Column Descriptions Relative Isotopic Standard Isotope Atomic Mass Composition Atomic Weight Notes _________________________________________________________________________ 6 C 12 12.0000000(00) 0.9893(8) [12.0096,12.0116] 13 13.00335483507(23) 0.0107(8) 14 14.0032419884(40) _________________________________________________________________________NIST | Physical Measurement Laboratory | Physical Reference Data | Atomic Weights and Isotopic Compositions Main Page