An AB4 molecule has one lone pair of electrons on the A atom (in addition to the four B atoms). What is the electron-domain geometry around the A atom?

• A. Trigonal bipyramidal
• B. Square planar
• C. Octahedral
• D. Tetrahedral

Answer: (A)

Verified step by step guidance

1. Identify the number of electron domains around the central atom A. In this case, there are four B atoms bonded to A and one lone pair, making a total of five electron domains.
2. Understand that electron-domain geometry considers both bonding pairs and lone pairs of electrons. The arrangement of these domains determines the geometry.
3. Recall that for five electron domains, the electron-domain geometry is trigonal bipyramidal. This geometry accommodates five regions of electron density around the central atom.
4. Visualize the trigonal bipyramidal geometry: it consists of three equatorial positions forming a triangle and two axial positions perpendicular to the equatorial plane.
5. Recognize that the presence of a lone pair affects the molecular geometry but not the electron-domain geometry. The lone pair will occupy one of the equatorial positions, but the electron-domain geometry remains trigonal bipyramidal.