Why are pi bonds weaker than sigma bonds?

Pi bonds are weaker than sigma bonds because they have a lesser extent of overlap between atomic orbitals, leading to less stability.

In molecular bonding, the strength of a bond is determined by the extent of overlap between the atomic orbitals of the bonding atoms. The greater the overlap, the stronger the bond. Sigma bonds are formed by the end-to-end or head-on overlap of atomic orbitals, which results in a greater extent of overlap and hence, a stronger bond.

On the other hand, pi bonds are formed by the side-to-side overlap of atomic orbitals. This side-to-side overlap is less effective than the end-to-end overlap in sigma bonds, resulting in a lesser extent of overlap and hence, a weaker bond. This is why pi bonds are generally weaker than sigma bonds.

Furthermore, the electron density in a pi bond is spread out more diffusely than in a sigma bond. This means that the electrons in a pi bond are not as tightly held and are more easily broken apart, contributing to the relative weakness of pi bonds.

In addition, pi bonds are more exposed to the surrounding environment than sigma bonds. This makes them more susceptible to attack by other molecules or ions, which can break the bond. This increased reactivity also contributes to the relative weakness of pi bonds.

It's also worth noting that a single bond is always a sigma bond, while a double bond consists of one sigma and one pi bond, and a triple bond consists of one sigma and two pi bonds. This means that while a molecule with a double or triple bond may appear to be stronger due to the presence of multiple bonds, the additional bonds are actually weaker pi bonds.

To understand the basics of molecular interactions that influence bonding, you can explore more about collision theory.

For further reading on how these atomic interactions are categorised in the periodic table, specifically in different blocks, see s, p, d, f blocks.

Also, the formation of covalent bonds provides a deeper insight into how different types of atomic orbitals overlap to form stronger or weaker bonds.

It's also worth noting that a single bond is always a sigma bond, while a double bond consists of one sigma and one pi bond, and a triple bond consists of one sigma and two pi bonds. This means that while a molecule with a double or triple bond may appear to be stronger due to the presence of multiple bonds, the additional bonds are actually weaker pi bonds.

In summary, the relative weakness of pi bonds compared to sigma bonds is due to their lesser extent of overlap between atomic orbitals, more diffuse electron density, and increased exposure to the surrounding environment.