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.
∎ 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 )