The Dissociation of the Molecule.—In the gaseous or in dependent state, molecules may be classed as heteropolar and homopolar or non-polar (see ELECTROLYSIS), the former having large electrical moments. In the homopolar molecules no such electrical dissymmetry appears. However, not only are molecules of all sorts of intermediate type known, but the polarity of one and the same molecule depends on its environment. If a hetero polar molecule, such as HBr, absorb energy, the increase of oscillation energy is small. An electron of the positive ion is raised to a higher quantum state, thereby inducing stronger, not weaker binding, and no dissociation, e.g., into positive and negative ions, can be expected from the absorption of light. It is not impossible, however, that an electron should be removed from the anion by radiation, although spectra corresponding to the reaction termed electron affinity spectra are not yet known. In such case we should have dissociation of the molecule into a positive ion, a neutral atom, and an electron, thus: In homopolar molecules Franck suggests two types of linkage. In one, electrons are shared in common (covalency), e.g., in H2 and ; "such molecules cannot separate from the normal state adiabatically into two atoms, and the electron jumps associated with the formation and dissociation of these molecules cannot be produced by radiation processes but only through collision."
In agreement with this no point of convergence of bands has been observed in the molecular spectrum of hydrogen. A second type of homopolar linkage is, however, of much smaller strength— sometimes termed van der Waal's forces, i.e., akin to the attrac tion between gas molecules and the chemist's residual affinity operative in molecular association. Here the molecule may take up so much oscillation energy through absorption that it will separate into a normal and an excited atom. There is evidence that the halogen molecules belong to this class. The absorption spectrum of iodine, which is approximately represented in fig. 2, indicates that the shorter the wave-length, the smaller the interval between the oscillation.quanta, until a real convergence limit appears at about 5,000 A.U. with a strong continuous spectrum beyond. The Angstrom unit, A.U.,= cm. = 0.1 A,u. Bromine and chlorine also show continuous spectra, of known wave-length limits, 5,200 A.U. for and 4,800 A.U. for This con