Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 57 La 138 137.9071149(37) 0.0008881(71) 138.90547(7) g 139 138.9063563(24) 0.9991119(71) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page