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How electron repulsion determines molecular shape
Water (H₂O) has two hydrogen atoms bonded to oxygen. Why aren't they in a straight line (H-O-H at 180°)?
Oxygen has two lone pairs of electrons that push the hydrogen atoms closer together. Result: 104.5° bent shape.
Rotate the molecules above. Shape determines properties — water's bent shape makes it polar, which is why it's a universal solvent.
Predict shapes from electron arrangements
Ammonia (NH₃) has a pyramidal shape with bond angles ~107°. If you removed the lone pair on nitrogen, the angle would become: