Anti bonding and bonding exists because electrons act like waves and can either constructively or deconstructively interfere with each other. The higher energy anti bonding molecular orbital destabilizes the molecule. The lower energy bonding molecular orbital stabilizes the molecule. Mo theory treats molecular bonds as a sharing of electrons between neulcie. This contrasts with vb theory, which treats the electrons as localized hybrid orbitals of electron density. Mo theory also says that the electrons are delocalized. This means that they are spread out over the entire molecule. Use atomic orbitals to construct bonding and anti-bonding mo. Sigma bond: covalent bond with the highest electron density along the bond axis. Pi bond: formed by mixing of atomic orbitals not oriented along axis in a molecule. Particle doesn"t move like a straight line but like a wave. Bond order is (# of electrons in mos- #of anti bonding electrons in mos)/2. A mo can accommodate two electrons with opposite spin.