Pi (π) bonds are a type of covalent chemical bond that forms when two parallel p orbitals overlap sideways, creating a region of electron density above and below the internuclear axis. Pi bonds are typically associated with multiple bonds, such as double bonds or triple bonds, and play a significant role in determining the overall electronic structure and reactivity of molecules.

Key features of pi (π) bonds:

1. Formation: Pi bonds are formed by the overlap of two parallel p orbitals from adjacent atoms. The overlap occurs perpendicular to the internuclear axis, creating a region of electron density above and below the bond axis.
2. Overlap: Unlike sigma (σ) bonds, where head-on overlap occurs along the bond axis, pi bonds involve side-to-side overlap of orbitals. This type of overlap leads to the formation of a bond between atoms.
3. Strength: Pi bonds are generally weaker than sigma bonds due to the lesser extent of overlap and the resulting weaker attraction between nuclei and electrons.
4. Multiple Bonds: Pi bonds are commonly found in molecules with multiple bonds, such as double bonds and triple bonds. Double bonds consist of one sigma (σ) bond and one pi (π) bond, while triple bonds consist of one sigma bond and two pi bonds.
5. Directionality: Pi bonds are not as directional as sigma bonds because the p orbitals involved in pi bonding are oriented parallel to each other. This allows for some degree of rotation around the bond axis.
6. Hybridization: Pi bonds can form between unhybridized p orbitals or between hybridized orbitals. The sigma bond in a multiple bond is usually formed by the overlap of hybrid orbitals, and the remaining unhybridized p orbitals are available for pi bond formation.
7. Weak Repulsion: Pi bonds can create a slight repulsion between electron clouds due to the concentration of electron density above and below the bond axis. This effect is important in larger molecules and impacts molecular geometry.
8. Overlap Regions: In a pi bond, the overlapping regions of the p orbitals are situated on either side of the internuclear axis.
9. Localized Electron Density: The pi bond creates localized regions of high electron density, which can be prone to attack by electrophiles in chemical reactions.
10. Bond Strength: Pi bonds are usually weaker than sigma bonds due to the nature of p orbital overlap and the resulting lower degree of effective overlap.

Pi bonds contribute to the overall stability, reactivity, and geometry of molecules. They play a crucial role in delocalized electron systems, such as conjugated systems found in many organic molecules, where pi bonds allow for the movement of electrons over multiple atoms. Understanding pi bonds is essential for explaining the properties of molecules and their behavior in chemical reactions.