Final answer:
A molecule with 5 electron domains and two lone pairs will have a trigonal bipyramidal electron-pair geometry with a seesaw molecular structure, where the lone pairs occupy the equatorial positions.
Step-by-step explanation:
When a molecule has 5 electron domains and two of these are lone pairs, the molecular geometry needs to minimize the repulsion between these lone pairs and the bonding pairs. In such a molecule, we expect the electron domains to adopt a trigonal bipyramidal electron-pair geometry. The two lone pairs will occupy the equatorial positions rather than the axial ones to minimize repulsions, leading to a seesaw molecular structure.
For example, in the case of SF4 (designated as AX4E), which has a total of five electron pairs, placing the lone pair in the equatorial position leads to fewer 90° lone pair-bonding pair (LP-BP) repulsions. This configuration is more stable than having the lone pair in the axial position, which would result in more LP-BP repulsions at 90°. Therefore, placing lone pairs in equatorial positions helps achieve a geometry that minimizes repulsions, balancing the distribution of electron density around the central atom.