The atomic number is defined as?

• A. The total number of nucleons
• B. The total number of protons and neutrons
• C. The number of electrons in a neutral atom
• D. The number of protons

Answer: (D)

Atomic number is the number of protons in the nucleus, and that count uniquely identifies the element. That’s why it’s the defining property Z of an atom. The mass number, by contrast, is the total number of nucleons (protons plus neutrons), so it describes the weight of the nucleus, not the element’s identity. Saying the atomic number is the total protons and neutrons would mix up identity with mass. The number of electrons in a neutral atom matches the number of protons, but the atomic number itself is defined by protons, not by electron count, and it remains fixed even if the atom is ionized (electrons can vary). For example, carbon always has 6 protons, so Z = 6, regardless of how many neutrons it has or whether it has gained or lost electrons. This is why the correct definition is the number of protons.