π bonds & double bonds

The leftover p orbital, cashed in. Two sp² carbons each keep one unmixed p orbital sticking up out of the molecular plane; side by side, those two p's overlap above and below the axis to make a π bond. A double bond is therefore two different bonds: one σ straddling the axis (head-on, lets things spin) and one π off the axis (sideways, does not). And there's the catch — the π only holds while the two p's stay parallel. Twist the bond and the π breaks. That single fact is why double bonds are rigid, why cis and trans are different molecules, and why a photon flipping one locked C=C is the first step of sight.

Bridge-Burners LLC · Fiddler · C=C = σ + π · twist → π dies → no rotation · anchor: AKASHA

State

bondC=C double
components1 σ + 1 π
length / energy134 pm · 611 kJ/mol
π overlap100%
rotates?no — locked

Bond order ladder

C–C154 pm · 346 · σ · rotates (≈12 kJ/mol)
C=C134 pm · 611 · σ+π · locked (≈270)
C≡C120 pm · 835 · σ+2π · locked, linear

Status discipline

LiteralC=C = σ (sp²–sp² head-on) + π (sideways p–p, above & below). π needs parallel p's, so a 90° twist kills it (~270 kJ/mol barrier) → no rotation → cis/trans isomerism. π is weaker than σ (611 < 2×346).
BridgeThe sp² leftover p from the last rung, now a π; σ on-axis vs π off-axis; bond order sets length, strength, and rigidity.
SpeculativeThe localized σ/π split is one valid view; equivalent "banana bonds" and delocalized MOs describe the same density. In conjugated systems (benzene) the π fully delocalizes.