The concept of resonance was introduced to deal with the type of difficulty experienced in the depiction of accurate structures of molecules like O . According to the concept of resonance, whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons are taken as the canonical structures of the hybrid which describes the molecule accurately. Thus for O , the two structures shown above constitute the canonical structures or resonance structures and their hybrid i.e., the III structure represents the structure of O more accurately. This is also called resonance hybrid .
Resonance is represented by a double headed arrow. Fig. . Resonance in the O molecule (structures I and II represent the two canonical forms while the structure III is the resonance hybrid) Some of the other examples of resonance structures are provided by the carbonate ion and the carbon dioxide molecule.
Problem . Explain the structure of CO – ion in terms of resonance. Solution The single Lewis structure based on the presence of two single bonds and one double bond between carbon and oxygen atoms is inadequate to represent the molecule accurately as it represents unequal bonds. According to the experimental findings, all carbon to oxygen bonds in CO – are equivalent.
Therefore the carbonate ion is best described as a resonance hybrid of the canonical forms I, II, and III shown below. Problem . Explain the structure of CO molecule. Solution The experimentally determined carbon to oxygen bond length in CO is pm.
The lengths of a normal carbon to oxygen double bond (C=O) and carbon to oxygen triple bond (C ≡ O) are pm and pm respectively. The carbon-oxygen bond lengths in CO ( pm) lie between the values for C = O and C ≡ O. Obviously, a single Lewis structure cannot depict this position and it becomes necessary to write more than one Lewis structures and to consider that the structure of CO