trigonal bipyramid
generic formula: AX5
example: phosphorus pentafluoride PF5
If there are no lone pairs then the molecular geometry matches the electronic and is trigonal bipyramid. The base bond angles are 180°, 120°, and 90°. There is no reason to tweak the bonds to other values.
POLARITY: NON-POLAR - As long as all five positions are the same, then the molecule cannot be polar due to perfect symmetry.
see-saw
generic formula: AX4E
example: sulfur tetrafluoride SF4
With one lone pair on this electronic geometry, the lone pair must occupy an equatorial position and the molecular geometry becomes see-saw shape. The base angles are still 180°, 120°, and 90° while the tweaked angle will now be slightly less in each case due to the extra repulsion from the lone pair.
POLARITY: POLAR - The lone pair electrons throw off the perfectly cancelling symmetry of the five trigonal bipyramidal regions thus making the overall molecule polar.
T-shaped
generic formula: AX3E2
example: bromine trichloride BrCl3
The two lone pairs on this electronic geometry have to be in equatorial positions which forces the molecular geometry to be T-shaped. The base angles are 180° and 90°. The standard tweaking of the angles applies and the actual angles will be slightly less than the standard angles.
POLARITY: POLAR - The T-shape and the two lone pairs will always lead to a polar (unsymmetrical) molecule.
linear
generic formula: AX2E3
example: triiodide ion I3–
With three lone pairs in the required (more roomy) equatorial positions electronic geometry, the molecular geometry is now linear. The only angle left is the 180° one with the peripheral atoms in the axial positions.
POLARITY: NON-POLAR - With all three equatorial positions occurpied by lone pairs, the resulting linear molecular species is perfectly symmetric and is therefore non-polar.