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Atomic Weights

The standard atomic weights of the chemical elements.

Recent changes (official status: 2017):

- 29.10.2018 for 14 chemical elements [cf. 9].
- 24.02.2016 for 19 elements (see CIAAW [8, open access]).
- 24.08.2015 for Ytterbium from 173,054 (5) to 173,045 (10) (see [7]).
- 29.04.2013 for bromine, magnesium, germanium, indium and mercury (see IUPAC report [6]).

Table with the current relative atomic weights (atomic weights) Ar (E), based on the carbon isotope 12C with Ar (12C) = 12 (exact).

In its technical report 'Atomic Weights of the Elements 2009' [3], the IUPAC has in particular the interval representation for the 10 chemical elements hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium and 2011 for bromine and Magnesium introduced: background is the variation in the isotopic composition of these elements (see isotope abundances). Simultaneously, rounded values ​​are indicated - indicated by '~' - which can be used for calculations that do not require high accuracy [cf. also 'application notes'].


Table of Atomic Weights

Abbreviations and symbols: see application notes below.


OZSymbolElementAr(E)I 1I 2Notes ABCDEFG 1HHydrogen1.0081.007841.00811c 2HeHelium4.002602(2)a. b 3LiLithium6.946.9386.997c 4BeBeryllium9.0121831(5) 5BBoron10.8110.80610.821c 6CCarbon12.01112.009612.0116 7NNitrogen14.00714.0064314.00728 8OOxygen15.99915.9990315.99977 9FFluorine18.998403163(6) 10NeNeon20.1797(6)a. c 11NaSodium22.98976928(2) 12MgMagnesium24.30524.30424.307g 13AlAluminium26.9815384(3)-- 14SiSilicon28.08528.08428.086 15PPhosphorus30.973761998(5) 16SSulfur32.0632.05932.076 17ClChlorine35.4535.44635.457c 18ArArgon39.8839.79239.963a. b 19KPotassium39.0983(1) 20CaCalcium40.078(4)a 21ScScandium44.955908(5) 22TiTitanium47.867(1) 23VVanadium50.9415(1) 24CrChromium51.9961(6) 25MnManganese54.938043(2) 26FeIron55.845(2) 27CoCobalt58.933194(3) 28NiNickel58.6934(4)b 29CuCopper63.546(3)b 30ZnZinc65.38(2)b 31GaGallium69.723(1) 32GeGermanium72.630(8)g 33AsArsenic74.921595(6) 34SeSelenium78.971(8)b 35BrBromine79.90479.90179.907g 36KrKrypton83.798(2)a. c 37RbRubidium85.4678(3)a 38SrStrontium87.62(1)a. b 39YYttrium88.90584(1) 40ZrZirconium91.224(2)a 41NbNiobium92.90637(1) 42MoMolybdenum95.95(1)a 43TcTechnetium[98]d 44RuRuthenium101.07(2)a 45RhRhodium102.90549(2) 46PdPalladium106.42(1)a 47AgSilver107.8682(2)a 48CdCadmium112.414(4)a 49InIndium114.818(1)g 50SnTin118.710(7)a 51SbAntimony121.760(1)a 52TeTellurium127.60(3)a 53IIodine126.90447(3) 54XeXenon131.293(6)a. c 55CsCaesium132.90545196(6) 56BaBarium137.327(7) 57LaLanthanum138.90547(7)a 58CeCerium140.116(1)a 59PrPraseodymium140.90766(1) 60NdNeodymium144.242(3)a 61PmPromethium[145]d 62SmSamarium150.36(2)a 63EuEuropium151.964(1)a 64GdGadolinium157.25(3)a 65TbTerbium158.925354(8) 66DyDysprosium162.500(1)a 67HoHolmium164.930328(7) 68ErErbium167.259(3)a 69TmThulium168.934218(6) 70YbYtterbium173.045(10)a 71LuLutetium174.9668(1)a 72HfHafnium178.49(2) 73TaTantalum180.94788(2) 74WTungsten183.84(1) 75ReRhenium186.207(1) 76OsOsmium190.23(3)a 77IrIridium192.217(2) 78PtPlatinum195.084(9) 79AuGold196.966570(4) 80PaMercury200.592(3)g 81TlThallium204.38204.382204.385 82PbLead207.2(1) 83BiBismut208.98040(1) 84PoPolonium[209]d 85AtAstatine[210]d 86RnRadon[222]d 87FrFrancium[223]d 88RaRadium[226]d 89AcActinium[227]d 90ThThorium232.0377(4)a. d. e 91PaProtactinium231.03588(1)d. e 92UUranium238.02891(3)a. c. d. e 93NpNeptunium[237]d 94PuPlutonium[244]d 95AmAmericium[243]d 96CmCurium[247]d 97BkBerkelium[247]d 98CfCalifornium[251]d 99EsEinsteinium[252]d 100FmFermium[257]d 101MdMendelevium[258]d 102NoNobelium[259]d 103LrLawrencium[266]d 104RfRutherfordium[267]d 105DbDubnium[268]d 106SgSeaborgium[271]d 107BhBohrium[270]d 108HsHassium[269]d 109MtMeitnerium[276]d 110DsDarmstadtium[281]d 111RgRoentgenium[280]d 112CnCopernicium[285]d 113NhNihonium[284]d 114FlFlerovium[289]d 115McMoscovium[288]d 116LvLivermorium[293]d 117TsTennessine[294]d. f 118OgOganesson[294]d


Application Notes

Column A - order number OZ.

Column B - element symbol.

Column C - name of the chemical element.

Column D - atomic weights.
* (x) - Paraphrased digits: Uncertainty representing the spread of the declared value (see there).
* [x] - mass number of the longest-lived isotope. See column 7, note d.

Column E and F - Intervals
* Atomic mass interval from I1 to I2.

Column G- Notes:

* a - For these chemical elements, geological occurrences are known in which the isotopic composition differs from that normally present.

* b - The bandwidth of the isotopic composition on earth varies and prevents a precise indication. The stated value should, however, be applicable to ordinary samples.

* c - divergent isotopic compositions may occur in commercially available preparations due to unknown or accidental isotopic fractionation; the deviation from the value mentioned here can be considerable.

* d - The element has no stable isotopes; the mass number of the longest lived isotope is given.

* e - The element does not have stable isotopes (see d). Thorium, protactinium and uranium, however, occur on Earth in characteristic isotopic compositions, the values ​​of which are listed here.

* f - Element 117 is not part of the IUPAC data [2]; the value widely used in the literature is indicated [occasionally also: 294].

* g - Amended by 2011 list [7].

Column H - isotopes
* References to data available here on the isotopes or isotope masses (nuclide masses) of the respective element.


Absolute atomic mass

The absolute atomic mass is given in kilograms (kg), grams (g) or atomic mass units (u). An atomic mass unit u corresponds to:

1 u = 1 mu = (1/12) m (12C) = 1.660538921 (73) × 10-27 kg (atomic mass constant, 1/12 of the mass of the 12C isotope) or

1 g = 6.022045 × 1023 u.

By numerical value, relative and absolute atomic masses are the same, since NA / 12 (Avogadro constant) = 1/12 mole of C-12 nuclide particles, as defined, weighs 1 gram.


Outdated values:

Depending on the publication date of a publication, the values ​​accepted at the time for the atomic masses were naturally used. IUPAC offers a PDF document in English documenting the history of changes for each element over the period 1882 to 1997 [see below: 6] and from 1993 to today [5].



[1] - The Atomic Mass Evaluation. Atomic Mass Data Center.

[2] - Atomic weights of the elements 2007 (IUPAC Technical Report); Pure Appl. Chem., 2009, Vol. 81, No. 11, pp. 2131-2156; doi:10.1351/PAC-REP-09-08-03.

[3] - Michael E. Wieser, Tyler B. Coplen: Atomic weights of the elements 2009 (IUPAC Technical Report). Online veröffentlicht am 12.12.2010, DOI 10.1351/PAC-REP-10-09-14.

[4] - Access to tables with older values: Atomic Weights of the Elements 2007, 2005, 2001, 1999, 1997, 1995, 1993.  IUPAC Commission on Atomic Weights and Isotopic Abundances.

[5] - T. B. Coplen, H. S. Peiser:
History of IUPAC Atomic Weight Values from 1882 to 1997: A Comparison of Differences from Currant Values of the Estimated Uncertainies of Earlier Values.
Pure and Appl. Chem., Vol. 70, No 1, pp 237 - 257, (1998).

[6] - Michael E. Wieser et al.:
Atomic weights of the elements 2011.
IUPAC Technical Report, DOI 10.1351/PAC-REP-13-03-02.

[7] - IUPAC:
Standard Atomic Weight of Ytterbium Revised.
published August 24th, 2015.

[8] - Juris Meija et al.:
Atomic weights of the elements 2013.
Pure and Applied Chemistry, 2016, DOI 10.1515/pac-2015-0305; Changes (previous values in brackets):
Cadmium (112.411(8)); Molybdenum (95.96(2)); Selenium (78.96(3)); Thorium (232.038 06(2)); Aluminium (26.9815386(8)); Arsenic (74.92160(2)); Beryllium (9.012182(3)); Cesium (132.905 4519(2)); Cobalt (58.933195(5)); Fluorine (18.9984032(5)); Gold (196.966569(4)); Holmium (164.93032(2)); Manganese (54.938045(5)); Niobium (92.90638(2)); Phosphorus (30.973762(2)); Praseodymium (140.90765(2)); Scandium (44.955912(6)); Thulium (168.934 21(2)) and Yttrium (88.90585(2)).

[9] - Juris Meija et al.:
Standard Atomic Weights of 14 Chemical Elements Revised.
Chemistry International, 2018, DOI 10.1515/ci-2018-0409; Changes (previous values in brackets):
Aluminium (26.981 5385(7)), Argon (39.948(1)), Cobalt (58.933 194(4)), Gold (196.966 569(5)), Holmium (164.930 33(2)), Iridium (192.217(3)), Manganese (54.938 044(3)), Niobium (92.906 37(2)), Praseodymium (140.907 66(2)), Protactinium (231.035 88(2)), Rhodium (102.905 50(2)), Terbium (158.925 35(2)), Thulium (168.934 22(2)), Yttrium (88.905 84(2)).


Last update: 07.12.2018.

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