Solution

Resonance effect is the polarity produced in a molecule due to interaction between a lone pair of electron and a pi bond or it is produced due to interaction of two pi bonds between two adjacent atoms. Resonance effect can be seen in molecules having conjugated double bond or in molecules having at least one lone pair of electrons and one double bond.

There are two types of Resonance effects namely positive resonance effect and negative resonance effect.

Positive Resonance Effect- Positive resonance effect occurs when the groups release electrons to the other molecules by the process of delocalization. The groups are usually denoted by +R or +M. In this process, the molecular electron density increases. For example- $$ -OH, -SH, -OR,-SR. $$

Negative Resonance Effect- Negative resonance effect occurs when the groups withdraw the electrons from other molecules by the process of delocalization. The groups are usually denoted by -R or -M. In this process, the molecular electron density is said to decrease. For example- $$-NO_{2}, C=O, -COOH, -C≡N. $$

The main difference between inductive effect and resonance effect is that inductive effect describes the transmission of electrical charges between atoms in a molecule whereas resonance effect describes the transmission of electron pairs between atoms in a molecule.