Chemistry · Chemical Bonding · NEET
Three. In CO3^2-, carbon is bonded to three oxygen atoms. One of them is a double bond and the other two are single bonds carrying the negative charges. Because the double bond can sit on any of the three oxygens, we can draw the same picture three ways. These three equal drawings are the three canonical (resonance) structures. This exact fact was asked in NEET 2024.
Because the real ion is NOT any single drawing. The true structure is an average (a hybrid) of all three resonance forms. Since the double bond is shared equally over all three oxygens, each C-O bond is really 'one and one-third' bonds. So all three bonds have the same length, which is in between a normal C-O single bond and a C=O double bond. Measured bond length is about 129 pm for all three.
CO2 (O=C=O) has two main resonance structures. Besides the normal O=C=O form, you can draw one with a triple bond to one oxygen and a single bond to the other, and another with the triple/single bond swapped. NEET remembers the count: CO2 has 2 resonance forms while CO3^2- has 3. Do not mix these numbers up.
No. This is the most common mistake. The molecule does NOT jump from one form to another. The single resonance forms do not exist by themselves at all. The real ion is one fixed, stable, average structure all the time. The three drawings are only a limitation of our simple bond-line pictures, not real changes happening in the ion.
When electrons are spread out (delocalised) over several atoms instead of being stuck in one place, the energy of the molecule goes down. This lowering of energy is called resonance energy. So CO3^2- and CO2 are more stable than any single Lewis structure would predict. More resonance structures of equal energy usually means more stability.
Identify the correct statement.
For the ozone (O3) Lewis structure with a central O doubly bonded to one terminal O (atom 2) and singly bonded to the other terminal O (atom 3), the correct formal charges on atoms 2, 1 and 3 respectively are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
CO3^2- is trigonal planar (flat, bond angles 120 degrees). Its C-O bond order is 4/3, or about 1.33, because three bonds are shared over three positions. That is why the bond is stronger and shorter than a single bond but weaker and longer than a pure double bond.
No. In resonance only electrons move and the atoms stay in the same place, giving one real average molecule. In tautomerism, atoms (usually a hydrogen) actually move, giving two different real molecules that can exist. Do not confuse the two for NEET.
A molecule with resonance is more stable. Spreading (delocalising) the electrons lowers the energy. The energy difference between the real molecule and its most stable single Lewis structure is called resonance energy.
No. When all forms are equal (like the three forms of CO3^2-), each contributes equally. When they are unequal (like CO2's triple-bond forms), the lower-energy, more stable form contributes more to the real hybrid.
