If an atom has 15 protons and 16 neutrons, what is its mass number?

• A. 31
• B. 15
• C. 16
• D. 46

Answer: (A)

Mass number is the total number of nucleons in the nucleus, which means protons plus neutrons. Here the atom has 15 protons and 16 neutrons, so you add them: 15 + 16 = 31. Therefore, the mass number is 31. This differs from the atomic number, which is just the number of protons (15), and from the neutron count (16). The mass number is not the atomic mass, but for this purpose, it’s the integer you get by adding protons and neutrons.