Question: why is the atomic number for chlorine 17?
Answer: The answer to why chlorine has an atomic weight of 35.5 not 35 is because of something called isotopes. Isotopes are atoms with the same number of protons (in the case of chlorine that means 17 protons) but different numbers of neutrons. Chlorine has two common isotopes, Chlorine-35 and Chlorine-37.