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Molecular Geometry: NH3

  1. Feb 12, 2010 #1
    1. The problem statement, all variables and given/known data
    I'm trying to figure out the electron pair geometry of NH3. I know that its molecular geometry is a trigonal plane, and it has 4 electron pairs total, including one lone pair.

    2. Relevant equations

    3. The attempt at a solution
    Should the e- pair geometry be a tetrahedron? I'm a little confused.
  2. jcsd
  3. Feb 12, 2010 #2
    I just came across another situation that confuses me. How do you rationalize the molecular geometries? For e- pair geometries, my understanding is that your goal is to minimize electron repulsion, therefore you use the geometry that will place the pairs as far away as possible from one another. What is the rule of thumb for molecular geometries, if there is one?

    For example, I am stuck on H2O. I completely understand why the e- pair geometry is a tetrahedron, but I have absolutely no idea how the molecular geometry comes out to be angular. The same goes for any of the compounds with pyramidal geometries.
  4. Feb 12, 2010 #3
    The shape of NH3 is not trigonal planar but trigonal pyramidal. Take a look at the images at wikipedia

    You know that with four electron pairs, they move as far away from each other as possible and form the vertices, or corners, of a tetrahedron. With H2O, there are two hydrogens, each at one of the corners of the tetrahedron; they are not in a straight line but rather at an angle
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