EARLY HISTORY OF LIFE 



27 



A specific atom behaves the way it does 

 because of the number of protons and 

 neutrons in its nucleus and the number of 

 electrons in its orbits. While there is always 

 the same number of protons in the nucleus 

 as there are electrons in the orbits, there 

 may be a varying number of neutrons pres- 

 ent in any specific atom. Since the chemical 

 characteristics of the atom are controlled 

 by the electrons in the outer shell or orbit, 



occurring atom but they have different 

 weights, and therefore can be identified or 

 "tagged." Tagging atoms has made it pos- 

 sible to trace various chemicals through the 

 animal body. This has been very helpful in 

 determining what happens to certain sub- 

 stances in normal life processes. If the iso- 

 tope happens to be radioactive, that is, if 

 it happens to give off radiations that can 

 be detected with a sensitive instnjment 



Hydro9«D 



H«|ium 



Nitrogen 



Oxygen 



Fluorine 



Neon 



Fig. 2-5. A small portion of the Periodic Table. The number at the center (nucleus) indicates the 

 atomic number. The electrons lie in the orbits, of which only two are shown here. The inner 

 orbit requires only two electrons while the second needs eight, which is satisfied in neon. 

 Of the ten elements indicated, hydrogen, carbon, nitrogen, and oxygen are important 

 constituents of living material. 



and since the number of electrons is con- 

 trolled by the number of protons at the 

 center, any additional neutrons will be 

 without effect on the chemical properties 

 of the atom. The only difference will be in 

 its weight. Physicists have found that they 

 can add or knock out neutrons as well as 

 electrons, and thus change the physical 

 properties of the atom itself. When only the 

 number of neutrons is changed the resulting 

 atom is called an isotope; isotopes have the 

 same chemical properties as the naturally 



(Geiger counter) or by some other means, 

 then the problem of tracing the chemical 

 becomes less difficult. We are gaining a 

 great deal of knowledge today from this 

 type of so-called "tracer research," and the 

 future holds out much promise in tliis field 

 of investigation. 



It would seem simple, then, to arrange 

 all of the various elements in a series from 

 1 to 98, according to the number of elec- 

 trons ( or protons ) in each individual atom. 

 This has been done and we call such an ar- 



