Chlorine 35 and chlorine 37 are two different types of chlorine atoms. They're called isotopes. The reason why one is 35 and 37 has to do with the subatomic particle called the neutron. Chlorine is atomic number 17 so both chlorine 35 and chlorine 37 has 17 protons and 17 electrons. But the chlorine 35 has 18 neutrons and the chlorine 37 has 20 neutrons, so the chlorine 37 is slightly heavier than the chlorine 35.