NH3 Molecular Geometry: Understanding Electron Pair Geometry and Lone Pairs

[chops369]

Homework Statement

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.

Homework Equations

N/A just nh3 molecular geometry

The Attempt at a Solution

Should the e- pair geometry be a tetrahedron? I'm a little confused.

 

[chops369]

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.

 

[Bohrok]

The shape of NH₃ is not trigonal planar but trigonal pyramidal. Take a look at the images at wikipedia
http://en.wikipedia.org/wiki/Nh3

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