Double bond rule

The empirical rule that elements beyond the second period rarely form stable multiple bonds because their sideways orbital overlap is weak.

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Pi bond: A covalent bond formed by sideways overlap of parallel orbitals, with electron density above and below the internuclear axis. Weak sideways overlap is the rule’s central explanation for unstable heavier-element multiple bonds.
Ethene: A hydrocarbon with a carbon–carbon double bond and molecular formula C₂H₄. Its stable carbon–carbon double bond exemplifies strong second-period sideways overlap.
Valence bond theory: A bonding theory that describes molecules through localized bonds formed by overlap of atomic orbitals. Its localized-overlap picture is the framework in which the rule is commonly introduced.
Main-group chemistry: The chemistry of elements in groups 1, 2, and 13–18 of the periodic table. The rule is a practical trend used to anticipate bonding across heavier p-block elements.
Sigma bond: A covalent bond formed by electron density concentrated along the axis between two nuclei. Head-on overlap remains effective for heavier atoms even when sideways overlap does not.
Dinitrogen: A diatomic molecule whose two nitrogen atoms are joined by a triple bond. Its stable triple bond illustrates how effectively second-period atoms can form multiple bonds.
Hypervalent molecule: A molecule whose central atom is conventionally described as having more than an octet of valence electrons. Hypervalent bonding in heavier elements shows that simple orbital-counting rules also have limits.
Multiple bond: A covalent bond involving two or three shared electron pairs between the same atoms. The rule predicts which elements commonly sustain these bonds.
Atomic orbital: A mathematical description of an electron’s spatial distribution in an atom. Orbital shape and extension determine how effectively atoms can form multiple bonds.
Disilene: A compound containing a silicon–silicon double bond, typically stabilized by bulky substituents. Its existence shows that heavier-element multiple bonds are rare, not impossible.