MCAT General Chemistry Question 15: Answer and Explanation
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Question: 15
15. Which of the following species is represented by the electron configuration 1s22s22p63s23p64s13d5?
I. Cr
II. Mn+
III. Fe2+
- A. I only
- B. I and II only
- C. II and III only
- D. I, II, and III
Correct Answer: B
Explanation:
When dealing with ions, you cannot directly approach electronic configurations based on the number of electrons they currently hold. First examine the neutral atom's configuration, and then determine which electrons are removed.
| Neutral Atom's Configuration | Ion's Configuration |
| Cr0: [Ar] 4s13d5 | — |
| Mn0: [Ar] 4s23d5 | Mn+: [Ar] 4s13d5 |
| Fe0: [Ar] 4s23d6 | Fe2+: [Ar] 4s03d6 |
Due to the stability of half-filled d-orbitals, neutral chromium assumes the electron configuration of [Ar] 4s13d5. Mn must lose one electron from its initial configuration to become the Mn+ cation. That electron would come from the 4s subshell, according to the rule that the first electron removed comes from the highest-energy shell. Fe must lose two electrons to become Fe2+. They'll both be lost from the same orbital; the only way Fe2+ could hold the configuration in the question stem would be if one d-electron and one s-electron were lost together.