Chlorine-35 and chlorine-37 are both isotopes of the element chlorine.The number after the name 'chlorine'is called the mass number.The mass number is a tally of the number of protons and the number of neutrons in an atom's nucleus.Since all atoms of chlorine contains 17 protons,chlorine-35 and chlorine-37 differ in the number of neutrons each one has.An atom of chlorine -35 contains 18 neutrons(17 Protons+18 Neutrons=35 particles in the nucleus) while an atom of chlorine -37 contains 20 neutrons (17 Protons+20 Neutrons=37 particles in the nucleus).