Existence of a molecule in more than one structure is called as Resonance. None of contributing structure is actual structure but actual structure is the hybrid of all the structure. Types of Resonance - (a) Pi-Bond resonance or Mesomeric effect (b) sigma bond resonance or Hyper conjugation . Condition - (a) Planer Molecule (b) Conjugation ........contd.. Note - Following video will be helpful for Understanding the concept of resonance