ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

When a molecule has three bonding electron domains and one non-bonding electron domain, it exhibits a bent shape due to the repulsion between the electron domains. In this arrangement, the three bonding domains, which can come from bonds to other atoms, create a geometric arrangement that would ideally be trigonal planar. However, the presence of the non-bonding domain (lone pair) alters this ideal shape, pushing down on the bonding pairs.

The lone pair occupies more space and exerts repulsive forces on the adjacent bonding pairs, which causes the bond angles between the atoms to decrease from the ideal angle of 120 degrees typical of a trigonal planar configuration. This results in a bent shape instead. A common example of a molecule with this geometry is water (H₂O), where the two hydrogen atoms and the lone pair on the oxygen result in a bent molecular shape.

The other options represent different molecular geometries that do not match the condition of having three bonding domains and one lone pair of electrons. Trigonal planar and tetrahedral geometries imply all domains are bonding, while the see-saw shape corresponds to a different arrangement often seen with five electron domains, typically involving a central atom with four bonding domains and one lone pair.