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Atomic Weights and Isotopic Compositions for All Elements*
Isotope Relative Atomic Mass    Isotopic Composition   Standard Atomic Weight 
1
H 1    1.007 825 032 23(9)   0.999 885(70)      1.007 94(7)
D 2    2.014 101 778 12(12)   0.000 115(70)
3 Li 6     6.015 122 8874(16)   0.0759(4)      6.941(2)
7     7.016 003 4366(45)   0.9241(4)
5 B 10   10.012 936 95(41)   0.199(7)    10.811(7)
11   11.009 305 36(45)   0.801(7)
6 C 12   12.000 000 0(0)   0.9893(8)    12.0107(8)
13   13.003 354 835 07(23)   0.0107(8)
7 N 14   14.003 074 004 43(20)   0.996 36(20)    14.0067(2)
15   15.000 108 898 88(64)   0.003 64(20)
8 O 16   15.994 914 619 57(17)   0.997 57(16)    15.9994(3)
17   16.999 131 756 50(69)   0.000 38(1)
18   17.999 159 612 86(76)   0.002 05(14)
11 Na 23   22.989 769 2820(19)   1.0000    22.989 769 28(2)
15 P 31   30.973 761 998 42(70)   1.0000    30.973 762(2)
16 S 32   31.972 071 1744(14)   0.9499(26)    32.065(5)
33   32.971 458 9098(15)   0.0075(2)
34   33.967 867 004(47)   0.0425(24)
36   35.967 080 71(20)   0.0001(1)
19 K 39   38.963 706 4864(49)   0.932 581(44)    39.0983(1)
40   39.963 998 166(60)   0.000 117(1)
41   40.961 825 2579(41)   0.067 302(44)
Atomic and Nuclear Constants* (unified atomic mass unit, u)
p
Proton, 1H+     1.007 276 466 879(91)
e electron     5.485 799 090 70(16)  x 10-4
*NIST Physical Measurement Laboratory (http://www.nist.gov/pml/data/comp.cfm)
NIST Standard Reference Database 144
Standard Atomic Weight from ATOMIC WEIGHTS OF THE ELEMENTS 2001
Go to amino acid molecular masses (http://www.riken.jp/BiomolChar/files/Aminoacidmolecularmasses.htm)