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Chemistry LibreTexts

3.2: MO Theory

  • Page ID
  • Molecular Orbital Theory

    MO Theory is an advanced theory of bonding and chemical reactivity. It is "advanced" because it is based on quantum mechanics, and it is important because it can predict the properties of molecules much better than any other model (including simpler models like Valence Bond Theory).

    Please give your best response to the following questions (be thorough and concise):

    1. What is a MO and how is it different from an AO?



    2. According to MO Theory, what is a bond?



    3. Rationalize the following using logic and by applying principles of physics:

    a. Bonding MO's are always lower in energy than contributing AOs.



    b. Anti-bonding MO's are always lower in energy than contributing AOs.




    4. Please describe the following terms in your own words:

    a) Bond order




    b) Constructive and destructive interference




    c) Paramagnetic and diamagnetic




    5. Examine the figure below:

    Screen Shot 2019-10-25 at 11.55.29 AM.png

    a) Draw appropriate orbitals in the empty boxes.

    b) For the sets of compatible AO's, please draw a sketch the bonding and antibonding MO that would result. Then label the new bonding and antibonding MO as sigma or pi.












    6. Give at least two reliable "rules" that you could use for determining whether a pair of orbitals is compatible or incompatible.





    Diatomic molecules

    Draw the molecular orbital diagrams of the diatomic molecules below. Label orbitals, draw sketches or orbitals, determine the bond order, and state whether it is diamagnetic or paramagnetic. 

    (a)  H2                                                                             (b) O2











    (c)  CO                                                                            (d)  HCl



    Polyatomic Molecules

    You already know how to draw MO diagrams for diatomic molecules (ie molecules that have only two atoms) by combining orbitals with compatible symmetry. Now, you are going to do the same for polyatomic molecules (ie molecules with more than two atoms). You will start with the examples BeH2, OH2, and BH3, shown below. These are nice examples to start with because they have only σ bonding.

    The approach to drawing a polyatomic MO diagram changes only slightly compared to what you’ve done before with diatomic molecules. The main difference now is that the orbitals on pendant atoms are treated as a set that is fixed in space with respect to the central atom. Each set of orbitals takes on its own symmetry identity. MOs form when the symmetry of the set is compatible with the atomic orbitals on the central atom.

    Draw the MO diagram of H2O


    Draw the MO diagram of BH3.