Which measurement reflects the number of protons in an atom?

• A. Atomic Number
• B. Mass Number
• C. Isotope
• D. Nucleus

Answer: (A)

The number of protons in an atom is given by its atomic number. This measurement directly counts protons and uniquely identifies the element on the periodic table, and in a neutral atom it corresponds to the number of electrons, which shapes how the atom bonds and interacts. The mass number is the total of protons and neutrons, so it varies among isotopes of the same element. An isotope refers to atoms with the same number of protons but different numbers of neutrons. The nucleus is the central region that contains protons and neutrons, not a count. So the measurement that reflects the number of protons is the atomic number.