MCAT General Chemistry Question 12: Answer and Explanation

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Question: 12

12. How many total electrons are in a ¹³³Cs cation?

A. 54
B. 55
C. 78
D. 133

Correct Answer: A

Explanation:

The quickest way to solve this problem is to use the Periodic Table and find out how many protons are in Cs atoms; there are 55. Neutral Cs atoms would also have 55 electrons. A stable Cs cation will have a single positive charge because it has one unpaired s-electron. This translates to one fewer electron than the number of protons, or 54 electrons.

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