198 RADIATION BIOLOGY 



of such properties are total energy, total angular momentum of the elec- 

 trons relative to the nucleus, total (average) lifetime for spontaneous 

 radiation, etc. Positions of the electrons are not well-defined properties. 

 By far the most important property is energy, and the energies cor- 

 responding to the various possible states of an atom are usefully repre- 

 sented on an energy-level diagram, in which the height of any level rela- 

 tive to the lowest one is proportional to the excess in energy of the cor- 

 responding state over that of the lowest or ground state. (This energy 

 difference is called the excitation energy of the state.) A symbol, affixed 

 to the level, indicates a number (but not all) of the other properties of 

 the state. (Very often a "state" is called conveniently, but improperly, 

 an "energy-level.") 



Knowledge of the different states of an atom, i.e., their energies and 

 other properties, is an object of spectroscopic investigation. The most 

 authoritative assembly of such information (not yet completed) is' the 

 work of Moore (1949, 1952) ; the book of Bacher and Goudsmit (1932) is 

 an older but still most useful compilation. In Fig. 3-1 are presented for 

 illustration the energy-level schemes of the three important atoms: hydro- 

 gen, sodium, and mercury. Only a few of the most important levels are 

 shown; sodium has, for example, a total of about 120 known levels 

 (Moore, 1949), and in principle there is indeed an infinite number of them. 

 The meaning of spectroscopic symbols such as those appearing in Fig. 3-1 

 is given in texts on the subject. 



Theoretical investigation of the laws which determine the binding of 

 electrons in the ground states, and especially in the excited states of 

 atoms, has made possible a broad understanding of the properties of these 

 states — in the most minute detail for simple (i.e., light) atoms, in pro- 

 portionately less detail for more complex atoms. It is a most useful and 

 usually very accurate approximation to view a given excited state as 

 arising from the excitation of one particular electron, or occasionally 

 several of them. Familiar spectroscopy deals almost exclusively with 

 excited states attributable to the excitation of the most weakly bound 

 (i.e., valence) electrons. Such states have excitation energies lying in 

 the range of about 3 to 20 electron volts (ev) for all atoms. States which 

 correspond to the excitation of more tightly bound electrons have been 

 much less extensively investigated and are usually not very important in 

 practice. The energies of the various electronic states of atoms all have 

 different, discrete values. It is this fact — perhaps the most striking con- 

 sequence of the quantum laws of atomic structure — which is responsible 

 for the existence of discrete (line) spectra. 



In addition to excitation energy arising from excitation of its electronic 

 system, an atom has some translational energy deriving from its motion 

 as a whole. For a medium in thermodynamical equihbrium the atoms 

 have the various translational energies given by the Maxwellian distribu- 



