Chlorine has two naturally occurring isotopes, Cl-35 and Cl-37. The atomic mass of chlorine is 35.45. Which of these two isotopes of chlorine is more abundant?

Cl-35, as the atomic mass of Chlorine (35.45) is closer to the number 35 than to the number 37. A higher abundance of CL-35 isotope would have caused the atomic number (which is an average of the values of all isotopes of a substances taking relative abundance into consideration) to decrease from 36, which would appear to be the average.