ATOMIC NUCLEI — HARNWELL 197 



fascinating phenomena that we observe. At present all we know 

 is that they do hold the fundamental particles together in stable 

 groups and these stable configurations represent the nuclei of all the 

 known elements. The simplest is the nucleus of hydrogen, which is 

 the proton itself. Then comes the nucleus of heavy hydrogen, which 

 is composed of one proton and one neutron. The combination of 

 three of these particles appears for some strange reason to be rela- 

 tively loosely held together or fragile, for we do not observe it in 

 nature, while the helium nucleus which is formed of four, two protons 

 and two neutrons, is a particularly strong and compact structure ; so 

 much so that it may even occur as a subunit in building other more 

 complex atomic nuclei. The next lightest atom, that of lithium, is 

 composed of three protons and either three or four neutrons and so 

 on through the entire periodic table till we reach uranium, which is 

 composed of 239 of these units, 92 being protons and the balance 

 neutrons. 



A study of the periodic table of the elements reveals many regu- 

 larities and periodicities among the atomic nuclei. In the early 

 portion of the table protons and neutrons occur in approximately 

 equal numbers in the nuclei but as the atoms become heavier and 

 more complex the laws of nuclear stability, of which we know very 

 little, apparently require that there must be many more neutrons 

 than protons in stable nuclei. We are just beginning to learn some- 

 thing about the forces that hold these complexes together, and in 

 our studies we have been able to make many new kinds of atoms which 

 are not stable but throw off a light fragment or absorb one of their 

 surrounding electrons, thus forming again a stable configuration. 

 The role of the ordinary electron and its positive counterpart is very 

 imperfectly understood, for they appear to be emitted or absorbed 

 by a nucleus, though we have good reason to believe that an electron 

 is quite unsuitable in nature to ever form part of a stable nucleus. 

 Until we have learned more, the mechanism of these processes must 

 remain pure speculation. As the proton and neutron differ char- 

 acteristically by their charge, the emission of a negative electron 

 may be thought of as the change of a nuclear neutron into a proton 

 accompanied by the simultaneous appearance and ejection of an elec- 

 tron. The emission of a positive electron is the converse process. 

 Small quantities of mass also appear or disappear, but with Einstein's 

 relation between mass and energy we can relate this with the kinetic 

 energy of the ejected particle or the very short wave length electro- 

 magnetic radiation that frequently accompanies such a process. 

 This relation {E=mc^), that may be considered either as a conse- 

 quence of the electromagnetic nature of mass or of the special theory 



