Other pages in this series (5)
A1
Number and spacing of revisions
How many times, and how far apart, each element's standard atomic weight changed
Takeaway
- The 84 elements with a standard atomic weight were revised 8.2 times on average between 1902 and 2024.
- Phosphorus was revised most often, 13 times.
- Definition
- The number of times CIAAW recommended a new standard atomic weight (the mean mass of the element's atoms in normal terrestrial material, relative to 1/12 of the mass of ¹²C) for each element, and the years between revisions; also revisions per year by era (1902–1960, 1961–2008, 2009–2024).
- Formula
- Revisions = rows per element. Spacing = difference between consecutive revision years.
- Denominator
- 701 revisions over 88 elements (84 with a standard atomic weight today); 74 report years.
- How to read it
- Elements with a standard atomic weight were revised 8.2 times on average (median 8); phosphorus most often, 13 times. Revisions per year: 5.71 in 1902–1960, 6.31 in 1961–2008, 3.81 in 2009–2024.
- Caveats
- The table lists only years with a revision (report years without one are marked separately). Values before 1961 are on the chemical scale (oxygen = 16), later ones on ¹²C = 12, so many 1961 revisions reflect the change of scale rather than new measurements.
| Era | Revisions | Report years | Per year |
|---|---|---|---|
| 1902–1960 | 337 | 41 | 5.71 |
| 1961–2008 | 303 | 24 | 6.31 |
| 2009–2024 | 61 | 9 | 3.81 |
| Symbol | Name | Revisions | Latest revision | Mean gap (years) |
|---|---|---|---|---|
| P | phosphorus | 13 | 2013 | 9.2 |
| Al | aluminium | 12 | 2017 | 10.5 |
| Ho | holmium | 12 | 2021 | 9.8 |
| S | sulfur | 12 | 2009 | 9.7 |
| Sc | scandium | 12 | 2021 | 10.8 |
| Tm | thulium | 12 | 2021 | 10.8 |
| C | carbon | 11 | 2009 | 10.7 |
| Cs | caesium | 11 | 2013 | 11.1 |
| K | potassium | 11 | 1979 | 7.7 |
| Ta | tantalum | 11 | 2005 | 10.3 |
| Y | yttrium | 11 | 2021 | 11.9 |
| Bi | bismuth | 10 | 2005 | 11.4 |
| Co | cobalt | 10 | 2017 | 12.8 |
| F | fluorine | 10 | 2021 | 13.2 |
| H | hydrogen | 10 | 2009 | 11.9 |
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