# Is this a possible explanation as to why sigma bonds are stronger than pi bonds?

Sigma bonds form due to the axial overlap of two orbitals whereas pi bonds form due to their lateral overlapping. Since the former is more effective, we can say that sigma is stronger than pi. We mean that the orbitals overlap to a further extent in sigma bonds and not as much in pi bonds(...or at least that is what I infer). To me this sounds like a circular proof/explanation, so I thought up another way to understand it.

In a sigma bond, the electron cloud of one atom is attracted to the nucleus of the other atom's nucleus.This happens because of the electrostatic forces of attraction between the positively charged protons in the nucleus of one atom and the negatively charged electron cloud of the other atom. The attractive force causes the electron clouds to the atoms to overlap, despite the fact that the electron clouds should repel each other. However, the electrons start to repel each other after a certain extent of overlapping has occurred. (Because the repulsive forces between the two nuclei and the electron clouds negate the attractive forces)

In a pi bond, there is no positively charged nucleus that would attract the electrons, so there must be another factor favoring the bond formation. Since I haven't studied the quantum mechanics of bonding yet, my best guess is that the pi bond (or lateral overlapping) occurs because of the pairing of electrons. So this means that sigma bonds are stronger because of electrostatic forces, which consequently causes more effective overlapping than pi bonds.

Is my reasoning correct to some (if any) extent?

• when you go through perturbation theory you will result in what is known as the klopman-Salem equation. In its simplified form this equation has two terms a Coulombic interaction and an orbital term. \begin{equation} \Delta E = -\frac{Q^2}{R}+\frac{[c_1c_2\beta]^2}{E_{Homo}-E_{Lumo}}\end{equation} this latter term is what we can look at. The c terms are orbital coefficients and beta is a resonance integral (basically how well the orbitals overlap) the denominator is the difference between the orbitals energies. Therefore a large gap means less contribution. – AngusTheMan Mar 18 '15 at 10:34
• From the orbital term. This is a good way to understand bonding. So we have the coulomb part. An overlap part. And an energy difference part. – AngusTheMan Mar 18 '15 at 10:36
• Look at the directionality of the overlap in the 2 cases. In a sigma bond the orbitals overlap is very effective as it is head-on. Pi overlap of orbitals is less effective because it is lateral. – ron Mar 18 '15 at 13:39
• Thanks so much @AngusTheMan and I hope I understand ur first comment when it's finally taught – tkhanna42 Mar 18 '15 at 18:09
• – LordStryker Mar 19 '15 at 13:20