Atomic number is?

• A. The number of protons.
• B. The total number of nucleons.
• C. The number of neutrons.
• D. The number of electrons.

Answer: (A)

The main idea is that the atomic number is defined by the number of protons in the nucleus. This count uniquely identifies each element and places it on the periodic table, shaping its chemical properties. The total number of nucleons is the mass number (protons plus neutrons), not the atomic number. Neutrons can vary and give different isotopes of the same element, while the number of electrons can differ in ions, but the atomic number remains the proton count. So, the atomic number being the number of protons is the correct definition.