Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 20 Ca 40 39.962590863(22) 0.96941(156) 40.078(4) g 42 41.95861783(16) 0.00647(23) 43 42.95876644(24) 0.00135(10) 44 43.95548156(35) 0.02086(110) 46 45.9536890(24) 0.00004(3) 48 47.95252276(13) 0.00187(21) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page