Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 51 Sb 121 120.9038120(30) 0.5721(5) 121.760(1) g 123 122.9042132(23) 0.4279(5) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page