atomic structure. 



