Final answer:
The repulsions between a lone pair and a bond pair at 90° are strong due to the overlap of large regions of space that they share. This strong repulsion affects the stability and geometry of molecules, as seen in cases like SF4. The geometry that minimizes different kinds of repulsions results in a more stable molecule structure.
Step-by-step explanation:
The repulsions between a lone pair and a bond pair at 90° are generally strong. This is because at this angle, the two electron pairs share a relatively large region of space, leading to increased electron-electron repulsion. In molecular geometry, this strong repulsion is taken into account to predict the shape of a molecule. For instance, in the case of SF4, which is designated as AX4E with a total of five electron pairs, if the lone pair is placed in an equatorial position instead of an axial position, it results in fewer 90° lone pair-bond pair (LP-BP) repulsions. As a result, the structure with the lone pair of electrons in the equatorial position is more stable than the one with the lone pair in the axial position.
Considering the repulsion order where a lone pair occupies a larger region of space than bonding pairs, the shape that minimizes repulsions when there are six electron groups around the central atom, with four bonding pairs and two lone pairs, is structured to allow as much spatial separation as possible thus reducing these LP-LP, LP-BP, and BP-BP repulsions.