Experimental Determination of the Molecular Constants

We now proceed to explain in detail how the molecular constants enumerated in the preceding section can be determined experimentally. There is first the molecular volume, the determination of which, when neutral molecules are in question, can be effected by the methods of the kinetic theory of gases, already referred to in Chapter I (viscosity, free path, diffusion and direct measurement by molecular rays). The following table shows some molecular diameters so determined, in Angstroms  

In liquids, where the induced moments of the molecules influence each other, the relation is somewhat more complicated. 

The polarizability of some neutral atoms (H, Li, K, Cs) has been recently determined by a method similar to the Stern-Gerlach experiment ( 7, p. 166), namely, by measuring the deflexion of a beam of atoms in an inhomogeneous electric field (Stark, 1936). The results do not agree very well with theoretical computations from atomic models. 

We go on now to consider molecules with permanent dipole moment q Here, in addition to the polarization efiect just considered, we have also the influence of the electric field on this permanent moment. In the absence of an external field, the moments of the individual molecules will have all possible directions, so that the gas is unpolarized. If an external field is applied, this tends to turn the individual dipoles round into the field direction (fig. 1) this ten- 

This is additional to the polarization determined by the polarizability of the molecules, and we therefore obtain a dielectric constant 

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The first experimental determination of the molecular constants of RCI2 was performed in two independent IR studies on SmCl2 (DeKock et al., 1972), EUCI2 (DeKock et al., 1972 Hastie et al., 1971), and YbCl2 (DeKock... 

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