The atomic number equals the number of protons and determines what property?

• A. The element's identity
• B. The element's mass
• C. The number of neutrons
• D. The number of electrons

Answer: (A)

The main idea is that the atomic number is the count of protons in an atom’s nucleus, and that exact count uniquely identifies which element the atom is. In other words, the number of protons tells you the element’s identity. If you change the number of protons, you don’t just get a heavier version of the same element—you get a different element entirely (its identity changes).

Isotopes all share the same number of protons, so they have the same identity, but they have different numbers of neutrons, which changes their mass. The atomic number doesn’t set the mass or the number of neutrons. The number of electrons can vary in ions, but that affects charge, not which element it is. So the property determined by the atomic number is the element’s identity. For example, 1 proton is hydrogen, 2 protons is helium, 6 protons is carbon—each number locks in a different element.