What is the mass number of Potassium-39?

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Multiple Choice

What is the mass number of Potassium-39?

Explanation:
Mass number is the total number of protons and neutrons in the nucleus (A = Z + N). Potassium has 19 protons (its atomic number). In Potassium-39, the total number of nucleons is 39, so the mass number is 39. The remaining neutrons would be 39 − 19 = 20, but the quantity asked for is the mass number itself, which is 39. The numbers shown in other options would correspond to different isotopes with different numbers of neutrons, not this isotope.

Mass number is the total number of protons and neutrons in the nucleus (A = Z + N). Potassium has 19 protons (its atomic number). In Potassium-39, the total number of nucleons is 39, so the mass number is 39. The remaining neutrons would be 39 − 19 = 20, but the quantity asked for is the mass number itself, which is 39. The numbers shown in other options would correspond to different isotopes with different numbers of neutrons, not this isotope.

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