• # question_answer 15) Explain the important aspects of resonance with reference to $CO_{3}^{2-}$ ion.

The following are some of the important aspects of resonance: (i) When a single Lewis structure cannot explain all the properties of the molecule and the molecule is supposed to have many structures with similar energy, position of nuclei, bonding and non-bonding pairs of electrons, each of which can explain most of the properties of the molecule, the molecule is said to have resonance. The actual structure is in between of all these contributing structures and is called resonance hybrid and the different structures are called canonical forms. (ii) The molecule as such has a single definite structure which cannot be depicted by a Lewis structure. (iii) The resonance hybrid is more stable than any one of the canonical forms, i.e., it has less energy. (iv) The canonical forms do not have any real existence. They are only imaginary. The resonance hybrid has only real existence. (v) It has been observed that a molecule which shows resonance has greater heat of formation than the calculated heat of formation of most stable canonical form. The difference is called resonance energy. Higher the resonance energy, more stable will be the molecule. Carbonate ion can be represented by the following structures: i.e., there are two single bonds and one double bond between carbon and oxygen atoms, showing unequal bonding. However, it is observed in experimental findings that all carbon and oxygen bonds are equivalent. Therefore, the $CO_{3}^{2-}$ ion is best described by a resonance hybrid of the various canonical forms.