PHYSICAL PEINCIPLES OF CHEMICAL REACTIONS 



213 



that the strength of attraction of two atoms will depend on the electronic 

 states in which they happen to be. Thus an ''energy-level diagram" of 

 this type includes a number of curves for a given molecule; for each elec- 

 tronic state there is a particular curve having its own value of electronic 

 (i.e., minimum) energy, r^, v„b, etc. The limit corresponding to very 

 great r is that of dissociation of the molecule. The ground electronic 

 state usually dissociates into atoms in their ground states; excited states 

 usually dissociate into atoms at least one of which is excited. This 

 behavior is illustrated in Fig. 3-3. For an attractive potential curve, the 

 difference between the energy limit at infinite r and the energy in the 

 lowest vibrational state is just the dissociation energy. Each electronic 

 state has its own dissociation energy ; the normal dissociation energy of a 

 molecule is that for the ground electronic state. i° 



Quantitative information on potential curves is obtained chiefly from 

 the study of spectra of molecules (Herzberg, 1950). Figure 3-3 presents 

 as an example some of the many known curves for the H2 molecule. Note 

 that two different states dissociate into H atoms in ground states: one 

 (the lower) is the ground state of the molecule; the other is a repulsive 

 state, and a molecule in that state will immediately fly apart. Figure 3-4 



Br + BrXPi/J 

 Br+Br ^ 



12 3 4 5 



INTERATOMIC DISTANCE, A 



Fig. 3-4. Potential curves of some of the electronic states of molecular bromine. 



presents several curves of Br2. The lowest curve is the normal molecule. 

 The uppermost one is a physically stable state formed when one of the 

 combining atoms is excited; note that its r^ is greater, and dissociation 

 energy smaller, than those of the ground state (indicating weaker binding). 



10 It is evident that the total vibrational energy of a diatomic molecule must be less 

 than its dissociation energy, for otherwise the atoms would fly apart. It can happen, 

 however, that the sum of vibrational energy and rotational energy exceeds the disso- 

 ciation energy ; a molecule in such a state is metastable, and dissociation will occur 

 eventually but usually (if the energy excess is small) only after the lapse of a signifi- 

 cant interval of time. This phenomenon is called dissociation by rotation, and can be 

 readily explained on the basis of potential curves so drawn as to include rotational 

 energy (cf. Herzberg, 1950). 



