Every chlorine atom has (1) 7 electrons (2) 17 neutrons

(3) a mass number of 35 (4) an atomic number of 17

The correct answer is (4). The atomic number of any element never changes. The atomic number is the number of protons an atom has and is the identifying characteristic of an atom. Hence, every chlorine atom has 17 protons, no more, no less, and consequently has an atomic number of 17. Different atoms of chlorine can have different numbers of electrons. This would give rise to ions. Different atoms of chlorine can have different number of neutrons, this results in isotopes. Because different atoms of chlorine can have different numbers of neutrons, they can have different mass numbers. The mass number is the sum of the number of protons (never changes for any given element) plus the number of neutrons (can change depending on the isotope of the element.)