Sigma bond

A covalent bond formed by end-to-end orbital overlap, with electron density concentrated along the axis joining the bonded nuclei. It is the first bond between two atoms and can form from s, p, or hybrid orbitals.

Atomic orbital: A quantum-mechanical description of an electron’s spatial distribution around an atomic nucleus. Sigma bonding depends on the shapes and orientations of atomic orbitals.
Covalent bond: A chemical bond formed when atoms share pairs of electrons. A sigma bond is one geometry-specific form of covalent bonding.
Pi bond: A covalent bond formed by sideways overlap of orbitals, with electron density above and below the internuclear axis. Unlike sigma bonds, pi bonds restrict rotation and accompany a sigma bond in multiple bonds.
Ethane: A two-carbon alkane with formula C₂H₆ and a single carbon–carbon bond. Its carbon–carbon bond is a basic example of a sigma bond formed by hybrid orbitals.
Orbital overlap: The sharing of spatial regions by atomic orbitals, enabling electron density between nuclei. End-to-end overlap places bonding electron density along the internuclear axis.
Bond order: A measure of the number of bonding interactions between two atoms. Every single bond is one sigma bond; multiple bonds add other bonding interactions.
Double bond: A covalent bond consisting of two shared electron-pair bonding interactions between the same atoms. Its two interactions are one sigma bond and one pi bond.
Ethene: A two-carbon alkene with formula C₂H₄ and one carbon–carbon double bond. Its double bond contains one sigma bond alongside one pi bond.
Hybrid orbital: An orbital formed by combining atomic orbitals on the same atom into directional bonding orbitals. Hybrid orbitals commonly point toward neighboring atoms to form sigma bonds.
Valence electron: An electron in an atom’s outer shell that can participate in chemical bonding. Shared valence electrons occupy the bonding orbital that defines a sigma bond.

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