How do you work out the mass number of an atom?

How do you work out the mass number of an atom?

• A. Protons minus Neutrons
• B. Protons plus Neutrons
• C. Neutrons plus Electrons
• D. Protons plus Electrons

Answer: (B)

Mass number tells you how many nucleons are in the nucleus, so you work it out by adding the number of protons and neutrons. Protons define what element it is, and neutrons add to the total count of nucleons. Electrons sit outside the nucleus and have negligible mass in this context, so they don’t affect the mass number. Therefore, mass number equals protons plus neutrons. The other ideas don’t fit because subtracting neutrons would give a smaller total, neutrons plus electrons mixes particles that aren’t counted together for the nucleus, and protons plus electrons would give a count of all charged particles rather than the nucleons in the nucleus.

Mass number tells you how many nucleons are in the nucleus, so you work it out by adding the number of protons and neutrons. Protons define what element it is, and neutrons add to the total count of nucleons. Electrons sit outside the nucleus and have negligible mass in this context, so they don’t affect the mass number. Therefore, mass number equals protons plus neutrons.

The other ideas don’t fit because subtracting neutrons would give a smaller total, neutrons plus electrons mixes particles that aren’t counted together for the nucleus, and protons plus electrons would give a count of all charged particles rather than the nucleons in the nucleus.