<P> In chemistry, pi bonds (π bonds) are covalent chemical bonds where two lobes of an orbital on one atom overlap two lobes of an orbital on another atom . Each of these atomic orbitals has zero electron density at a shared nodal plane, passing through the two bonded nuclei . The same plane is also a nodal plane for the molecular orbital of the pi bond . </P> <P> The Greek letter π in their name refers to p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital when seen down the bond axis . One common form of this sort of bonding involves p orbitals themselves, though d orbitals also engage in pi bonding . This latter mode forms part of the basis for metal - metal multiple bonding . </P> <P> Pi bonds are usually weaker than sigma bonds . The C-C double bond, composed of one sigma and one pi bond, has a bond energy less than twice that of a C-C single bond, indicating that the stability added by the pi bond is less than the stability of a sigma bond . From the perspective of quantum mechanics, this bond's weakness is explained by significantly less overlap between the component p - orbitals due to their parallel orientation . This is contrasted by sigma bonds which form bonding orbitals directly between the nuclei of the bonding atoms, resulting in greater overlap and a strong sigma bond . </P> <P> Pi bonds result from overlap of atomic orbitals that are in contact through two areas of overlap . Pi - bonds are more diffuse bonds than the sigma bonds . Electrons in pi bonds are sometimes referred to as pi electrons . Molecular fragments joined by a pi bond cannot rotate about that bond without breaking the pi bond, because rotation involves destroying the parallel orientation of the constituent p orbitals . </P>

What is pi and sigma bond in chemistry