# An atom of chlorine is represented by ""_17^37 Cl. How many neutrons are in the nucleus of this atom?

""_17^37Cl contains 20 neutrons. Why? Because $17 + 20 = 37$.
We know that EVERY $C l$ contains $17$ protons, massive, positively charged nuclear particles. This is what defines it as a chlorine atom (and in fact, given the atomic symbol $C l$, 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, $35.45$ $\text{amu}$, is the weighted average of the individual isotopes.