= Carbon-carbon double bond
A <carbon-carbon double bond> is a <covalent bond> of <bond order> two between <carbon> atoms, conventionally written $\mathrm{C=C}$. In a localized orbital description it consists of one <sigma bond> and one <pi bond>. The distinction describes the symmetry of the bonding orbitals: the localized pi interaction has a nodal plane containing the internuclear axis, whereas the sigma interaction does not.<Sigma and pi bonds in IUPAC Gold Book>[S05434, localized two-centre symmetry definitions]{ref}
<Ethylene>, $\mathrm{H_2C=CH_2}$, illustrates the counting. Its four carbon–hydrogen connections are <sigma bonds>, and the carbon–carbon connection contributes another <sigma bond> plus one <pi bond>. The localized picture therefore has five sigma bonds and one pi bond, representing six shared electron pairs. Calling the central connection a double bond does not mean that the two bonding interactions are geometrically identical.
Maintaining the <pi bond> favors an aligned arrangement of the participating orbitals. Rotation that destroys this overlap has an energetic cost, which helps make <cis-trans isomerism> meaningful when each double-bonded carbon has suitable distinct substituents. Ethylene itself has identical hydrogens at each end, so it does not furnish that geometric distinction.
Chemical reactions can change the connection while retaining the carbon skeleton. <Fumarate hydratase> adds <water> across the <carbon-carbon double bond> of <fumarate> to produce <L-malate>; the reverse reaction removes water and restores the double bond.<Fumarate hydratase in IUBMB Enzyme Nomenclature>[EC4.2.1.2, reversible reaction]{ref} Counting atoms and identifying bond order thus reveal different parts of the transformation.
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