Pi bonds are generally weaker than sigma bonds, primarily due to their reduced overlap between atomic orbitals, which results in lower stability.

In molecular bonding, the strength of a bond is largely 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 through end-to-end or head-on overlap of atomic orbitals, which allows for a significant degree of overlap and consequently, a stronger bond.

Conversely, pi bonds are created by side-to-side overlap of atomic orbitals. This type of overlap is less effective than the end-to-end overlap found in sigma bonds, leading to a decreased extent of overlap and, therefore, a weaker bond. This is the fundamental reason why pi bonds are typically weaker than sigma bonds.

Additionally, the electron density in a pi bond is more spread out compared to that in a sigma bond. As a result, the electrons in a pi bond are not held as tightly and can be more easily disrupted, further contributing to the relative weakness of these bonds.

Moreover, pi bonds are more exposed to the surrounding environment than sigma bonds, making them more vulnerable to interactions with other molecules or ions. This increased reactivity can lead to the breaking of the bond, which adds to the weakness of pi bonds.

It’s also important to note that a single bond is always a sigma bond, while a double bond consists of one sigma bond and one pi bond, and a triple bond comprises one sigma bond and two pi bonds. Thus, while a molecule with a double or triple bond may seem stronger due to the presence of multiple bonds, the additional bonds consist of weaker pi bonds.

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