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5.5B: \(NH_3\)

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  • NH3, also known commonly as Ammonia, is a colorless gas that carries with it a pungent smell. It hydrogen bonds extremely well due to the fact that it has an accessible lone pair, making it miscible with water. It has a molecular configuration of trigonal pyramidal and with its lone pair experiences a dipole moment making the molecule polar.


    Molecular Orbital Theory builds on the aspects of VSEPR Theory, VB Theory, the Pauli Exclusion Principle and Hund's Rule to explain how valence electrons fill in each molecular orbital. The Molecular Orbital Theory explains, for polyatomic molecules, how hybridization occurs. Molecular Orbital Theory has advantages over VB Theory in that it can explain para/dia-magnetic properties of molecules and easily deduce their bond orders from the coinciding molecular orbital diagrams.

    Molecular Orbital Diagram

    NH3 has 8 valence electrons. As was already discussed there is one lone pair and 3 N-H bonds resulting in a trigonal pyramidal shape. Based on knowledge of Molecular Orbital Theory we fill the orbitals to obtain the lowest possible energy. Resulting in 2 paired electrons in the σ2s, σ*2s,...

    (PICTURE HERE: MO Diagram)

    Symmetry Elements

    With the resulting predicted trigonal pyramidal shape we can find the primary axis of rotation to be a C3 , and there are 2 C3's. We also can see 3σv's along the N-H bonds.

    (PICTURE HERE: Symmetry elements trig planar model)

    Character Table

    C3v E 2C3 v Linear Rotations Quadratic
    A1 1 1 1 z x2+y2,z2
    A2 1 1 -1 Rz
    E 2 -1 0 (x,y)(Rx, Ry) (x2-y2, xy)(xz,yz)


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    1. What are the benefits of using MO theory over VB theory?
    2. What is the predicted molecular geometry based on VSEPR?
    3. Based on the MO diagram what is NH3 paramagnetic or diamagnetic?
    4. Based on the MO diagram what is the bond order for NH3?
    5. What is NH3 point group characterization?

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