MCAT General Chemistry Question 5: Answer and Explanation
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Question: 5
5. Which of the following atoms only has paired electrons in its ground state?
- A. Sodium
- B. Iron
- C. Cobalt
- D. Helium
Correct Answer: D
Explanation:
The only answer choice without unpaired electrons in its ground state is helium. Recall from the chapter that a diamagnetic substance is identified by the lack of unpaired electrons in its shell. A substance without unpaired electrons, like helium, cannot be magnetized by an external magnetic field and is actually slightly repelled. Elements that come at the end of a block (Group IIA, the group containing Zn, and the noble gases, most notably) have only paired electrons.