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Valency : Molecular Compounds

Saturated molecules often have the capacity of uniting with each other, although they cannot take up additional atoms of elements. Hydrofluoric acid, HF, and potassium fluoride, KF, although both saturated compounds, combine to form the salt potassium hydrogen fluoride, KHF2. This is readily broken up on heating into KF and HF, and hence is often formulated as KF,HF, and called a molecular compound.

An explanation of the formation of compounds from apparently saturated molecules may be given on the hypothesis of residual valencies. The free positive valency of potassium is not quite neutralised by the free negative valency of fluorine when the elements combine atom for atom; in order to bring about complete neutralisation, a fraction of an atom more of fluorine would be required. The addition of this fraction of an atom is impossible, hence the KF molecule exhibits a residual positive valency. The electronegative valency of fluorine is not entirely neutralised by the positive valency of hydrogen, hence the HF molecule exhibits a residual negative valency. These residual valencies may be represented by dotted lines instead of by bonds; they are less than a unit of free valency as exhibited by a hydrogen atom: K+F... and H-F... The two residual valencies, although not capable of uniting with a univalent atom, can unite with each other, forming the molecular compound KF... HF. The constituents of molecular compounds are usually separated by commas, e.g., KF,HF.

A more satisfactory explanation of the formation of molecular compounds is usually given by the electronic theory of valency, although association by electrostatic attraction may sometimes occur.

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