Double Bond
Definition and meaning of Double Bond in chemistry.
A double bond is a covalent bond in which two atoms share two pairs of electrons (four electrons total), formed from one sigma (σ) bond plus one pi (π) bond.
In more detail
The sigma bond arises from head-on orbital overlap directly along the axis connecting the two nuclei, while the pi bond forms from sideways overlap of adjacent, unhybridized p orbitals above and below that axis. Because breaking the pi bond's overlap requires substantial energy, rotation around a double bond is restricted, locking the bonded atoms and their substituents into a fixed, planar arrangement that gives rise to cis/trans (E/Z) isomerism. Double bonds are shorter and stronger than a corresponding single bond but weaker than a triple bond, and their exposed pi electron density makes them especially reactive toward addition reactions.
Key facts
| Field | General Chemistry |
|---|---|
| Composition | 1 sigma (σ) bond + 1 pi (π) bond |
| Bond order | 2 |
| Classic example | Ethylene, C2H4 |
In ethylene, H2C=CH2, each carbon is sp2 hybridized and forms one sigma bond and one pi bond to the other carbon, along with two sigma bonds to hydrogen atoms, giving a rigid, planar molecule.
Frequently asked questions
Why can't atoms rotate freely around a double bond?
Rotation would require breaking the sideways overlap of the p orbitals that form the pi bond, which costs significant energy, so the atoms stay locked in a fixed, planar geometry.
Is a carbon-carbon double bond stronger than two single bonds?
No. A C=C double bond (about 614 kJ/mol) is stronger than one C-C single bond (about 347 kJ/mol) but weaker than two single bonds combined, since the pi component is weaker than the sigma component.