Explain why a sigma (σ) bond is stronger than a pi (π) bond?

A sigma (σ) bond is stronger than a pi (π) bond due to the nature of their overlapping orbitals and bonding interactions. Let's delve into the details to understand this difference.

In a covalent bond, atoms share electrons to form a stable molecule. A sigma bond arises from the head-on overlap of atomic orbitals, specifically the overlap of two s orbitals or the overlap of an s orbital with a p or d orbital. This head-on overlap allows for maximum electron density between the two nuclei, resulting in a strong bond.

On the other hand, a pi bond arises from the sideways overlap of atomic orbitals, typically p orbitals. In a pi bond, the overlapping orbitals are parallel to each other and lie above and below the axis connecting the nuclei. This sideways overlap creates a region of electron density above and below the internuclear axis, forming what is often referred to as a "cloud" of electron density. The pi bond is formed in addition to the sigma bond in a double or triple bond situation.

The key difference between a sigma bond and a pi bond lies in the nature of their overlapping orbitals. In a sigma bond, the overlap occurs along a greater surface area of the atomic orbitals, resulting in a more effective sharing of electrons and stronger bonding. In contrast, a pi bond has a smaller overlap area since it occurs along a single line or axis, leading to weaker bonding compared to the sigma bond.

Additionally, the orientation of the electron density in sigma and pi bonds affects their strength. In a sigma bond, the electron density is concentrated between the nuclei, creating a strong electrostatic attraction. In a pi bond, the electron density is distributed above and below the internuclear axis, resulting in a less effective attraction between the nuclei and weaker bonding.

Overall, the combination of the head-on overlap, larger surface area of overlapping orbitals, and concentrated electron density between nuclei in a sigma bond contribute to its greater strength compared to the sideways overlap, smaller overlap area, and distributed electron density of a pi bond.