The mesomeric effect operates through conjugated pi systems and does NOT diminish with distance along the conjugation pathway (unlike inductive effect).

+M groups (electron-donating via lone pairs): -NH2 > -OH > -OR > -F > -Cl > -Br > -I These groups increase electron density at ortho/para positions in benzene.

-M groups (electron-withdrawing via pi bonds): -NO2 > -CN > -CHO > -COR > -COOH > -COOR > -SO3H These groups decrease electron density, especially at ortho/para positions.

Critical JEE point: Halogens are -I but weak +M. The net effect on benzene is deactivating (-I wins), but the +M effect directs to ortho/para positions. This is why halogens are ortho/para directors but ring deactivators.