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    Chemistry Point Groups

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    For questions 1-5 below, determine the point group for each of the following molecules, ions and objects. Draw a clear picture of each molecule, ion and object and clearly show or describe all symmetry elements necessary to determine the point group. Remember, only the connectivity of the atoms determines the symmetry, not the bond order or lone-pair electrons. Be very careful that you have drawn the correct structure when required.
    You do not need to find all symmetry elements, just those necessary to identify the point group. Drawing symmetry elements is great, but I would expect quite a few to be described in brief words based on your drawings.

    1. a) borazine, B3N3H6 b) a paper clip

    2. a) a trash can b) cyanide ion, CN-

    3. a) FeCl63- b) a Pringles® potato chip

    4. a) NiCl2H2 (tetrahedral) b) NiCl2H2 (square planar)

    5. a) a dxy orbital (shading counts!) b) a pz orbital (shading counts!)

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    Solution Preview

    Dear Student,

    To answer point group questions you need to first know how the basic symmetry functions work (C2, σh etc.) work. I'll assume you know this. If not then please let me know! It'll also help if you have a molecular modeling kit if you're having trouble seeing molecules in 3D in your head.

    Next, what I like to do is ask myself the following questions in a stepwise fashion. You might want to memorize these before an exam.

    • Is this a special group?
    o If yes, then Td, Oh or Ih (This only applies if the molecule is a perfect tetrahedron, octahedron or icosahedron, respectively)
    o (If no, then move to next question)
    • Is the highest order Cn axis greater than 1?
    o (If yes, then move to next question)
    o If no, then Cs, Ci or C1
    • Is there a ...

    Solution Summary

    This contains questions regarding point groups. Answers are given mainly as guidelines. A very useful exam tip is also given