Forces and Molecular Properties

It will now be expedient to discuss briefly the relation between inter-atomie forces and certain other molecular characteristics which are more or less closely connected with them. 

The forces between atoms are, of course, only indirectly accessible to experimental study. What is most commonly measured is the energy change accompanying a transformation, but this quantity introduces the force as a complicated int ral, the constitution of which is seldom clear. The nearest approach to an immediate manifestation is perhaps in the vibration frequency of a diatomic molecule. 

When two masses joined by a spring execute a simple harmonic motion about their equilibrium position, the frequency is given by the equation i f 

If the vibrations which a diatomic molecule manifests in its spectrum are assumed to be simple harmonic, then / may be calculated from V. 

For polyatomic molecules the situation is more complicated, and the vibrations are characteristic not of individual pairs of atoms but of the molecule as a whole. If it contains N atoms, their positions in space are describable by ZN Cartesian coordinates which may be written qp. These may conveniently be measured from 

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