28 



RADIATION BIOLOGY 



Both isotopes have the atomic number Z = 7, but their mass numbers are, 

 respectively, 14 and 15. Nitrogen nuclei of mass number 14 consist of 7 

 protons and 7 neutrons; those of mass number 15 consist of 7 protons and 

 8 neutrons. All nitrogen atoms contain a cloud of 7 electrons. 



Nuclei can absorb or eject protons, neutrons, and other particles, as well 

 as photons of electromagnetic radiation. Nuclear processes involve the 

 uptake or release of energy in amounts which are generally of the order of 

 1 Mev or more, i.e., far larger than the amounts involved in most extra- 

 nuclear processes. 



The energy required to extract an electron from an atom is called the 

 "binding energy" or the "ionization potential" of that electron. The 



order of magnitude of the electronic bind- 

 ing energies can be estimated from a 

 knowledge of the electric charges of the 

 atomic particles and of their average dis- 

 tances. Two particles bearing an electric 

 charge of magnitude e and lying 1 A apart 

 have a potential energy of 15 ev. This 

 energy is comparable to the lower ioniza- 

 tion potentials of atomic electrons and a 

 little larger than the energies of chemical 

 bonds between atoms. 



Each electron of a neutral atom is at- 

 tracted by the positive charge Ze of the 

 nucleus and repelled by the other Z — 1 

 electrons. However, the repulsions ex- 

 erted by the other electrons act in different 

 directions and cancel each other in part. 

 It may be stated with close approximation 

 that each electron is effectively repelled away from the nucleus only by 

 that portion of the total electronic charge which is closer to the nucleus 

 than the electron itself (Fig. 1-19). The remaining portion of the elec- 

 tronic charge does not exert any substantial net force. Therefore any 

 electron appears to be attracted toward the nucleus by an "effective 

 charge," Ze/fC, which is smaller than the actual charge and depends on the 

 distance of that electron from the nucleus. 



An electron on the edge of the atom is kept away from the nucleus 

 practically by all its fellow electrons, so that Z^ ~ Z — (Z — 1) = 1. 

 Hence the binding energy of this electron must be comparable to the 

 binding energy of the single electron of an H atom (Z = 1), i.e., ~10 ev. 

 The actual value of the binding energy of the outermost electron in 

 different atoms depends on the number of electrons in the external layers 

 of the atom (see Fig. 1-20). This number depends, in turn, on the total 



^d 



^6 

 Fig. 1-19. Diagram of inter- 

 actions among atomic electrons. 

 The forces on the innermost 

 electron (a) tend to cancel out. 

 The forces on the outermost 

 electron (f) combine to yield a 

 net repulsion away from the 

 nucleus. 



