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Sigma bond
strongest type of covalent chemical bond; formed by head-on overlapping between atomic orbitals. / From Wikipedia, the free encyclopedia
A sigma bond (σ bond) is a type of chemical bond. It is the strongest one of the covalent type.[1] In a sigma bond, the atomic orbitals from two atoms which are nearby go one on top of each other. This is different from a pi bond where they touch sideways. This is why a sigma bond is stronger. Sigma bonds can involve s, p, d or f orbitals. They must however be of the right symmetry to be able to form the bond.
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Since the σ bond is strong, the bonding orbital is low in energy. This also means that the antibonding orbital σ* is high in energy.