Average atomic mass
Most elements exist as mixtures of isotopes — atoms with the same number of protons but different numbers of neutrons. The average atomic mass is the weighted average based on each isotope's natural abundance.
Average Mass = Σ (Isotope Mass × Fractional Abundance)
Example: Chlorine has two isotopes: ³⁵Cl (75.77%, 34.969 amu) and ³⁷Cl (24.23%, 36.966 amu). Average = 34.969 × 0.7577 + 36.966 × 0.2423 = 35.453 amu.