The figure shows the orbital diagram for the formation of the carbonyl group.
Which of the following correctly describes the π bond of the carbonyl group shown in the figure?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → It is formed by overlap of a p orbital of carbon with a p orbital of oxygen, and its electron cloud lies above and below the plane of the sp2 orbitals.
In the first panel the carbon uses its three sp2 orbitals in the plane: two are labelled sp2 and the third forms the C–O σ bond. The upright lobes standing above and below the plane on C and on O are the unhybridised p orbitals.
Orbitals that overlap — a p orbital of C and a p orbital of O. In the second panel these two upright p orbitals overlap sideways to give the π bond. The sp2 orbitals of carbon stay in the plane and are used only for σ bonds.
Position of the π-electron cloud — above and below the plane. The π bond is drawn as two lobes, one above and one below the plane. The region along the C–O axis, in the plane, is taken by the σ bond.
Why the other options are incorrect
Option (1) It is formed by overlap of an sp2 orbital of carbon with a p orbital of oxygen, and its electron cloud lies above and below the plane of the sp2 orbitals. — Incorrect. The position of the cloud is right, but an sp2 orbital of carbon forms a σ bond, not the π bond.
Option (2) It is formed by overlap of a p orbital of carbon with a p orbital of oxygen, and its electron cloud lies in the plane of the sp2 orbitals, along the C–O axis. — Incorrect. The orbitals are right, but a cloud along the C–O axis in the plane describes the σ bond.
Option (4) It is formed by overlap of an sp2 orbital of carbon with a p orbital of oxygen, and its electron cloud lies in the plane of the sp2 orbitals, along the C–O axis. — Incorrect. Both parts are wrong: this describes a σ bond formed from an sp2 orbital.
NCERT (2026-27 edition):
8.1.2 Structure of the Carbonyl Group – “The carbonyl carbon atom is sp2-hybridised and forms three sigma (σ) bonds. The fourth valence electron of carbon remains in its p-orbital and forms a π-bond with oxygen by overlap with p-orbital of an oxygen. In addition, the oxygen atom also has two non bonding electron pairs. Thus, the carbonyl carbon and the three atoms attached to it lie in the same plane and the π-electron cloud is above and below this plane. The bond angles are approximately 120° as expected of a trigonal coplanar structure (Figure 8.1).”
Fig. 8.1: Orbital diagram for the formation of carbonyl group