MCAT General Chemistry Question 44: Answer and Explanation

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Question: 44

14. Both BF3 and NH3 have three atoms bonded to the central atom. Which of the following is the best explanation for why the geometry of these two molecules is different?

  • A. BF3 has three bonded atoms and no lone pairs, which makes its geometry trigonal pyramidal.
  • B. NH3 is nonpolar, while BF3 is polar.
  • C. NH3 has three bonded atoms and one lone pair, which makes its geometry trigonal pyramidal.
  • D. BF3 is nonpolar, while NH3 is polar.

Correct Answer: C

Explanation:

NH3 has three hydrogen atoms bonded to the central nitrogen, which also has a lone pair. These four groups—three atoms, one lone pair—lead NH3 to assume tetrahedral electronic geometry yet trigonal pyramidal molecular geometry. The nitrogen in ammonia is sp3-hybridized. By hybridizing all three p-orbitals and the one s-orbital, four groups are arranged about the central atom, maximizing the distances between the groups to minimize the energy of the configuration with a tetrahedral configuration. In contrast, BF3 has three atoms and no lone pairs, resulting in sp2-hybridization. Its shape is called trigonal planar.

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