Experimentally, O2 is found to have two unpaired spins – VBT can not explain a simple molecule like O2 !
Assumptions:
1) Molecular Orbitals look like atomic orbitals, especially near the nucleus
2) Molecular Orbitals are constructed by interference patterns of atomic orbitals
3) The total number of molecular orbitals = the total number of atomic orbitals
4) Molecular wavefunctions are products of the molecular orbital functions
constructive interference gives bonding orbital, no nodes
destructive interference gives antibonding orbital one node, perpendicular to the internuclear axis
molecular orbitals are labeled by nodal properties:
0 nodes parallel to the internuclear axis: σ
1 node parallel to the internuclear axis: π
2 nodes parallel to the internuclear axis: δ
0 nodes perpendicular to the internuclear axis: bonding
1 node perpendicular to the internuclear axis: antibonding