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Homework Help: VSEPR model

  1. May 13, 2008 #1
    1. The problem statement, all variables and given/known data

    What is the geometry of AsH3 according to the VSEPR model?

    3. The attempt at a solution

    As has 5 valence electrons and H has one. The molecule therefore has four sets of electron pairs, whereof one is unshared. This gives a tetrahedral structure where the As-H angles are 107 degrees.

    However, according to the solutions manual the geometry is triangular planar. Why?

    I also have a problem with the geometry of OF2. Here, there are four sets of electron pairs, whereof two are unshared. I would expect the geometry of this molecule to be bent, but it's linear according to my book.
    Last edited: May 13, 2008
  2. jcsd
  3. May 13, 2008 #2
    For the OF2, I think it is due to the formal charges. O and F are both negative, where in the case of H2O the O is neg and the H is +. This is my first year of chem, so I may be wrong.
  4. May 13, 2008 #3

    For AsH3 you are on the right track - keep thinking about the shape - but your solution manual is wrong

    For OF2 you are correct and the manual is wrong again
  5. May 13, 2008 #4
    Hm, I couldn't figure it out.
  6. May 13, 2008 #5
    shape depends on atoms
  7. May 13, 2008 #6
    Last edited: May 13, 2008
  8. May 14, 2008 #7
    yes electrons are in the tetrahedral arrangement and dictate the arrangement of the outside atoms. so what shape has 4 pairs of electrons, 1 unshared and 3 outside atoms?
  9. Oct 30, 2010 #8
    Sorry to be digging up old threads but is the answer Trigonal Pyramidal for AsH3 ?
  10. Oct 31, 2010 #9
    I can't say with absolute certainty; but yes, I would conclude that AsH3 is trigonal pyramidal, like Ammonia. As it has three bonds to Hydrogen, and a lone pair.
  11. Nov 10, 2010 #10
    Yes AsH3 is pyramidal, as the lone pair wants as much space as it can get, and the largest angle it could have from the other atoms would be on top of the molecule. leaving the other hydrogen atoms in a trigonal pyramidal shape.

    shows what it would look like, w/ the yellow atom being the lone pair.
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