Ace the ACS Chemistry Challenge 2025 – Ignite Your Inner Scientist!

The correct answer is trigonal bipyramidal because the electron domain geometry depends on the number of electron domains surrounding the central atom in a molecule. When there are five electron domains, regardless of whether they are bonding or nonbonding (lone pairs), the geometry is trigonal bipyramidal.

In a trigonal bipyramidal arrangement, five regions of electron density are organized around the central atom, with three of them forming a plane (equatorial positions) and the other two positioned above and below this plane (axial positions). In the case presented, there are four bonding pairs and one lone pair. The presence of the lone pair will affect the molecular shape, causing the bonding pairs to adjust their positions slightly, but the electron domain geometry remains trigonal bipyramidal.

Understanding the distinctions between electron domain geometry and molecular shape is important. Electron domain geometry considers all electron domains, while molecular shape focuses only on the positions of the atoms. In this scenario, with the lone pair present, the molecular shape would actually be seesaw due to the repulsion between the lone pair and the bonding pairs, but the electron domain geometry still classifies the arrangement as trigonal bipyramidal.

The other choices do not fit the criteria for five

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