Lexico Unisciel english version

Strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. 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).