Sigma bond

In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2 (where z is defined as the axis of the bond). Quantum theory also indicates that molecular orbitals (MO) of identical symmetry actually mix. As a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and pz+pz molecular orbitals become blended. The extent of this mixing (or blending) depends on the relative energies of the like-symmetry MO's.
For homodiatomics, bonding σ orbitals have no nodal planes between the bonded atoms. The corresponding antibonding, or σ* orbital, is defined by the presence of a nodal plane between these two bonded atoms.

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