electron density on a ring is greater or less this will be decided by the presence of functional group in it if it is attached by a functional group which donates electron by mesomeric effect then electron density in the ring will be more and if it is attached by a functional group which shows -M effect then electron density will be less on ring.. some +M group are -OH , -NH2 etc . some -M group are -NO2,-COOH,-CN etc
