5.2.1: Molecular Orbitals

• a molecular orbital diagram shows the relative energies and identities of each molecular orbital in a molecule
• there are the same number of molecular orbitals as there are atomic orbitals
• each orbital in the diagram is rigorously labeled with labels including the subscripts u and g
• sigma symbol indicates that the orbital is symmetric with respect to the internuclear axis
• pi sympbol indicates that there is one node along that axis
• the subscript g is given to orbitals that are even, or symmetric, with respect to an inversion center
• the subscript u is given to orbitals that are uneven, or antisymetric, with respect to an inversion center
• a complete molecular orbital diagram will include the electrons for each atom and for the molecule
• electrons in molecular orbitals are filled in the same way an atomic orbital diagram would be filled: electrons occupy lower energy orbitals before higher energy orbitals and electrons occupy empty degenerate orbitals before pairing
• a complete molecular orbital diagram would show whether the molecule is diamagnetic or paramagnetic and can be used to calculate the bond order of the molecule
• bond order = (number of electrons in bonding orbitals - number of electrons in antibonding orbitals)/2
• non-valence (core) electrons can be ignored because they do not contribute to the bond order