Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 34 Se 74 73.922475934(15) 0.0089(4) 78.971(8) r 76 75.919213704(17) 0.0937(29) 77 76.919914154(67) 0.0763(16) 78 77.91730928(20) 0.2377(28) 80 79.9165218(13) 0.4961(41) 82 81.9166995(15) 0.0873(22) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page