THE GROWTH OF CHEMICAL IDEAS 139 



Structure as regards protons and electrons but has 12 neutrons 

 instead of 10. It differs from its twin only by being slightly 

 heavier, which makes it possible to achieve a separation in 

 the mass spectrograph. 



The most interesting isotopic element discovered is the 

 isotope of hydrogen, which has an atomic weight of 2. It was 

 isolated by Harold Urey at Columbia University in 1931 after 

 its existence had been predicted by R. Birge and D. Menzel 

 at the University of California to explain the difference be- 

 tween the chemical atomic ^veight of hydrogen, w^hich repre- 

 sents, of course, the average weight of the atoms of the mixed 

 isotopes, and the atomic weight as determined in the mass 

 spectrograph, w^hich sho^vs only the weight of the proton it- 

 self. This isotope of hydrogen has t^vice the atomic weight 

 of hydrogen, since the neutron ^veighs as much as the proton, 

 and it is consequently not very difficult to separate it from 

 ordinary hydrogen. Moreover, the difference in ^veight is 

 sufficient to make it behave somewhat differently from hy- 

 drogen itself. The hydrogen isotope has even been dignified 

 by a separate name, deuterium. 



As a result of the clarification of atomic structure, chemists 

 were able to make a new attack on the nature of the valence 

 bond. The valence bonds of Kekule and Couper w^ere rep- 

 resented by a line drawn bet\veen the symbols of two chemical 

 elements, indicating that the elements were connected in 

 some way, but the nature of the bond "^\^as completely un- 

 known. Indeed, its nature could not possibly be known be- 

 fore something was known of the structure of the atoms. 



In 1916 G. N. Lewis worked out the electron theory of 

 valence, in which he emphasized the stability of the group of 

 8 electrons in the case of the lighter atoms. If the outer ring 

 contains exactly 8 electrons, the element has zero valence; 

 that is, it is one of the rare gases and is incapable of forming 

 molecules or compounds. AV'hen the outer electron ring of 

 the element contains less than 8 electrons, it can form com- 

 pounds in w^hich the electron ring of the one element is com- 

 pleted by electrons from another element, making 8 electrons 



