Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 2 He 3 3.0160293201(25) 0.00000134(3) 4.002602(2) g,r 4 4.00260325413(6) 0.99999866(3) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page