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16.18: Atomic Weights of the Elements

  • Page ID
    220815
  • The atomic weight of any isotope of an element is referenced to 12C, which is assigned an exact atomic weight of 12. The atomic weight of an element, therefore, is calculated using the atomic weights of its isotopes and the known abundance of those isotopes. For some elements the isotopic abundance varies slightly from material- to-material such that the element’s atomic weight in any specific material falls within a range of possible value; this is the case for carbon, for which the range of atomic masses is reported as [12.0096, 12.0116]. For such elements, a conventional, or representative atomic weight often is reported, chosen such that it falls within the range with an uncertainty of \(\pm 1\) in the last reported digit; in the case of carbon, for example, the representative atomic weight is 12.011. The atomic weights reported here—most to five significant figures, but a few to just three or four significant figures—are taken from the IUPAC technical report (“Atomic Weights of the Elements 2011,” Pure Appl.Chem. 2013, 85, 1047–1078). Values in ( ) are uncertainties in the last significant figure quoted and values in [ ] are the mass number for the longest lived isotope for elements that have no stable isotopes. The atomic weights for the elements B, Br, C, Cl, H, Li, Mg, N, O, Si, S, Tl are representative values.

    At. No. Symbol Name At. Wt. At. No. Symbol Name At. Wt.
    1 H hydrogen 1.008 60 Nd neodymium 144.24
    2 He helium 4.0026 61 Pm promethium [145]
    3 Li lithium 6.94 62 Sm samarium 150.36(2)
    4 Be beryllium 9.0122 63 Eu europium 151.96
    5 B boron 10.81 64 Gd gadolinium 157.25(3)
    6 C carbon 12.011 65 Tb terbium 158.93
    7 N nitrogen 14.007 66 Dy dysprosium 162.50
    8 O oxygen 15.999 67 Ho holmium 164.93
    9 F fluorine 18.998 68 Er erbium 167.26
    10 Ne neon 20.180 69 Tm thulium 168.93
    11 Na sodium 22.990 70 Yb ytterbium 173.05
    12 Mg magnesium 24.305 71 Lu lutetium 174.97
    13 Al aluminum 26.982 72 Hf halfnium 178.49(2)
    14 Si silicon 28.085 73 Ta tantalum 180.95
    15 P phosphorous 30.974 74 W tungsten 183.84
    16 S sulfur 32.06 75 Re rhenium 186.21
    17 Cl chlorine 35.45 76 Os osmium 190.23(3)
    18 Ar argon 39.948 77 Ir iridium 192.22
    19 K potassium 39.098 78 Pt platinum 195.08
    20 Ca calcium 40.078(4) 79 Au gold 196.97
    21 Sc scandium 44.956 80 Hg mercury 200.59
    22 Ti titanium 47.867 81 Tl thallium 204.38
    23 V vanadium 50.942 82 Pb lead 207.2
    24 Cr chromium 51.996 83 Bi bismuth 208.98
    25 Mn manganese 54.938 84 Po polonium [209]
    26 Fe iron 55.845(2) 85 At astatine [210]
    27 Co cobalt 58.933 86 Rn radon [222]
    28 Ni nickel 58.693 87 Fr francium [223]
    29 Cu copper 63.546(3) 88 Ra radium [226]
    30 Zn zinc 65.38(2) 89 Ac actinium [227]
    31 Ga gallium 69.723 90 T thoriium 232.04
    32 Ge germanium 72.630 91 Pa protactinium 231.04
    33 As arsenic 74.922 92 U uranium 238.03
    34 Se selenium 78.96(3) 93 Np neptunium [237]
    35 Br bromine 79.904 94 Pu plutonium [244]
    36 Kr krypton 83.798(2) 95 Am americium [243]
    37 Rb rubidium 85.468 96 Cm curium [247]
    38 Sr strontium 87.62 97 Bk berkelium [247]
    39 Y yttrium 88.906 98 Cf californium [251]
    40 Zr zirconium 91.224(2) 99 Es einsteinium [252]
    41 Nb niobium 92.906(2) 100 Fm fermium [257]
    42 Mo molybdenum 95.96(2) 101 Md mendelevium [258]
    43 Tc technetium [97] 102 No nobelium [259]
    44 Ru ruthenium 101.07(2) 103 Lr lawrencium [262]
    45 Rh rhodium 102.91 104 Rf futherfordium [267]
    46 Pa palladium 106.42 105 Db dubnium [270]
    47 Ag silver 107.87 106 Sg seaborgiuim [271]
    48 Cd cadmium 112.41 107 Bh bohrium [270]
    49 In indium 114.82 108 Hs hassium [277]
    50 Sn tin 118.71 109 Mt meitnerium [276]
    51 Sb antimony 121.76 110 Ds darmstadium [281]
    52 Te tellurium 127.60(3) 111 Rg roentgenium [282]
    53 I iodine 126.90 112 Cn copernicium [285]
    54 Xe xenon 131.29 113 Uut ununtrium [285]
    55 Cs cesium 132.91 114 Fl flerovium [289]
    56 Ba barium 137.33 115 Uup ununpentium [289]
    57 La lanthanum 138.91 116 Lv livermorium [293]
    58 Ce cerium 140.12 117 Uus ununseptium [294]
    59 Pr praseodymium 140.91 118 Uno ununoctium [294]
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