Keep in mind that when they ask for shape it's usually molecular geometry. Molecular and electron geometry are two different things!! This is copied and pasted from an answer I gave to someone a while back. Hope it helps!
Ok so I actually just reviewed everything on VSEPR so I'm gonna try to explain it better now:
1. Count number of groups surrounding central atom (or atom of interest) in a molecule. A group=lone pair or another atom bonded to the atom of interest (Keep in mind that it doesn't matter if it's single or triple bonded it still only counts as ONE group)
2. If you have 4 groups surrounding, sp3 hybrid orbitals and tetrahedral electron geometry. If you have 3 groups surrounding, sp2 hybrid orbitals and trigonal planar electron geometry. Finally, if you have 2 groups surrounding, sp hybrid orbitals and linear electron geometry. Can you have more? Of course but I doubt DAT is gonna ask us about sp3d and sp3d2 hybrid orbitals so I'll skip that.
3. Electron geometry does NOT determine molecular geometry. After determining the electron geometry now you must take into account the IDENTITY of those groups surrounding your central atom (whether they are bonding electrons or lone pairs). If you have sp hybrid orbitals and linear electron geometry, molecular geometry also is linear whether you have two atoms bonded to your central atom (CO2) or you have an atom and a lone pair (N2, which has N's triple bonded and each N also has a lone pair 180 degrees from the triple bond). If you have sp2 hybrid orbitals and trigonal planar electron geometry, you can have trigonal planar molecular geometry (if all groups are bonded atoms, ie: BF3) or bent molecular geometry (ie: SnCl2, where there is a lone pair on Sn and thus Sn has 3 groups: 2 bonding groups and one lone pair). Finally, if you have sp3 hybrid orbitals and tetrahedral electron geometry you can have tetrahedral molecular geometry (CH4 with all 4 groups as bonded atoms), trigonal pyramidal molecular geometry (NH3 with 3 groups as bonded atoms and one group as a lone pair), or bent molecular geometry (H20 with 2 groups as bonded atoms and two lone pairs). Phew!
Hope that clears everything up.
