The atomic number is equal to the number of what particles?

• A. Mass Number
• B. Atomic Number
• C. Neutron Count
• D. Electron Count

Answer: (B)

The key idea is that the atomic number tells you how many protons are in the nucleus, and this proton count is what defines an element. By definition, the atomic number equals the number of protons in an atom. In the context of the choices, the best match is the option that names the quantity itself because the atomic number is the label for that proton count. The other options describe different counts: mass number is protons plus neutrons, neutron count is just the neutrons, and electron count varies with charge (though it can equal the proton count in a neutral atom, it’s not the defining measure). So the family of ideas centers on protons determining the element, and the atomic number is the term used for that proton count.