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Valency : Saturated And Unsaturated Compounds |
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In some cases it is assumed that two or more valencies of an atom of an element can unite with a corresponding number of an atom of the same element:
 CH, or usually, CH3۰CH3, CH2:CH2, CHCH (There is some chance of confusion of these with electronic formulae, In modern literature, for bond order lines are usually used: -, =, or ≡).The propriety of this representation is shown by the fact that molecules with multiple bonds are unsaturated; they can add on other atoms to form saturated compounds: + H2 +H2 CH CH - +H2 -> CH2:CH2 - +H2 -> CH3۰CH3.Groups united by a double (or triple) bond are supposed to be restrained from free rotation, whilst those united by a single bond are supposed, in most cases, to be capable of rotation. Thus, the two compounds:  and  in which the carbon atoms are linked by a double bond, are different isomeric compounds. Isomerism of this type is called stereoisomerism, since it is due to the different arrangements of the atoms or groups in space (Greek στερεός). Kekule assumed that variable valency is always due to latent bonds. Phosphorus was supposed to be always quinquevalent, but in compounds in which it is apparently tervalent two bonds are latent or unsaturated:   Support was lent to this idea by the circumstance, pointed out by Odling, that when the valency of an element changes, it usually does so two units at a time. This, however, is not always the case. The discovery of such compounds as InCl, InCl2, and InCl3 vitiated this hypothesis. In some cases the atoms in a molecule form closed rings; compounds of this kind, called cyclic or ring compounds, are more common in the case of certain groups of carbon compounds (organic compounds), but are known also in the case of some other elements:   
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, single bond, or linkage, between carbon atoms.
, double bond, or linkage, between carbon atoms.