Which statement correctly describes mass number and atomic number?

• A. Mass number equals protons; atomic number equals neutrons.
• B. Mass number equals protons plus neutrons; atomic number equals protons.
• C. Mass number equals electrons; atomic number equals protons and neutrons.
• D. Mass number equals protons plus electrons; atomic number equals neutrons

Answer: (B)

Mass number is the total count of nucleons in the nucleus, which means protons plus neutrons. Atomic number is the number of protons, and that value uniquely identifies an element. Neutrons add to the mass but do not affect the atomic number, so changing neutrons changes the mass number without altering the element’s identity. In a neutral atom, the number of electrons equals the number of protons, but mass number is not electrons. This makes the statement that mass number equals protons plus neutrons and atomic number equals protons the correct description.