PRINCIPLES OF RADIOLOGICAL PHYSICS 



29 



number Z of electrons and therefore on the position of the atom in the 

 periodic system. In general, the properties of the external electrons of an 

 atom are identified with its chemical characteristics. 



Electrons with Z^^ > 1 are attracted by the nucleus Z^^ times more 

 strongly and held approximately Zeff times closer to the nucleus than an 

 electron with Z = 1. Consequently the binding energy is approxi- 

 mately Zpff times larger than in the case of the H atom. 



The innermost electron experiences the full attraction of the nucleus; 

 consequently Ze(f « Z. 



10 



en 100 



I- 



_i 



o 



> 



o 1000 

 \~ 



O 



UJ 



10,000 



100,000 







100 



20 40 60 80 



ATOMIC NUMBER, Z 

 Fig. ]-20. Lowest and highest ionization potentials of the elements. 



In conclusion, it may be stated that the binding energy of individual 

 electrons within an atom of atomic number Z varies between '^'10 and 



lOZ^ 



ev. 



It follows from these data that physical phenomena taking place in the 

 inner and outer portions of an atom remain rather independent of one 

 another. On the one hand the motion of the inner electrons is hardly 

 affected by the presence, absence, or arrangement of the outer electrons, 

 but it is affected greatly by the strong nuclear attraction. On the other 

 hand the outer electrons experience a weaker attraction and move more 

 slowly; they "see" the inner electrons as forming a fixed cloud of negative 

 charge around the nucleus. 



The motion of the atomic electrons can proceed steadily according to a 

 variety of possible patterns, which are called the characteristic "sta- 



