structure and bonding: MO theory for N2 and heterodiatomics

trend in atomic orbital energy for first row of p block elements

• at beginning of first row of p block elements 2s and 2p atomic orbitals are relatively close together

• 2p less penetrating than 2s so has smaller effective nuclear charge

• this means that 2p is higher in energy than 2s

• effective nuclear charge increases from Li to Ne (since nuclear charge increases) so s-p energy gap gets larger from left to right

sp mixing

• can mix molecular orbitals of similar symmetry and energy to give different molecular orbitals

• the 2s and 2p sigma bonding orbitals become close enough in energy so that they interact

• 2p higher in energy than 2s so moves up whilst 2s moves down - 2p sigma bonding orbital ends up above the pi bonding orbital

• 2s and 2p antibonding orbitals also close enough in energy so they interact

• pi molecular orbitals do not move

• electrons will occupy lower energy pi orbitals before occupying sigma orbitals

homonuclear diatomics

• as the bond order increases, the bond lengths decreases and bond strength increases

• as the bond length increases the bond dissociation enthalpy decreases

formation of cations and anions

• you can use an MO diagram from a homonuclear diatomic molecule to predict properties of a species derived from that molecule

• add/remove electrons to the MO diagram and calculate bond order for new species that has been derived

• for isoeletronic atoms and ions - if atom and ion different, one with greater nuclear charge will have a slightly shorter bond

heteronuclear diatomics

• energy levels for the orbitals on each atom are not the same

• more electronegative atom has lower energy orbital - skewed energy level diagram

• as electronegativity difference increases, the E_covalent term becomes smaller - less efficient interference between atomic orbitals

• greater difference in electronegativity between 2 atoms, energy gap increases