164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



the discovery of the electron and the proton as smaller and more 

 fundamental constituents of matter and the atom itself became the 

 happy hunting ground of the experimental physicist. Atomic physics 

 developed rapidly ; for the atom was found to be a domain of almost 

 incredible richness, and today, thanks perhaps to the newspapers, 

 our children speak knowingly of smashing atoms ! 



To explain the wonderful phenomenon of radioactivity, Eutherf ord 

 came forward in 1904 with a revolutionary hypothesis which reduced 

 the complicated and mysterious observations of radioactivity to simple 

 order. According to Rutherford, not all of the atoms have existed 

 for ages and will exist for all time, but there are some atoms in nature 

 that are energetically unstable and in the course of time, of their own 

 accord, blow up with explosive violence. These are the natural radio- 

 active substances, and the fragments given off in the atomic explosions 

 are the observed penetrating rays. 



It was not long before Rutherford's hypothesis was established as 

 a law of nature and formed a greater keystone, replacing the chem- 

 ists' conception of the atom and serving as a foundation for a new 

 science, the science of the atomic nucleus. 



Time does not permit an adequate historical resume of the develop- 

 ment of nuclear physics, but for the present purpose it is sufficient 

 to say that the ideas of Rutherford and Bohr on the structure of 

 atoms are now firmly established. There is an abundance of evidence 

 that an atom consists of a nebulous cloud of planetary electrons 

 whirling about a very dense sun, the positively charged nucleus, 

 and that it is in the nucleus that the atomic explosions of radioactiv- 

 ity occur. Indeed, our assurance that this is so rivals our confidence 

 that the planets revolve about the sun ! 



ATOMIC NUCLEUS 



Let us now proceed immediately to a consideration of the structure 

 of the nucleus. The nucleus consists of a closely packed group of 

 protons and neutrons, elementary building blocks of nature some 

 2,000 times heavier than the electrons. The neutrons are electri- 

 cally neutral while the protons carry positive charges, and for each 

 proton in the nucleus there is a corresponding negative electron out- 

 side, for the atom as a whole is uncharged. Since the number of elec- 

 trons outside determines the ordinary chemical and physical proper- 

 ties of the atom, it follows that the nuclear charge determines the 

 place of the atom in the periodic table of the elements. 



Thus, the nucleus is the body and soul of the atom. More than 

 99.9 percent of the atom's mass is in the nucleus and the nuclear 

 charge determines the nature of the atom, its chemical and physical 

 properties. 



