Which of the following molecules has a trigonal planar electron geometry?

• A. H_2O
• B. BF_3
• C. CH_4
• D. NH_3

Answer: (B)

Verified step by step guidance

1. Recall that the electron geometry of a molecule depends on the number of electron groups (bonding and lone pairs) around the central atom, according to the VSEPR (Valence Shell Electron Pair Repulsion) theory.
2. Identify the central atom in each molecule and count the total number of electron groups around it. Electron groups include bonded atoms and lone pairs of electrons.
3. For H_2O (water), the central atom is oxygen, which has 2 bonded hydrogen atoms and 2 lone pairs, totaling 4 electron groups. This corresponds to a tetrahedral electron geometry, not trigonal planar.
4. For BF_3 (boron trifluoride), the central atom is boron, which has 3 bonded fluorine atoms and no lone pairs, totaling 3 electron groups. This corresponds to a trigonal planar electron geometry.
5. For CH_4 (methane) and NH_3 (ammonia), the central atoms have 4 and 3 electron groups respectively, but NH_3 has one lone pair, making its electron geometry tetrahedral, and CH_4 has 4 bonded atoms with no lone pairs, also tetrahedral. Therefore, only BF_3 has trigonal planar electron geometry.