We know that EVERY #Cl# contains #17# protons, massive, positively charged nuclear particles. This is what defines it as a chlorine atom (and in fact, given the atomic symbol #Cl#, the subscript is a bit superfluous). However, the nucleus can also contain NEUTRONS, massive, neutrally charged nuclear particles. Interactions between neutrons and protons help stabilize the nucleus (and I am not a particle physicist). The sum of the massive particles, #17+20#, gives the mass number, #37#, which is conveniently SUPERSCRIPTED.
Other chlorine nuclei may contain different numbers of neutrons; this is what gives rise to the existence of isotopes. The atomic mass mass quoted on the Periodic Table, #35.45# #"amu"#, is the weighted average of the individual isotopes.
