The number of electrons around chlorine may vary, making it an ion. Chlorine would be negatively charged if it had additional electrons. Different isotopes of chlorine have different mass numbers (varying neutron counts). So the atomic mass is not necessarily 35. But one thing that can never be different is the proton count, also known as the atomic number. For chlorine this is always 17, and any atom with 17 protons in its nucleus is chlorine. (4) is the answer.