Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 19 K 39 38.9637064864(49) 0.932581(44) 39.0983(1) 40 39.963998166(60) 0.000117(1) 41 40.9618252579(41) 0.067302(44) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page