180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 7 



ber is 2 and its mass number 4. Nature's heaviest atom has 92 protons 

 and 146 neutrons for its nucleus, and 92 satellite electrons. Its atomic 

 number is 92 and its mass number 238. This atom turns out to be 

 the basic source of atomic energy. 



Atoms themselves are also exceedingly small, for the combined di- 

 ameters of 200 million of them would be an inch long. Even at that 

 they consist mostly of emptiness, for if the nucleus were enlarged 

 to a baseball, the satellite electrons would be specks some 2,000 feet 

 away. As we know, the solar system is also mostly empty. In round 

 numbers for both the atom and the solar system the radius of the 

 whole is 10,000 times the radius of the central sun. The analogy is 

 striking. 



The chemical properties of any substance are determined by the 

 satellite electrons. In this sense then, chemistry is concerned only 

 with the superstructure of the atoms, and never comes to grips with 



HYDROGEN ATOM 



HEUtUM ATOM 

 FiGUKE 1. 



URANIUM ATOM 



the nucleus. The 92 elements of chemistry have the 92 kinds of 

 superstructure. Though the satellite electrons move at high speed 

 their mass is negligible, so that the energy values involved are rela- 

 tively small and chemical changes can yield but little energy. 



ISOTOPES 



An architect with only 92 house elevations could design many more 

 than 92 interiors. Similarly nature assisted by man has designed 

 upwards of 600 atoms despite the fact that only 92 exteriors seemed 

 possible. This is accomplished by the addition to or subtraction from 

 the nucleus of neutrons, thus changing the mass number. These new 

 atoms are called isotopes of the original and are chemically indis- 

 tinguishable from them. Thus heavy hydrogen has 1 proton and 1 

 neutron in its nucleus and 1 satellite electron, so that it is almost 

 twice as heavy as ordinary hydrogen, but has the same chemical prop- 



