Chlorine's listed atomic mass is 35.45, a non-whole number. What does this decimal reflect?

1. Atomic mass and mass number (a whole number for any single isotope) are not the same quantity. 2. The text explains: "scientists also determine the atomic mass, which is the calculated mean of the mass number for its naturally occurring isotopes," giving the example that "the atomic mass of chlorine (Cl) is 35.45 because chlorine is composed of several isotopes." 3. So the fractional value arises precisely because it is a weighted average across multiple isotopes, not a value tied to a single isotope or to the atomic number. 4. This is why chlorine's atomic mass (35.45) does not match any single isotope's whole-number mass number. _Source: OpenStax Biology Ch 2 "The Chemical Foundation of Life", p.34 §2.1 | Atoms, Isotopes, Ions, and Molecules: The Building Blocks_