It has been known for a long time that the relative atomic mass, A_r, of lead, Pb, changes slightly depending upon the origin of the lead as it is the end product of natural radioactive decay series. If the lead originates from an area rich in 238-U then the end product will be richer in 206-Pb whereas if the area was originally rich in 232-Th then the resulting lead will have a slightly higher relative atomic mass as the end product of the decay of 232-Th is 208-Pb. However, the working assumption has always been that the relative atomic masses of the other elements are fixed and precise values are given in all Periodic Tables.  This has now changed. Now the only fixed values are for those elements with only one stable isotope such as fluorine, aluminum, sodium and gold.

In a recent publication by Michael E. Wieser and Tyler B. Coplen (who work at the University of Calgary,Canada and the U.S. Geological Survey, Reston, USA respectively), the values for the relative atomic masses of eleven common elements have been revised. These are hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, germanium and thallium. For germanium the change is straightforward – from 72.63 to 72.64. The change is anything but straightforward for the other ten elements. Instead of a single value the relative atomic masses for these ten elements are now given as a range depending upon the precise isotopic make up of the particular source of the sample i.e. its physical, chemical and nuclear history. This will not affect IB chemistry calculations to any great degree as the range is quite small and in fact to two decimal places it often has the same value as that given in the IB Periodic Table (Table 5 of the IB Data Booklet). For example, the range for hydrogen is 1.007 84 to 1.008 11. The old value was 1.007 95 which is actually unchanged at 1.01 when quoted to two decimal places. For some of the other elements the change is just significant to register at two decimal places. Thus boron now will be quoted in the range 10.806 to10.821. So the relative atomic mass to two decimal places could be 10.81 or 10.82 (the old value was 10.811 which corresponds to the value of 10.81 given to two decimal places in the current IB Chemistry Data Booklet).

In a news report about the published article the International Union of Pure and Applied Chemistry, IUPAC, explains that advances in modern analytical techniques (Option A) means that  relative atomic masses can now be measured much more precisely. These small changes, due to the relative abundances of particular stable isotopes in the sample, are important in research and industry. For the IB they can also be related to particular options.  For example precise measurements of isotopic carbon abundance and be used to determine purity in food products (Option F) and precise measurements of the nitrogen content can be used for tracing pollutants in streams or underground water systems (Option E). There are uses for Option B: Human biochemistry too. Performance enhancing steroids can be detected in the human body because the relative atomic mass of carbon in natural testosterone is higher than in testosterone from pharmaceutical sources.

This is an interesting TOK aspect to Chemistry in that it shows how scientific ‘knowledge’ is constantly changing and indeed that the Law of Constant Composition is not as immutable as it was once thought to be (see the page on Topic 1 under Including International-mindedness, TOK, and 'Utilization' in Stoichiometric relationships). One other noteworthy aside is that if you look at the articles nowhere is relative atomic mass mentioned. IUAPC still calls it relative atomic weight!