Mass Number is defined as what?

• A. Number of electrons in the outer shell
• B. Number of protons in the nucleus
• C. Sum of protons and neutrons in the nucleus
• D. Total number of electrons in the atom

Answer: (C)

Mass number is the total count of nucleons in the nucleus, which means the sum of protons and neutrons. This number identifies the isotope of an element, since protons determine the element’s identity (atomic number) while neutrons can vary, changing the mass number. The correct description is the one that states protons plus neutrons in the nucleus. For example, carbon-12 has 6 protons and 6 neutrons (mass number 12), whereas carbon-14 has 6 protons and 8 neutrons (mass number 14). The other statements describe electrons or just protons, which do not account for the mass contributed by neutrons in the nucleus.