In a neutral chlorine atom, the number of protons equals the number of electrons.

• A. Protons equal neutrons
• B. Protons equal electrons
• C. Neutrons equal electrons
• D. Protons equal mass number

Answer: (B)

When an atom is neutral, its positive charges from protons balance exactly with the negative charges from electrons, so the numbers of these two particles are equal. For chlorine, the atomic number is 17, meaning there are 17 protons. In a neutral chlorine atom, there are also 17 electrons, giving no overall charge. Neutrons contribute to the mass but not the charge, and chlorine has neutrons in the range that gives mass numbers of about 35 or 37 (A = Z + N).

So the statement reflects why a neutral atom has equal numbers of protons and electrons, which keeps the atom electrically neutral. The other pairings aren’t generally true: protons don’t have to equal neutrons, neutrons don’t have to equal electrons, and the mass number isn’t equal to the number of protons (it’s the sum of protons and neutrons).