5.2.3: Diatomic Molecules of the First and Second Periods

• Dihydrogen is the simplest diatomic molecule with only two molecular orbitals (og and ou*), two electrons, a bond order of 1, and is diamagnetic.
• Dihelium has a bond order of 0 due to equal number of electrons in bonding and antibonding orbitals
• Valence orbitals of second period homonuclear diatomic molecules are 2s and 2p
• In MO diagrams, second period homonuclear diatomic molecules disregard the non-valence orbitals (1s)
• Orbital mixing affects the order of filling of the og (2p) and piu (2p) orbitals
• Early in period 2 (up to and including nitrogen), the piu(2p) orbitals are lower in energy than the og(2p)
• Later in period 2, the og (2p) orbitals are pulled to a lower energy, along with all of the o orbitals in the molecule due to the increasing positive charge of the nucleus
• as nuclear charge increases, the energy of the og (2p) orbital is lowered significantly more than the energy of the piu (2p) orbitals
• Dilithium has a bond order of 1 and is observed experimentally to have one Li-Li bond
• Diberylium has a bond order of 0 and can be produced in a lab although the bond is very weak
• Diboron has a bond order of 1 and is paramagnetic as a consequence of orbital mixing, resulting in the og orbitals being at a higher energy than the two degenerate piu* orbitals
• Dicarbon has a bond order of 2 and its MO predicts two bonds with pi symmetry and no sigma bonding
• Dinitrogen is predicted to have a triple bond which is consistent with a short bond length and bond dissociation energy
• Dioxygen is a case where valence bond theory fails to predict actual properties. MO theory correctly predicts that dioxygen is paramagnetic with a bond order of 2
• DIfluorine has a bond order of 1 and the og is lower in energy than piu
• Dineon exists in the atomic form and does not form bonds at ordinary temperatures and pressures
• trends in experimental bond lengths are predicted by MO theory, specifically by the calculated bond order
• Bond length correlates with bond order, with a minimum bond length occurring where the bond order is greatest (shorter bond distance with greater bond order)