sigma bond

Every covalent bond between two atoms begins with one direct, head-on connection, like two people gripping hands palm to palm right between them. That straight-line, end-to-end overlap is a sigma bond. It is the strongest and most basic kind of covalent bond, and it forms the rigid framework that holds an organic molecule's atoms in place. Every single bond is a sigma bond, and even multiple bonds contain exactly one sigma bond among them.

More precisely, a sigma bond forms when two atomic orbitals overlap end-to-end along the axis directly joining the two nuclei, concentrating the shared electrons right between them. Because the electron density sits along the bond axis, atoms joined by a single sigma bond can spin freely around it, like two beads rotating on a shared rod — this is why single bonds allow free rotation and the floppy conformations of alkane chains. Sigma bonds can form from s orbitals, from hybrid orbitals (sp3, sp2, sp), or from the head-on part of a p orbital overlap.

Sigma bonds matter because they are the skeleton of organic chemistry: the C-C and C-H bonds of every chain and ring are sigma bonds. They are strong precisely because the electron density is concentrated between the nuclei where it binds most effectively. The key contrast to remember: a single bond is one sigma bond; a double bond is one sigma plus one pi bond; a triple bond is one sigma plus two pi bonds. The sigma is always the first, strongest, and most rotationally free of the bonds linking any two atoms.