The atomic number is determined by the number of which subatomic particle in the nucleus?

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

Answer: (C)

The number of protons in the nucleus determines the atomic number. Protons are the positively charged particles that define what element an atom is. Each element has a unique number of protons, and that count is what appears as the atomic number on the periodic table, identifying the element itself. Neutrons also live in the nucleus, but they don’t change which element it is; adding or removing neutrons changes the mass and creates isotopes of the same element, which share the same atomic number. Electrons orbit the nucleus and balance the atom’s charge, but they don’t determine the atomic number. So, the atomic number is specifically the count of protons in the nucleus.