Every bond has exactly one sigma (σ) bond; a double adds one pi (π) bond, a triple adds two. σ allows free rotation and holds atoms together; π bonds lock rotation and carry the reactive electrons.
1. A Single Bond Is Exactly One Sigma Bond
A σ bond comes from end-on overlap of hybrid orbitals along the internuclear axis, so every single bond in an alkane is one σ.
2. A Double Bond Is One Sigma Bond Plus One Pi Bond
A double bond is one σ plus one π bond, formed by side-on overlap of the leftover p orbitals above and below the plane of the sp²-hybridized carbons.
3. A Triple Bond Is One Sigma Bond Plus Two Pi Bonds
A triple bond is 1 σ + 2 π: each sp-hybridized carbon has two p orbitals that form two perpendicular π bonds — count every "extra" line beyond the first as a π.
4. More Pi Bonds Mean Shorter, Stronger, Less Rotatable Bonds
Each added π shortens the bond and raises total strength (single > double > triple in length), yet a lone π is weaker than a σ — so π breaks first in reactions.
5. Pi Bonds Block Rotation — the Basis of Cis/Trans
Single bonds spin freely, but rotating a double bond would break its side-on p-orbital overlap — that locked C=C is what makes cis/trans (E/Z) isomers possible.
6. Pi Electrons Are the Reactive, Nucleophilic Site
Loosely-held π electrons sit exposed outside the bond axis, making the π bond the nucleophilic site that drives most reactions while the σ skeleton stays intact.
7. Summary
Single = 1 σ · double = 1 σ + 1 π · triple = 1 σ + 2 π · more π → shorter, stronger · σ rotates, π locks (cis/trans) · π electrons react.
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One σ bond and two π bonds. The σ forms from end-on overlap of sp hybrid orbitals; the two π bonds come from two pairs of perpendicular unhybridized p orbitals.
A σ bond's end-on overlap is unchanged by rotation, so single bonds spin freely. Twisting a double bond would break the side-on p-orbital overlap of its π bond, which costs too much energy — so double bonds are locked, giving cis/trans isomers.
The C–C single bond is longer and weaker. Adding a π bond (making it C=C) raises the bond order, pulling the atoms closer and increasing total strength; a triple bond is shorter and stronger still.
π electrons lie above and below the bond axis and are held more loosely than σ electrons, so they are exposed and easy to donate. That makes the π bond the nucleophilic, reactive site while the strong σ framework stays intact.
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