Atomic weights — the revision record of standard atomic weights
How the standard atomic weights recommended by CIAAW since 1902 changed, and whether each new value lay within the previous recommendation's uncertainty range. The question of the physical-constants chapter, asked of a record that defines its uncertainties differently.
Takeaway
- CIAAW has recommended or revised standard atomic weights 701 times since 1902; the 84 elements with a standard atomic weight changed 8.2 times on average.
- CIAAW uncertainties are not standard uncertainties (1σ) but expanded ranges (wide ranges that normal materials fall in with great certitude). So only 12 of 204 revisions (5.9%) put the new value outside the previous range, and 2 had ranges that do not overlap at all (Br 1965, Zn 2007).
- Since 2009, 14 elements have been given as an interval rather than a single value; in all 14 the new interval contains the previous value.
- Physical constants (standard uncertainties) and atomic weights (expanded ranges) define uncertainty differently, so their miss rates are not read on one scale; that difference is why the two chapters sit side by side.
12/ 204revisions with the new value outside the previous range (criterion 1)
2revisions whose ranges do not overlap (criterion 2): Br 1965, Zn 2007
8.2mean revisions per element (the 84 with a standard atomic weight)
14elements switched to an interval
Standard atomic weight — the mean mass of an element's atoms in normal terrestrial material, relative to 1/12 of the mass of a ¹²C atom. Because it averages over isotopic mixtures, unlike a physical constant it genuinely varies a little from sample to sample.
CIAAW uncertainty = expanded range — the uncertainty CIAAW attaches to an atomic weight (the 0.02 in 65.38(2)) is not a standard uncertainty (1σ) but a wide range within which the atomic weight of normal materials is expected to lie "with great certitude" (nor is it an expanded uncertainty with a stated coverage factor or confidence level). It is not read on the same scale as the ±1σ of the physical-constants chapter.
Interval — since 2009, elements whose isotopic composition varies widely in nature are given as [lower, upper] rather than a single value (hydrogen: [1.007 84, 1.008 11]). An interval is not an uncertainty but the range of values actually found.
Metrics
A1 · Number and spacing of revisionsHow many times, and how far apart, each element's standard atomic weight changedA2 · Revisions outside the previous rangeShare of revisions in which the new value left the previous recommendation's uncertainty rangeA3 · Revision size relative to the previous uncertaintyDistribution of revision size divided by the previous recommendation's uncertaintyA4 · Elements left unchanged longestDistribution of the year of the latest revisionA5 · Switch to interval notationElements whose single value was replaced by an interval
Elements
H · hydrogenZ=1 · 10× · [1.007 84, 1.008 11]He · heliumZ=2 · 8× · 4.002 602(2)Li · lithiumZ=3 · 8× · [6.938, 6.997]Be · berylliumZ=4 · 7× · 9.012 1831(5)B · boronZ=5 · 8× · [10.806, 10.821]C · carbonZ=6 · 11× · [12.0096, 12.0116]N · nitrogenZ=7 · 8× · [14.006 43, 14.007 28]O · oxygenZ=8 · 4× · [15.999 03, 15.999 77]F · fluorineZ=9 · 10× · 18.998 403 162(5)Ne · neonZ=10 · 7× · 20.1797(6)Na · sodiumZ=11 · 10× · 22.989 769 28(2)Mg · magnesiumZ=12 · 7× · [24.304, 24.307]Al · aluminiumZ=13 · 12× · 26.981 5384(3)Si · siliconZ=14 · 9× · [28.084, 28.086]P · phosphorusZ=15 · 13× · 30.973 761 998(5)S · sulfurZ=16 · 12× · [32.059, 32.076]Cl · chlorineZ=17 · 7× · [35.446, 35.457]Ar · argonZ=18 · 9× · [39.792, 39.963]K · potassiumZ=19 · 11× · 39.0983(1)Ca · calciumZ=20 · 6× · 40.078(4)Sc · scandiumZ=21 · 12× · 44.955 907(4)Ti · titaniumZ=22 · 5× · 47.867(1)V · vanadiumZ=23 · 8× · 50.9415(1)Cr · chromiumZ=24 · 7× · 51.9961(6)Mn · manganeseZ=25 · 10× · 54.938 043(2)Fe · ironZ=26 · 6× · 55.845(2)Co · cobaltZ=27 · 10× · 58.933 194(3)Ni · nickelZ=28 · 9× · 58.6934(4)Cu · copperZ=29 · 5× · 63.546(3)Zn · zincZ=30 · 9× · 65.38(2)Ga · galliumZ=31 · 7× · 69.723(1)Ge · germaniumZ=32 · 9× · 72.630(8)As · arsenicZ=33 · 9× · 74.921 595(6)Se · seleniumZ=34 · 5× · 78.971(8)Br · bromineZ=35 · 6× · [79.901, 79.907]Kr · kryptonZ=36 · 8× · 83.798(2)Rb · rubidiumZ=37 · 7× · 85.4678(3)Sr · strontiumZ=38 · 5× · 87.62(1)Y · yttriumZ=39 · 11× · 88.905 838(2)Zr · zirconiumZ=40 · 7× · 91.222(3)Nb · niobiumZ=41 · 10× · 92.906 37(1)Mo · molybdenumZ=42 · 8× · 95.95(1)Tc · technetiumZ=43 · 1× · 98.9062(1)Ru · rutheniumZ=44 · 5× · 101.07(2)Rh · rhodiumZ=45 · 8× · 102.905 49(2)Pd · palladiumZ=46 · 6× · 106.42(1)Ag · silverZ=47 · 7× · 107.8682(2)Cd · cadmiumZ=48 · 8× · 112.414(4)In · indiumZ=49 · 8× · 114.818(1)Sn · tinZ=50 · 7× · 118.710(7)Sb · antimonyZ=51 · 8× · 121.760(1)Te · telluriumZ=52 · 5× · 127.60(3)I · iodineZ=53 · 9× · 126.904 47(3)Xe · xenonZ=54 · 10× · 131.293(6)Cs · caesiumZ=55 · 11× · 132.905 451 96(6)Ba · bariumZ=56 · 7× · 137.327(7)La · lanthanumZ=57 · 9× · 138.905 47(7)Ce · ceriumZ=58 · 7× · 140.116(1)Pr · praseodymiumZ=59 · 9× · 140.907 66(1)Nd · neodymiumZ=60 · 6× · 144.242(3)Sm · samariumZ=62 · 8× · 150.36(2)Eu · europiumZ=63 · 6× · 151.964(1)Gd · gadoliniumZ=64 · 9× · 157.249(2)Tb · terbiumZ=65 · 10× · 158.925 354(7)Dy · dysprosiumZ=66 · 7× · 162.500(1)Ho · holmiumZ=67 · 12× · 164.930 329(5)Er · erbiumZ=68 · 9× · 167.259(3)Tm · thuliumZ=69 · 12× · 168.934 219(5)Yb · ytterbiumZ=70 · 9× · 173.045(10)Lu · lutetiumZ=71 · 9× · 174.966 69(5)Hf · hafniumZ=72 · 6× · 178.486(6)Ta · tantalumZ=73 · 11× · 180.947 88(2)W · tungstenZ=74 · 6× · 183.84(1)Re · rheniumZ=75 · 5× · 186.207(1)Os · osmiumZ=76 · 7× · 190.23(3)Ir · iridiumZ=77 · 6× · 192.217(2)Pt · platinumZ=78 · 9× · 195.084(9)Au · goldZ=79 · 9× · 196.966 570(4)Hg · mercuryZ=80 · 8× · 200.592(3)Tl · thalliumZ=81 · 8× · [204.382, 204.385]Pb · leadZ=82 · 8× · [206.14, 207.94]Bi · bismuthZ=83 · 10× · 208.980 40(1)Rn · radonZ=86 · 2× · 222Ra · radiumZ=88 · 7× · withdrawnTh · thoriumZ=90 · 10× · 232.0377(4)Pa · protactiniumZ=91 · 6× · 231.035 88(1)U · uraniumZ=92 · 10× · 238.028 91(3)Np · neptuniumZ=93 · 1× · 237.0482(1)
Source: Standard atomic weights © IUPAC/CIAAW 2016–2024. Commission on Isotopic Abundances and Atomic Weights, Historical changes of standard atomic weights (compiled by J. Meija), https://www.ciaaw.org/historical-atomic-weights.htm