PRINCIPLES OF RADIOLOGICAL PHYSICS 31 



Oscillating currents accompany also the transitions of an atom from a 

 stationary state to the "ionized" states in which an electron moves 

 away with sufhcient energy to overcome the attraction of the nucleus. 

 The frequency of these currents relates to the energy difference of the 

 initial and final states according to Eq. (12a). Here the energy A^ 

 gained by the atom in the process of ionization may have any value 

 larger than the binding energy of the outermost electron because an 

 electron which breaks loose from an atom may carry away any amount of 

 excess energy. 



2-lb. Aggregates of Atoms. Chemical bonds 

 arise when the combined external electrons of 

 two or more adjacent atoms can take up a 4- 



coordinated motion such that their total energy I 



is lower than it would be if the atoms remained -^—-—4. 



separate. The bond formation may involve an 

 excess concentration of electrons on some atoms 

 and an equivalent defect on other atoms. The 

 energy released by the formation of a chemical 

 bond is of the same order of magnitude as the 

 energy differences of the stationary states of \_^ 



outer electrons in an atom, i.e., of the order of p^, i_22. Diagram of the 

 a few electron volts. mutual orientation of a 



Molecules and crystals of various constitu- group of water molecules 

 tion and stability result from the aggregation of ^^ich yields a net attrac- 

 , , , .11 1 T-. . 1 tion between their positive 



atoms through chemical bonds. Rather gen- ^^^ ^^^^^.^^ ^j^^^^^^ 



eral arguments of atomic mechanics suffice to 



explain why and under what conditions chemical bonds are actually 



formed. 



Other types of atomic aggregates, such as liquids, plastics, or col- 

 loidal systems, are less rigid and stable than molecules or crystals. They 

 result from cohesive forces among atoms and molecules, which are weaker 

 and less specific than chemical bonds. These forces result from a suit- 

 able mutual adjustment of the electric charges within separate but 

 adjacent molecules. In essence, the electrons of each atom or molecule 

 draw particularly close to one of the nuclei of another atom. In some 

 substances, for example, water, the center of mass of the electrons of each 

 molecule does not normally coincide with the center of mass of the 

 nuclei; therefore there results an excess concentration of negative charge 

 on one side of the molecule and an excess positive charge on the opposite 

 side. Intermolecular attraction arises then merely from a suitable 

 orientation of adjacent molecules (see Fig. 1-22). This attraction is 

 called a "polar force." In other substances, where the electrons tend 

 to remain more evenly distributed about the nuclear charges, the elec- 

 tronic distribution becomes nevertheless distorted by the very proximity 



