How are sigma bonds different from pi bonds?

Sigma bonds are formed by the end-to-end overlapping of atomic orbitals, while pi bonds are formed by the side-to-side overlapping.

Sigma bonds are the strongest type of covalent bond, formed by the direct overlap of atomic orbitals, resulting in electron density concentrated between the nuclei of the bonding atoms. They are formed when two atomic orbitals combine and the electrons in the bond are located in the area directly between the two nuclei. This type of bond allows for free rotation because the electron density is symmetrically distributed around the bond axis. Sigma bonds are present in all types of covalent bonds - single, double, and triple.

On the other hand, pi bonds are formed by the side-to-side overlapping of atomic orbitals, resulting in electron density above and below the plane of the nuclei of the bonding atoms. The electron cloud in a pi bond is not symmetrically distributed, but it is concentrated on either side of the line joining the two nuclei. This type of bond does not allow for free rotation. This is because rotating the bond would require shifting the position of the electron cloud, which requires a significant amount of energy. Pi bonds are present in double and triple covalent bonds, but not in single bonds.

In terms of strength, sigma bonds are stronger than pi bonds. This is because the end-to-end overlapping of atomic orbitals in sigma bonds allows for a greater area of overlap, leading to a stronger bond. In contrast, the side-to-side overlapping in pi bonds results in a lesser area of overlap, leading to a weaker bond.

In summary, sigma and pi bonds differ in the way they are formed, their electron distribution, their presence in different types of covalent bonds, their allowance for rotation, and their strength. Understanding these differences is crucial for understanding the properties of molecules and how they react in chemical reactions.