The atomic number of an element is the number of ____ in the nucleus.

• A. Electrons
• B. Protons
• C. Neutrons
• D. Quarks

Answer: (B)

The atomic number is the number of protons in the nucleus. This count uniquely identifies an element because protons carry positive charge and determine the element’s identity on the periodic table. In a neutral atom, the number of electrons matches the number of protons, but electrons are not what the atomic number measures. Neutrons affect the atom’s mass but not its identity or charge, and quarks are even smaller particles that make up protons and neutrons, not a quantity used to define the element. For example, hydrogen has one proton, carbon has six protons, and so on.