Mass Number is defined as what?

• A. The total number of protons and neutrons
• B. The number of protons
• C. The number of neutrons
• D. The total number of electrons

Answer: (A)

Mass number is the total number of protons and neutrons in the nucleus. This reflects the nucleus’s mass because protons and neutrons make up nearly all of it, while electrons contribute very little mass. The atomic number is the number of protons, and the mass number satisfies A = Z + N, where N is the number of neutrons. Isotopes differ in neutron count, so they have different mass numbers. Therefore, the mass number is the total number of protons and neutrons.