Mass number is the sum of the number of neutrons and protons in an atomic nucleus.

Mass number is the sum of the number of neutrons and protons in an atomic nucleus.

• A. Sum of neutrons and protons
• B. Sum of protons and electrons
• C. Sum of protons and neutrons minus electrons
• D. Number of electrons

Answer: (A)

Mass number measures the total number of nucleons in the nucleus, that is, protons plus neutrons. The nucleus contains nearly all the atom’s mass, while electrons lie outside the nucleus and have negligible mass by comparison, so they don’t contribute to the mass number. Therefore, the mass number is simply the sum of protons and neutrons. The other options mix in electrons or subtract them, which does not reflect what the mass number represents. In neutral atoms, electrons equal protons, but that doesn’t change the fact that mass number accounts for nucleons only.

Mass number measures the total number of nucleons in the nucleus, that is, protons plus neutrons. The nucleus contains nearly all the atom’s mass, while electrons lie outside the nucleus and have negligible mass by comparison, so they don’t contribute to the mass number. Therefore, the mass number is simply the sum of protons and neutrons. The other options mix in electrons or subtract them, which does not reflect what the mass number represents. In neutral atoms, electrons equal protons, but that doesn’t change the fact that mass number accounts for nucleons only.