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The possible structures drawn by moving either the charge on the atoms or the lone pair of electrons having the least formal charge are called resonance structures.

There are four bonding patterns for which more than one Lewis structure can be drawn.

a. In this conjugated system, the lone pair and the double bond are shifted such that resonance gives conjugated carbanion.

Resonance structure for a

b. In this compound, the double bond has one atom more electronegative than the other. The resonance structure is drawn by moving the pi-electrons.

Resonance structure for b

c. In this carbocation, even though the chlorine atom is fairly electronegative, the lone pair would migrate to form a pi-bond.

Resonance structure for c

d. In this 鈥渢hree atom allyl鈥� system, two resonance structures are possible that would differ on the location of the double bond and the charge.

Resonance structure for d