Mesomeric effect

Due to the increase in electron density, the conjugate system will develop a more negative charge.

As a result, the system under the +M effect will be more reactive towards electrophiles, which can take away the negative charge, than a nucleophile.

Specifically the inductive effect is the tendency for the substituents to repel or attract electrons purely based on electronegativity and not dealing with restructuring.

The mesomeric effect however, deals with restructuring and occurs when the electron pair of the substituents shift around.

The inductive effect only acts on alpha carbons, while the mesomeric utilizes pi bonds between atoms.

Mesomeric effect can be transmitted along any number of carbon atoms in a conjugated system.

Some of these properties include different reactivities, local diamagnetic shielding in aromatics, deshielding, and acid and base strengths.