What does the atomic number equal?

• A. Number of Protons
• B. Number of Neutrons
• C. Atomic Mass
• D. Number of Electrons

Answer: (A)

The atomic number is the count of protons in the nucleus. This number uniquely identifies each element and stays fixed for a given element, which is why it’s used to place the element in the periodic table. The number of neutrons can vary between isotopes, so it does not define the element. Atomic mass is protons plus neutrons, so it changes with different isotopes as well. The number of electrons can vary in ions, so it isn’t what the atomic number measures. In a neutral atom, electrons equal protons, but the defining quantity of the atomic number is still the number of protons.