What is the sum of the protons and neutrons in an atom called?

• A. Atomic number
• B. Mass number
• C. Number of electrons
• D. Isotopes

Answer: (B)

Think of the nucleus as made up of protons and neutrons. The total number of these particles is the mass number, since it adds protons and neutrons together. For example, a carbon atom with 6 protons and 6 neutrons has a mass number of 12 (often written as carbon-12). The atomic number, by contrast, counts only the protons and identifies the element. The number of electrons equals the number of protons in a neutral atom, not the sum of nucleons. Isotopes are versions of the same element with different numbers of neutrons, which is why their mass numbers differ. Therefore, the sum of protons and neutrons is called the mass number.