Mass number is defined as which of the following?

Mass number is defined as which of the following?

• A. the total number of electrons
• B. the number of protons
• C. the total number of protons and neutrons in the nucleus of an atom
• D. the average mass of all isotopes

Answer: (C)

Mass number is the total count of nucleons in the nucleus, that is, protons plus neutrons. This number is an integer and stays fixed for a given isotope because it counts the particles that contribute most to the atom’s mass (electrons are far lighter and not included). For isotopes of the same element, the mass number changes with different numbers of neutrons (for example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons). The other statements describe either the number of electrons (which relates to charge, not mass) or the atomic number (protons) or the atomic mass (a weighted average across isotopes), not the mass number.

Mass number is the total count of nucleons in the nucleus, that is, protons plus neutrons. This number is an integer and stays fixed for a given isotope because it counts the particles that contribute most to the atom’s mass (electrons are far lighter and not included). For isotopes of the same element, the mass number changes with different numbers of neutrons (for example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons). The other statements describe either the number of electrons (which relates to charge, not mass) or the atomic number (protons) or the atomic mass (a weighted average across isotopes), not the mass number.