Weighted average from isotope data.
The atomic mass on the periodic table is a weighted average of all naturally occurring isotopes. For chlorine: Cl-35 (34.969 amu, 75.77%) and Cl-37 (36.966 amu, 24.23%) give an average of 35.453 amu. Formula: average mass = Σ(isotope mass × fractional abundance). This is why atomic masses are rarely whole numbers — they reflect the mixture of isotopes found in nature. Carbon-12 is defined as exactly 12 amu, and all other masses are relative to this standard. Mass spectrometry measures exact isotope masses and abundances, which is how these values are determined experimentally.