A pi (π) bond is a bond formed by the sideways overlap of two p-orbitals, with the electron density above and below the plane of the bonding atoms.

In a double bond, there is one sigma (σ) bond and one pi (π) bond.

C_nH_2n

Alkenes have one or more carbon-carbon double bond.

In an alkene, each carbon atom involved in the double bond forms three single bonds, either with hydrogen atoms (C-H) or other carbon atoms (C-C), and one double bond with the adjacent carbon atom. The three single bonds arrange themselves in a trigonal planar geometry, lying in a single plane with bond angles of ≈120°.

The trigonal planar arrangement of the bonds is due to the equal repulsion between the bonding pairs of electrons.

The high reactivity of alkenes is due to three key features of the π-bond in the C=C double bond:

1. Its high electron density makes the π-electrons highly accessible for reactions.
2. Its protruding shape allows electrophiles to readily attack the π-electrons.
3. Its lower bond enthalpy requires less energy to break compared to σ-bonds.

The pi (π) bond formed by the sideways overlap of the p-orbitals on the carbon atoms locks the carbons in position and prevents rotation around the double bond.

Alkenes undergo addition reactions because they are unsaturated molecules - a molecule can add across the C=C double bond to form a single saturated compound.

Alkenes react with electrophiles (electron pair-acceptors) because there is an area of high electron density around the C=C double bond.

Sigma (σ) bonds result from head-on overlap of atomic orbitals, while pi (π) bonds result from side-to-side overlap of p-orbitals.