MCAT General Chemistry Question 8: Answer and Explanation

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Question: 8

8. Which of the following isotopes of carbon is LEAST likely to be found in nature?

A. ⁶C
B. ¹²C
C. ¹³C
D. ¹⁴C

Correct Answer: A

Explanation:

Recall that the superscript refers to the mass number of an atom, which is equal to the number of protons plus the number of neutrons present in an element. Sometimes a text will list the atomic number, Z, as a subscript under the mass number, A. According to the Periodic Table, carbon contains six protons; therefore, its atomic number is 6. Isotopes all have the same number of protons, but differ in number of neutrons. Almost all atoms with Z > 1 have at least one neutron. Carbon is most likely to have a mass number of 12, for six protons and six neutrons, as in choice (B). Choices (C) and (D) are possible isotopes that would have more neutrons than ¹²C. The ⁶C isotope is unlikely. It would mean that there are 6 protons and 0 neutrons. As shown in Figure 1.4, this would be a highly unstable isotope.

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