Periodic table to change

Textbooks and wallcharts all over the world will need to be altered, follwing a decision to change the atomic weights of some elements listed in the Periodic Table for the first time ever.     

The new table, outlined in a report released this month, will express the atomic weights of 10 elements – hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium – in a new way, to reflect more accurately how they are found in nature.

“For more than a century and a half, many were taught to use standard atomic weights — a single value — found on the inside cover of chemistry textbooks and on the periodic table of the elements. As technology improved, we have discovered that the numbers on our chart are not as static as we have previously believed,” says Dr Michael Wieser, an associate professor at the University of Calgary.

Wieser also serves as secretary of the International Union of Pure and Applied Chemistry’s (IUPAC) Commission on Isotopic Abundances and Atomic Weights, which oversees the evaluation and dissemination of atomic-weight values.

The variable atomic weight of some elements is already widely used to deduce the origins of a particular element in nature. In sports doping investigations, for erxample, performance-enhancing testosterone can be identified in the human body because the atomic weight of carbon in natural human testosterone is higher than that in pharmaceutical testosterone.


The atomic weights of these 10 elements will now be expressed as intervals, having upper and lower bounds, reflected to more accurately convey this variation in atomic weight.

For example, sulfur is commonly known to have a standard atomic weight of 32.065. However, its actual atomic weight can be anywhere between 32.059 and 32.076, depending on where the element is found.

Elements with only one stable isotope do not exhibit variations in their atomic weights. For example, the standard atomic weights for fluorine, aluminum, sodium and gold are constant, and their values are known to better than six decimal places.

“Though this change offers significant benefits in the understanding of chemistry, one can imagine the challenge now to educators and students who will have to select a single value out of an interval when doing chemistry calculations,” says Dr Fabienne Meyers, associate director of IUPAC.

“We hope that chemists and educators will take this challenge as a unique opportunity to encourage the interest of young people in chemistry and generate enthusiasm for the creative future of chemistry.”