A covalent bond is the electrostatic attraction between two nuclei and a shared pair of electrons. Covalent bonds are formed from the overlap of two single electrons in two atomic s or p orbitals.Sigma bonds are formed from the end-on overlap of two s-orbitals, two p-orbitals or an s-orbital and a p-orbital. Pi bonds arise from the sideways overlap of 2 atomic p-orbitals.Pi bonds cannot form until sigma bonds are formed so lead to the formation of double or triple bonds.