Chlorine atoms each have 17 protons.

How many neutrons does each Cl-37 atom have?

A. 17

B. 20

C. 37

D. 54

The chlorine-37 has 20 neutrons.

Explanations:

1) When an atom of chlorine-35, then it contains 18 neutrons.

mass = 17 protons + 18 neutrons = 35

2) When an atom of chlorine-37 then it contains 20 neutrons

mass = 17 protons + 20 neutrons = 37

Note: Adding or removing a neutron from an atom's nucleus creates isotopes of a particular element