82 Introductory Biology 



The chemical and physical behaviors of the atom are determined 

 largely by the number and arrangements of the orbital electrons. It is 

 common knowledge that proper bombardment of certain atoms (with 

 neutral neutrons, protons, etc.) results in the release of tremendous 

 amounts of atomic energy by the process of nuclear fission. For example, 

 the energy released by the fission of one pound of U^^^ (a fissionable 

 isotope of uranium) is roughly equivalent to that secured from burning 

 10,000 tons of coal. It is unknown how, or if, comparable energy re- 

 leases occur in phenomena outside of the artificially conducted experi- 

 ments of recent years. 



Each kind of element has an atomic number which is specific for that 

 kind of element, but some of these same elements have been discovered 

 to have different atomic weights and consequently are known as isotopes 

 fi' so tope) (Gr. isos, equal; topos, place). Isotopes having the same 

 atomic number are identical as far as their chemical properties and their 

 extranuclear structures are concerned, but they differ in their atomic 

 weights and with regard to the structure of the atomic nucleus (number 

 of neutrons). 



Artificially produced radioactive isotopes are extremely valuable in 

 the study of certain biologic problems. The use of such radioactive 

 isotopes as "tracers" is valuable because they emit certain radiations 

 whose presence can be detected in various parts of an organism by sen- 

 sitive Geiger counters. Hence, the rate of absorption of iodine by the 

 thyroid gland has been determined by the use of radioactive iodine, and 

 this has assisted in the treatment of goiter. Radioactive phosphorus 

 has been traced to the stems and certain parts of the leaves of tomato 

 plants, while radioactive zinc concentrates in tomato seeds. The many 

 uses of radioactive isotopes will be of great value in the study of animal 

 and plant metabolism, diagnosis and treatment of certain diseases, etc. 



Atoms with less than one-half of the maximum number of electrons 

 in the outer orbit mav under certain conditions even lose those which 

 they have, while atoms with more than one-half the maximum number 

 of electrons in the outer orbit may add electrons until the outer orbit 

 is filled to its maximum. The additions or losses of electrons (negatively 

 charged) in the orbits docs not affect the structure of the atomic nucleus, 

 but the latter can no longer be electrically neutral after such changes. 

 Normally, the positively charged protons and the negatively charged 

 electrons are balanced. Hence, the loss of electrons makes the atom 

 positively charged and the addition of electrons makes the atom nega- 



