Basics of molecular orbital theory

Molecular orbitals arise from overlap of atomic orbitals on adjacent atoms. The most significant overlap is for the valence orbitals.
 

• For diatomic molecules where both atoms are the same (homonuclear diatomics), overlap occurs only if the valence orbitals are of similar energy.
   
  • a 1s valence orbital on atom A overlaps with a valence 1s orbital on atom B
     
  • a 2s valence orbital on atom A overlaps with a valence 2s orbital on atom B
     
  • a 2p valence orbital on atom A overlaps with a valence 2p orbital on atom B 
      
     
• Overlap does not result in either loss or gain in the overall number of orbitals; therefore the number of molecular orbitals formed equals the number of atomic orbitals that overlap.
   
  • Overlap of one s orbital on atom A with one s orbital on atom B gives two molecular orbitals
     
  • Overlap of the three 2p orbitals on atom A with the three 2p orbitals on atom B gives six molecular orbitals.
     
  
• Each molecular orbital may contain a maximum of two electrons. As shown at the right the two electrons must have different spin states.