ACS Organic Chemistry Practice Test

What does the Atomic Mass Number represent?

Number of protons Number of neutrons Number of protons and neutrons Number of electrons

Correct Answer:

Number of protons and neutrons

Explanation:

The mass number is the total count of nucleons in the nucleus, which means protons plus neutrons. It equals Z + N, where Z is the number of protons (defining the element) and N is the number of neutrons. Electrons don’t contribute to this count because their mass is negligible and they sit outside the nucleus. Different isotopes have the same number of protons but different numbers of neutrons, giving different mass numbers. So the mass number represents protons plus neutrons.

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