You may have seen the news about some changes in atomic weights in the periodic table lately. Watts has one of the clearest summaries about this in The periodic table becomes a weight watcher.
What it boils down to is that the isotope ratio of elements that have more than one stable isotope can vary depending upon where it is found. Since each isotope has a different weight, the atomic weight will vary between samples from different places with different histories.
This shouldn’t bother basic Chemistry instruction. The differences are out in the four and five digits of precision range and need modern analytic equipment and methods to measure accurately. A high school chem lab is probably doing OK with 3 digits unless it has pretensions.
It is rather sad that some, like Dr. Michael Wieser, an associate professor at the University of Calgary think that this means “we have discovered that the numbers on our chart are not as static as we have previously believed.” The whole thing with isotopes goes way back when folks started noting weight differences in elements. Its use in centrifuges to separate uranium isotopes for nuclear weapons and fuel has a long history.
The fact that this is such an unusual change in a basic table of properties indicates just how good the properties measurements has been going back to even before the periodic table was invented. The hype about it being unusual misses the fact that elements have been added and other changes made over time.
Another insight in these modifications is that it is now easier to see which elements are commonly available with multiple isotopes. These are targets for diagnostic forensics. Carbon dating is perhaps the most well known technique. That is because its isotope ratio varies over time and the decay rate from one to another isotope is convenient for many geological time frames and it is a critical component of the biosphere. The other 9 elements have differing isotope decay rates and environmental involvements that may make them useful for other determinations.
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