December 26, 1919] 



SCIENCE 



579 



THE BUILDING OF THE NUCLEI OF COMPLEX ATOMS 



Suppose that we consider the specific prob- 

 lem of the building of a carbon atom. The 

 characteristic chemical and physical behavior 

 of carbon are due to its six planetary negative 

 electrons, and these will arrange themselves 

 around any nucleus which carries a positive 

 charge of six, so our problem reduces to that 

 of putting six positive charges of electricity 

 into the extremely minute space occupied by 

 the nucleus of an atom, with a diameter of 

 the order of 10'^^ cm., that is one millionth 

 of a millionth of a centimeter. These six 

 positive charges must not only be put into 

 this ultra-uUra-microscopic space, but must 

 unite to form an intra-nuclear compound of 

 extreme stability. 



Now, up to the present time, the smallest 

 mass ever found associated with one positive 

 electrical charge, is that of the hydrogen ion, 

 which is associated primarily with the mass 

 of the hydrogen nucleus, with a value of 

 1.0078.^ If six of these hydrogen nuclei could 

 be packed tightly enough together to form the 

 nucleus of a new atom they would form the 

 nucleus of a carbon atom, which would have a 

 mass of approximately six. That no carbon 

 atom of this mass exists, is not because such a 

 nucleus if formed, would not give a true car- 

 bon atom, but because six positive hydrogen 

 nuclei undoubtedly repel each other, and can 

 not be made to form a stable system. 



In order to make a complex nucleus stable 

 it is necessary to include not only hydrogen 

 nuclei, but also negative electrons. Since the 

 mass of the ordinary carbon atom is 12.00, it 

 could be built up from 12 hydrogen nuclei, 

 bound together by six negative electrons. 

 Such a nucleus would have a positive charge 

 of 6, it would therefore take up six negative 

 electrons, and would thus form a complete 

 carbon atom. The only objection to this idea 

 is that 12 times 1.00Y8 is 12.036, while the 

 weight, and probably the mass, of the carbon 

 atom is only 12.005, or the actual carbon atom 

 is 0.76 per cent, lighter than it should be if 

 built from 12 hydrogen nuclei without any 

 resulting change of mass. Now Professor 



2 Equal to 1.66 X 10-2* grams. 



A. C. Lunn has worked out the mathematical 

 expression for this effect for the writer, and 

 this shows that according to the electromag- 

 netic theory, if such a nucleus is held together 

 in a very small space by attractive forces there 

 should be a loss of mass in its formation, and 

 that in a simple atom the observed loss of 

 mass would result if the center of the nega- 

 tive electron has a distance 400 times the 

 radius of the positive electron. 



The alpha particle, or the nucleus of a 

 helium atom, carries two positive charges, has 

 a mass of four, and is probably made up of 

 four positive hydrogen nuclei bound together 

 by two negative electrons into what is prob- 

 ably by far the most stable nucleus of any 

 known atom, except that of hydrogen itself. 



If we make the atomic number of the 

 element equal to the positive charge on the 

 nucleus, then the atomic number of hydrogen 

 would be one, that of helium two, that of 

 carbon six, of lead eighty-two, and of 

 uraniimi, ninety-two. Now the mass of the 

 carbon atom (atomic number 6) is exactly 

 what it should be if its nucleus consists of 

 3 alpha particles. Also, 3 times the charge 

 on the alpha particle is just the charge on 

 the carbon nucleus. It is easily seen that 

 there is a possibility that the carbon nucleus 

 is simply a compound made up of 3 alpha 

 particles, of a formula 3 a. Now it is obvious 

 that if the nuclei of complex atoms were 

 simply structures built up from alpha par- 

 ticles, that, since the positive charge on the 

 alpha particle is two, there would be no 

 nuclei with an odd number of charges. The 

 work of Mosely indicates, however, that the 

 elements of odd atomic number also exist, 

 but it is certain that the nuclei of such odd 

 numibered atoms can not be compounds made 

 of alpha particles alone. 



On the other hand, it is quite evident, as I 

 announced four years ago, that the nuclei of 

 the atoms of even atomic number are mostly 

 intra-nuclear compounds of helium nuclei. 

 For this there is much evidence which will 

 be found in my printed papers in the Journal 

 of the American Chemical Society and in 

 Science. This evidence can only be hinted at 

 here, since the time for the paper is short. 



