In
chemistry,
sigma bonds (
s bonds) are the strongest type of
covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for
diatomic molecules using the language and tools of
symmetry groups. In this formal approach, a s-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, p
z+p
z, s+p
z and d
z2+d
z2 (where z is defined as the axis of the bond). Quantum theory also indicates that
molecular orbitals (MO) of identical symmetry actually mix or
hybridize. As a practical consequence of this mixing of diatomic molecules, the
wavefunctions s+s and p
z+p
z molecular orbitals become blended. The extent of this mixing (or hybridization or blending) depends on the relative energies of the MO's of like symmetry.