In VSEPR theory, what is the geometry for AB2E2 and AB3?

• A. AB2E2 is linear; AB3 is tetrahedral.
• B. AB2E2 is trigonal planar; AB3 is bent.
• C. AB2E2 is bent; AB3 is trigonal planar.
• D. AB2E2 is tetrahedral; AB3 is linear.

Answer: (C)

In VSEPR, the shape around a central atom is determined by the repulsion between all electron pairs (bonding and lone pairs). The number of electron groups around the central atom sets the electron-pair geometry, and the presence of lone pairs can distort the final molecular shape.

For AB2E2, there are two bonding pairs and two lone pairs, making four electron groups total. That arrangement gives a tetrahedral electron-pair geometry, but the two lone pairs occupy more space and push the two bonds closer together, resulting in a bent molecular geometry with bond angles reduced from the ideal tetrahedral angle (as seen in water).

For AB3, there are three bonding pairs and no lone pairs, so there are three electron groups. The geometry is trigonal planar, with bonds arranged in a plane at about 120° to each other, since there’s no lone-pair repulsion to distort the arrangement.

Thus AB2E2 is bent, and AB3 is trigonal planar.