Atomic Number is defined as which of the following?

• A. The number of neutrons
• B. The sum of protons and neutrons
• C. The number of protons
• D. The number of electrons

Answer: (C)

The atomic number tells you how many protons are in the nucleus. This count uniquely identifies the element (for example, an element with 6 protons is carbon). It does not include neutrons, and it isn’t the total of protons and neutrons—the latter is the mass number. The number of electrons equals the number of protons only in a neutral atom, but that still isn’t how the atomic number is defined. So the defining quantity is the number of protons.