Answer:
See below.
Step-by-step explanation:
The two isotopes are Cl-35 and Cl-37.
The average atomic mass (35.45 u) is closer to 35 u, so most of the atoms are Cl-35.
mass number = number of protons + number of neutrons
A = p + n
35 = 17 + n
n = 35 – 17
n = 18
∴ Most chlorine atoms contain 18 neutrons.