246 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



A first approximation to the solution of the problem regarding 

 the structure of the atom was given, in 1911, by the late Sir Ernest 

 Eutherford. As a result of a brilliant series of investigations, he 

 reached the conclusion that the atom consists of a positively charged 

 nucleus (or core) surrounded by as many electrons as the number of 

 unit positive charges on the nucleus. This number is known as the 

 atomic number of an element, and an element of atomic number X 

 will be referred to in this article as element X. This number varies 

 in value from 1 for hydrogen to 92 for uranium. Thus the uranium 

 atom consists of a nucleus having 92 units of positive electricity and 

 92 electrons, revolving in orbits about the nucleus. Since the mass of 

 even 92 electrons is about one-fiftieth of 1 percent of the mass of the 

 uranium atom, it follows that nearly the whole mass of the atom is 

 concentrated in the nucleus. And yet, since the diameter of the nucleus 

 is only about one hundred-thousandth of that of the atom, we must 

 conceive of the atom as an extremely miniature solarlike system in 

 which most of the volume is just empty space. 



The electrons external to the nucleus are arranged in shells or 

 groups in a manner somewhat similar to the concentric crystal spheres 

 in which, according to the medieval astronomers, the stars, sun, and 

 planets were supposed to revolve about the earth as the center of the 

 universe. With increase in atomic number, that is the charge on 

 the nucleus, the electrons fill up first the innermost shells, and then the 

 outer shells. There is a periodicity with increasing atomic number 

 in the distribution of the electrons which corresponds to the perio- 

 dicity in chemical and physical properties of the elements that had 

 been previously observed in the latter half of the nineteenth century. 

 Thus hydrogen consists of a nucleus of unit +ve charge and an 

 electron located in the innermost shell. When one more +ve charge 

 is added to the nucleus, the second electron which is required to 

 neutralize the added +ve charge on the nucleus goes into the same 

 shell as that occupied by the electron in the hydrogen atom and thus 

 forms the atom of helium. With the addition of a third unit of 

 positive charge to the nucleus the third electron enters into a second 

 shell and we have the atom of lithium which is atomic number 3. 

 As more positive charges are added to the nucleus the corresponding 

 number of electrons enter into the same shell until we reach the atom 

 of atomic number 10. This element is neon, which is chemically inert 

 like He. This atom has two electrons in the innermost shell and 

 eight electrons in the next outer shell. If we add eight more pos- 

 itive units to the nucleus the added eight electrons fill up a third 

 shell, and the element of atomic number 18 is argon, which is a chem- 

 ically inert gas similar to neon and helium. This periodicity of 

 eight leads to groups of chemically similar elements. Thus the 



