Which quantity is defined as the number of protons in the nucleus of an atom?

• A. Mass number
• B. Atomic number
• C. Molar mass
• D. Isotopes

Answer: (B)

The quantity defined as the number of protons in the nucleus is the atomic number. This count uniquely identifies an element and also matches the number of electrons in a neutral atom, balancing the nuclear charge with the surrounding electron cloud. The mass number, by contrast, is the total of protons and neutrons, so it can vary even for atoms of the same element. Isotopes are atoms with the same atomic number but different numbers of neutrons, which changes mass but not the identity. Molar mass is a bulk property representing mass per mole of atoms or molecules, not the nuclear composition. Hence, the defining quantity for the proton count is the atomic number.