Chemistry CFE Practice Test

In the isotopes Potassium-39 and Potassium-40, which quantity is different between the two?

Protons Electrons Neutrons Mass number

Correct Answer:

Neutrons

Explanation:

Isotopes differ in the number of neutrons while keeping the same number of protons. Potassium has 19 protons, so a neutral potassium atom also has 19 electrons. The two isotopes shown have mass numbers 39 and 40, which means their neutron counts are 39 − 19 = 20 and 40 − 19 = 21, respectively. Therefore, the quantity that differs is the number of neutrons. Since neutrons change, the mass number also changes, but the defining difference between isotopes is the neutron count.

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