Chemistry · Chemical Bonding · NEET
Resonance means one Lewis structure is not enough to show a molecule correctly. So we draw two or more valid structures. Each one is called a canonical form or resonance form. The true molecule is not any single form. It is a blend of all of them, called the resonance hybrid. NCERT says: whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, same positions of nuclei, and same bonding and non-bonding electron pairs are taken together as the canonical forms.
Ozone has two main resonance (canonical) structures. In one form the double bond is on the left O-O and the single bond on the right. In the other form they swap. This is important for NEET: the answer is TWO, not three. NEET 2024 used exactly this trap. The two forms blend into one resonance hybrid, so both oxygen-oxygen bonds become equal.
A single Lewis structure of O3 shows one O-O single bond and one O=O double bond. A normal single O-O bond is 148 pm and a double O=O bond is 121 pm. But experiments show both bonds in O3 are equal, at 128 pm. This is between a single and a double bond. One Lewis structure cannot explain equal bonds, so we need resonance. The real O3 is a hybrid of the two forms.
No. This is a very common NEET trap. The canonical forms have NO real existence. The molecule does not spend part of its time in one form and part in another form. The molecule has ONE single real structure, which is the resonance hybrid. The separate forms are only drawings we use because one Lewis structure is not enough. Resonance is shown by a double-headed arrow, not an equilibrium arrow.
The carbonate ion CO3 2- has THREE equivalent resonance (canonical) structures. In each form, the C=O double bond is on a different oxygen, and the other two are single C-O bonds. Because all three forms are equal, all three carbon-oxygen bonds become the same length in the real ion. NEET 2024 tested this: three canonical forms for CO3 2- is a correct statement.
Resonance makes a molecule MORE stable. NCERT states clearly: resonance stabilizes the molecule because the energy of the resonance hybrid is lower than the energy of any single canonical structure. This extra stability is called resonance energy. Resonance also averages the bond characters, so bonds that look different in single structures become equal in the real molecule.
Identify the correct answer.
For the ozone (O3) Lewis structure where the central O atom (1) is doubly bonded to one terminal O atom (2) and singly bonded to the other terminal O atom (3), the correct formal charges on the oxygen atoms numbered 2, 1 and 3 respectively are:
Which of the following pairs of ions is isoelectronic and isostructural?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Resonance structures (canonical forms) are the separate drawings we make when one Lewis structure is not enough. They are only on paper and do not exist by themselves. The resonance hybrid is the ONE real structure of the molecule, a blend of all the canonical forms. Only the hybrid is real.
Resonance is shown by a double-headed arrow (a single arrow with a head on both ends). This is different from the two half arrows used for equilibrium. Resonance is NOT an equilibrium; the molecule does not move between forms.
The NCERT examples are ozone (O3, two forms), the carbonate ion CO3 2- (three forms), and carbon dioxide CO2. Other frequently asked ions are NO3 - (three forms), NO2, and SO3. Benzene is the classic organic example.
Resonance energy is the extra stability a molecule gains from resonance. It is the difference between the energy of the real resonance hybrid and the energy of the most stable single canonical structure. A larger resonance energy means a more stable molecule.
Because resonance averages the bond characters over the whole molecule. In ozone both O-O bonds become 128 pm, between single (148 pm) and double (121 pm). In CO3 2- and NO3 - all three bonds become equal. This equal bond length is direct proof of resonance, and NEET loves asking about it.
