I get the basics; about how a molecule will try and minimise repulsion. And I get why the repulsion between two lone pairs is greater than two bond pairs.
I don't see how a lone pair of electrons can contribute to the shape of a molecule, the bonds pairs yes, but not the lone pairs. In other words, I'd expect something like NH3 to be planar (like boron trichloride is). In NH3, the lone pair is associated purely with the N atom, so shouldn't the extra net force be in all directions, giving the same shape as boron trichloride?
Or is it that the lone pair electrons move slower when at a certain side of the N atom or what? And if this is the case, I don't see why they would?