Periodic Table changed – students everywhere stunned

Atomic weights revised, reality stays the same
[Via Ars Technica]

Yesterday, the University of Calgary announced that there will be a significant revision to the periodic table, a phrasing that implies a radical upheaval of our understanding of basic matter. The reality is quite a bit less dramatic—so tame, in fact, that the publication that announced the revision was released on Sunday without causing any disturbance. But the announcement provides a good opportunity to give everyone a refresher on the whole concept of atomic weight.

A good periodic table (like this one) will typically have two numbers associated with each element. The first is the atomic number, which is the number of protons in the nucleus, and thus the number of electrons present when an atom isn’t ionized. Since there are no fractional electrons, these numbers are integers. These electrons dictate the element’s chemical properties, so they tell us something about its behavior.

The second is the atomic weight. This tells us, in grams, how much a mole (6.023 x 1023) of atoms will weigh, and is thus proportional to the weight of a single atom. These numbers are not integers, and that’s not because atoms come in fractions. Instead, atoms have different isotopes that contain different numbers of neutrons in the nucleus. A pure isotope will have a well-defined weight, but in the natural world, most elements appear as a mixture of isotopes. A mass of hydrogen, for example, will mostly contain atoms with a single proton and no neutrons, but will have a few with one or two neutrons mixed in. As a result, when you have over 1023 atoms of hydrogen around, these heavier isotopes ensure that it weighs a touch more than just a gram.

So, it’s possible for the atomic weight to change simply because we have a better idea of the typical isotope ratios found in the natural world, and that change wouldn’t reflect any new knowledge of the atom’s internal structure.

The changes announced this week, however, are a mix of new knowledge and a bookkeeping decision. The new knowledge is a better understanding of how isotope mixtures can differ based on the history of the material. This is easiest to understand in terms of carbon. Geological processes generally don’t care what isotope of carbon gets incorporated into rocks or dissolved into the oceans. But biological processes are heavily biased towards the lighter 12C isotope. So, if you’re looking at a bacteria-rich sediment,


Actually, the changes reflect a few differences in how the atomic weight is presented, since the relative ratios of isotopes can be different depending on the material. This is particularly true for biological samples because living things prefer to use one isotope of carbon – 12C – than others – such as 13C.

This difference in ratios is one way we can tell whether carbon is derived from living material or from geologic material. Thus the changing carbon ratios seen for carbon dioxide in the atmosphere as we burn fossil fuels that were once living matter. An increase in the relative amount of 12C over time is one of the bits of data telling us just how humans have altered the atmosphere.

Nice to see biology cause changes in such an basic table of science.