A pi bond is a type of covalent bond that occurs when two atomic orbitals overlap sideways. This bond typically forms in addition to a sigma bond, which is the first bond between two atoms. Pi bonds are found in double and triple bonds, such as those in ethylene (C2H4) and acetylene (C2H2), where they contribute to the overall stability and reactivity of the molecule. In a pi bond, the electron density is concentrated above and below the plane of the bonded atoms, rather than directly between them. This unique arrangement allows for certain properties, such as restricted rotation around the bond axis, which is important in the structure of organic compounds. Pi bonds play a crucial role in the chemistry of unsaturated hydrocarbons and other molecules.