Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 81 Tl 203 202.9723446(14) 0.2952(1) [204.382,204.385] 205 204.9744278(14) 0.7048(1) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page