Which statement defines the atomic number of an element?

Which statement defines the atomic number of an element?

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

Answer: (B)

The atomic number is defined by the number of protons in the nucleus. This count uniquely identifies the element and is what sits as Z on the periodic table. Mass number equals protons plus neutrons, so it tells you the total nucleons, not the identity. The number of electrons in a neutral atom equals the number of protons, but that only holds when the atom is neutral; the atomic number itself is defined regardless of charge. So the statement that it is the number of protons best defines the atomic number.

The atomic number is defined by the number of protons in the nucleus. This count uniquely identifies the element and is what sits as Z on the periodic table. Mass number equals protons plus neutrons, so it tells you the total nucleons, not the identity. The number of electrons in a neutral atom equals the number of protons, but that only holds when the atom is neutral; the atomic number itself is defined regardless of charge. So the statement that it is the number of protons best defines the atomic number.