What is the most common isotope of chlorine?
As you may know, chlorine's relative atomic mass is 35.5, but it has two significantly occurring isotopes: chlorine-35 and chlorine-37. From this information we can determine that chlorine-35 must be the most common of the two, and so is chlorine's most common isotope.
Specifically, chlorine-35 comprises 75% of chlorine in the universe whilst chlorine-37 makes up 25%. Mathematically, this can be expressed as: