20 



GENERAL CONCEPTS 



Electron in Hs 

 orbit 



Electron in 

 outer orbit - 

 in inner orbit /' 



r 

 I 





Proton In 

 the nucleus 



Hydrogen Atom 



0" 



^^Q 



Ceu-bon Atom 







;o 



4 





.^ 



^v 



sT 



Nitrogen Atom 



Oxygen Atom. 



Figure 2.3. Diagrams of the structure of the atoms of the four chief elements of 

 protoplasm: hydrogen, carbon, nitrogen and oxygen. The symbols used are O, neutron; 

 +, proton; ©, electron. 



electrons from other atoms and become negatively charged ions or 

 anions (e.g., Cl~, chloride ion). Anions migrate to the anode or posi- 

 tively charged electrode of an electrolytic cell. Because they bear opposite 

 electric charges, anions and cations are attracted to each other. Atoms 

 such as carbon, which have lour electrons in the outer orbit, neither lose 

 nor gain electrons, but share them with adjacent atoms. 



Physical research has shown that most of these elements are com- 

 posed of two or more kinds of atoms, which differ in the number of 

 neutrons in the atomic nucleus. The different kinds of atoms of an 

 element are called isotopes (iso =i equal, tope = place), because they 

 occupy the same place in the periodic table of the elements. All the 

 isotopes of a given element have the same number of electrons circling 

 the atomic nucleus. The development of the cyclotron and nuclear 

 reactor made possible the artificial production of a host of new isotopes. 

 The availability of these new isotopes, in turn, made possible a new 

 type of biologic research, that of tracing particular elements and com- 

 pounds through their many devious metabolic pathways, and of measur- 

 ing the time required for any given substance in the body to be replaced 



