MCAT General Chemistry Question 35: Answer and Explanation

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Question: 35

5. Both CO32– and ClF3 have three atoms bonded to a central atom. What is the best explanation for why CO32– has trigonal planar electronic geometry, while ClF3 has trigonal bipyramidal electronic geometry?

  • A. CO32– has multiple resonance structures, while ClF3 does not.
  • B. CO32– has a charge of -2, while ClF3 has no charge.
  • C. ClF3 has lone pairs on its central atom, while CO32– has none.
  • D. CO32– has lone pairs on its central atom, while ClF3 has none.

Correct Answer: C

Explanation:

The central carbon in carbonate has no lone pairs. It has three resonance structures, each of which involves a double bond between carbon and one of the three oxygens. Having made four bonds, carbon has no further orbitals for bonding or to carry lone pairs. This makes carbonate's geometry trigonal planar. Alternatively, ClF3 also has three bonds; however, chloride still maintains two extra lone pairs. These lone pairs each inhabit one orbital, meaning that the central chloride must organize five items about itself: three bonds to fluorides and two lone pairs. The best configuration for maximizing the distance between all of these groups is trigonal bipyramidal. Choices (A) and (B) are true statements but do not account for the difference in geometry.

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