The atomic number is defined as the number of what in the nucleus?

• A. Protons
• B. Neutrons
• C. Electrons
• D. Quarks

Answer: (A)

The atomic number is the number of protons in the nucleus. This count uniquely identifies the element, since each element has a specific number of protons. The total mass comes from protons plus neutrons, so changing neutrons alters the isotope but not the element. Electrons orbit outside the nucleus (and in a neutral atom their number matches the protons), but that number doesn’t define the element. Quarks are the fundamental constituents inside protons and neutrons, not a label for the atomic number. For example, carbon has six protons, so its atomic number is six.