Understanding Resonance Structure: It has been confirmed by the scientist that a single structural formula of certain compounds cannot satisfactorily explain its all properties. For example, the structure of benzene ordinarily represented either as I or II. These structures have two types of carbon-carbon bonds, three C-C (bond length 1:54 Å) and three C = C (bond length 1:34 Å). But experimentally it has been found that all the carbon-carbon bonds in benzene are found to be identical in all respects, e.g., their bond length is 1:39 Å. The various properties of benzene and several other compounds which are derived from benzene can only be explained if we consider that benzene molecule is having a structure in midway between the two, i.e. I and II.
What is a Resonance Structure
When a molecule has more than one structure and none of these structures define it’s all properties, then the actual structure lies between these structures said to represent it uniquely, is referred to as resonance structures.
- This phenomenon is also known as resonance or mesomerism or canonical Structure.
- The various resonating structures for a particular molecule are separated by the notation (sign for resonance).
Resonance Structures Definition
|If a compound is having a certain molecular formula can be represented by different structural formulae which differ only in the arrangement of the electron pair and not the atoms, such like structures are called as resonating structure.|
The difference between the energy of resonance hybrid and internal energy of the most stable contributing resonating structure is known as resonance energy. One thing must be remembered that the hybrid is always more stable than the contributing structures. Thus benzene is said to be stabilized by a resonance energy of 36 kcal/mole.It has been experimentally seen :
|Maximum Canonical Structures||Resonance Energy ↑||Stability ↑|
This is because of the delocalization of the electron charge which gets distributed over a greater area because of resonance. The charge density decreases and consequently the energy of the hybrid decreases. Hence it’s stability increases.
Some important conditions concerning resonance are listed below:
- It differ in the position of electrons and not in the position of atoms.
- The resonating structures must have same or nearly the same energies.
- All these structures should have the same number of unpaired electrons.
- Structure in which negative charge is on an electronegative atom and positive charge is on the more electropositive atom, contribute more towards hybrid than the other ones.
- Structure with the greater number of covalent bonds contributes more towards hybrid than the other.
Resonance Structure of (CO) Carbon Monoxide
Resonance Structure of (CO2) Carbon Dioxide
Resonance Structure of (NO) Nitrogen oxide
Resonance Structure of (NO2) Nitrogen dioxide
Resonance Structure of (N2) Nitrogen
(HN3) Hydrazoic acid
Resonance Structure of (O3) Ozone
(SO3) sulphur trioxide
Resonance Structure of (CO32-) Carbonate Ion
Resonance Structure of (NO3–) Nitrate Ion
Resonance Structure of (SO42-) Sulphate Ion
(BCl3) Boron trichloride
Resonance Structures Examples
Arrange the following resonating structures in the order of decreasing stability.
- (a) (i) 1 is the most stable because it has no formal charge.
- 3 is the least stable because it has an electron-deficient carbon. Thus the order of stability is I > II > III
- (b) 5 and 6 have the greater number of covalent bonds, hence these are more stable than either 7 or 8. Further, between 5 and 6, 5th has no formal charge and hence is more stable than the 6.
- 8 is less stable than 7 since in 8th positive charge is on oxygen which is more electronegative atom than carbon on which positive charge is present in 7. Thus the order of stability is V > VII > VII > VIII.
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