Maximum Covalency , Resonance

Maximum Covalency:

Elements which have vacant d-orbital can expand their octet by transferring electrons, which arise after unpairing, to these vacant d-orbital e.g. in sulphur.

In excited state sulphur has six unpaired electrons and shows a valency of six e.g. in SF6. Thus an element can show a maximum covalency equal to its group number e.g. chlorine shows maximum covalency of seven.


There may be many molecules and ions for which it is not possible to draw a single Lewis structure. For example we can write two electronic structures of O3.

In (A) the oxygen – oxygen bond on the left is a double bond and the oxygen – oxygen bond on the right is a single bond. In B the situation is just opposite. Experiment shows however, that the two bonds are identical. Therefore neither structure A nor B can be correct.

One of the bonding pairs in ozone is spread over the region of all the three atom rather than associated with particular oxygen-oxygen bond. This delocalised bonding a type of bonding in which bonding pair of electrons is spread over a number of atoms rather than localised between two.

Structures (A) and (B) are called resonating or canonical structures and C is the resonance hybrid. This phenomenon is called resonance a situation in which more than one plausible structure can be written for a species and in which the true structure cannot be written at all.

Some other examples

∎ CO32- ion

∎ Carbon-oxygen bond lengths in carboxylate ion are equal due to resonance.

∎ Benzene

∎ Vinyl Chloride

Difference in the energies of the canonical forms and resonance hybrid is called resonance stabilization energy and provides stability to species.

Rules for writing Resonating Structures

∎ Only electrons (not atoms) may be shifted and they may be shifted only to adjacent atoms or bond positions.

∎ The number of unpaired electrons should be same in all the canonical form.

∎ The positive charge should reside as far as possible on less electronegative atom and positive charge on more electronegative atom.

∎ Like charge should not reside on adjacent atom

∎ The larger the number of the resonating structures greater the stability of species.

∎ Greater number of covalent add to the stability of the molecule.

Illustration : Why bond length in 1, 3 butadiene is shorter than C – C bond length in n – butane.


Due to resonance in 1,3 butadiene , there is some sort of double bond character present in C2 – C3single bond, which decreases the bond length.

Also Read :

Deviation from ideal behaviour & FAJAN’S RULE
Role of φ ( ionic Potential )
Hydrogen Bonding
Intermolecular Forces

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