Extreme makeover: The Periodic Table edition

While this change is unlikely to shake up the public as much as Pluto’s demotion did, the field of chemistry is quietly making a significant shift in the periodic table of elements.

For a century the periodic table has looked something like this, listing a single atomic weight for each element. Mercury, for example, has an atomic weight of 200.59. This is a representation of the “average mass” of each element.

As it turns out, atomic weights are not absolute constants. Rather, the atomic weight of an element depends on the physical, chemical, and nuclear history of the material.


To express some of the complexity and variability of atomic weights, chemists have decided to change the standard atomic weight of 10 elements — hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium — as they appear in the periodic table.

This pre-print of a journal article provides a very nice overview of the history of the atomic weights and the periodic table, and why the changes are now being made.

The image below shows how the new elements will look in the revised periodic table:


This figure shows four classifications of elements:

A. Those whose standard atomic weights are not constants of nature and are assigned an interval (for chlorine it’s 35.446 to 35.457).

B. Those whose standard atomic weights are not constants of nature and they are not assigned an interval.

C. Those whose atomic weight is a constant of nature because they have a single stable isotope.

D. those who have no standard atomic weight because they have no stable isotopes.

This means a couple of things to me. First of all, it’s yet another example of science as an organic process that changes to reflect the accumulation of evidence and a deeper understanding of the natural world.

Second, a lot of parents are going to be confused when their kids start bringing home their chemistry homework in the next few years.

Eric Berger