PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 



199 



tion; at 20°C, the mean translational energy is 0.038 ev. For a medium at 

 thermal equiUbrium, even at a very high temperature, the translational 

 energy is usually very small compared to the electronic excitation energy. 

 This need by no means be true, however, if nonequilibrium conditions 



>- 



cr 



UJ 



I5(- 



14 



13 



12 



10 



<D 8 



CONTINUUM 



t ttt i2 



S^S. 2P, ^D 



CONTINUUM 



e^s. 2p 



RESONANCE 

 LINE 



etc. 



7'Sr 



I^S 



Fig. 3-1. Energy-level diagrams of atomic hydrogen, sodium, and mercury. (Only 

 the lowest levels are shown. The doublet separations for sodium are not to scale.) 



obtain — for example, if a chemical reaction is proceeding. The transla- 

 tional energy is never included in energy-level diagrams, because it cannot 

 change appreciably during a transition. There is no analogous rota- 

 tional energy from rotation of the atom as a whole; since only rotation of 



