tetrahedral
generic formula: AX4
example: methane CH4
If there are no lone pairs then the molecular geometry matches the electronic and is tetrahedral. The base bond angle is 109.5° and there is no reason to tweak the bond to another value.
POLARITY: NON-POLAR - As long as all four positions are the same, then the molecule cannot be polar due to perfect symmetry.
trigonal pyramid
generic formula: AX3E
example: ammonia NH3
If there is one lone pair on this electronic geometry, the molecular geometry is now trigonal pyramid. The base angles are still 109.5° while the tweaked angle will now be slightly less (107°) due to the extra repulsing power of the lone pair.
POLARITY: POLAR - The lone pair electrons throw off the perfectly cancelling symmetry of the four tetrahedral regions thus making the molecule polar.
bent or angular
generic formula: AX2E2
example: water H2O
If there are two lone pairs on this electronic geometry, the molecular geometry is now bent or angular. The base angles are still 109.5° although the tweaked angle will now be even smaller by about 4-5 degrees (104.5° for water) due to the extra repulsing power of the two lone pairs.
POLARITY: POLAR - The two sets of lone pair electrons throw off the perfectly cancelling symmetry of the four tetrahedral regions thus making the molecule polar.