The number of neutrons in a nucleus can be determined by which relation?

• A. Mass number minus atomic number
• B. Mass number plus atomic number
• C. Atomic weight minus mass number
• D. Atomic number minus mass number

Answer: (D)

The number of neutrons is found from the fact that the mass number equals the total count of protons and neutrons in the nucleus. Since mass number A = Z + N, where Z is the number of protons (atomic number) and N is the number of neutrons, solving for neutrons gives N = A − Z. So, subtracting the atomic number from the mass number yields the neutron count.

For example, carbon-12 has A = 12 and Z = 6, so N = 12 − 6 = 6 neutrons.

Other options don’t fit because adding mass number and atomic number would not represent a nucleon count, and using atomic weight (a weighted average) with mass number isn’t the direct, exact relationship for a specific nucleus.