Atomic weights of 10 elements on periodic table about to make an historic change

Dec 15, 2010
Michael Wieser is a scientist from the University of Calgary who is helping to update periodic table. Credit: Riley Brandt/University of Calgary

For the first time in history, a change will be made to the atomic weights of some elements listed on the Periodic table of the chemical elements posted on walls of chemistry classrooms and on the inside covers of chemistry textbooks worldwide.

The new table, outlined in a report released this month, will express atomic weights of 10 elements - , , , carbon, , , , sulfur, and thallium - in a new manner that will reflect more accurately how these elements 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 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, who serves as secretary of the International Union of Pure and Applied Chemistry's (IUPAC) Commission on Isotopic Abundances and Atomic Weights. This organization oversees the evaluation and dissemination of atomic-weight values.

Modern analytical techniques can measure the atomic weight of many elements precisely, and these small variations in an element's atomic weight are important in research and industry. For example, precise measurements of the abundances of isotopes of carbon can be used to determine purity and source of food, such as vanilla and honey. Isotopic measurements of nitrogen, chlorine and other elements are used for tracing pollutants in streams and groundwater. In sports doping investigations, 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 now will be expressed as intervals, having upper and lower bounds, reflected to more accurately convey this variation in atomic weight. The changes to be made to the Table of Standard Atomic Weights have been published in Pure and Applied Chemistry and a companion article in Chemistry International.

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. "In other words, knowing the atomic weight can be used to decode the origins and the history of a particular element in nature," says Wieser who co-authored the report.

Michael Wieser, a professor at the University of Calgary, is contributing to changes to the periodic table. He works with a thermal ionization mass spectrometer used to measure the isotope abundance of an element. Credit: Riley Brandt/University of Calgary

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."

The University of Calgary has and continues to contribute substantially in the study of atomic weight variations. Professor H. Roy Krouse created the Stable Isotope Laboratory in the Department of Physics and Astronomy in 1971. Early work by Krouse established the wide natural range in the atomic weight of significant elements including carbon and . Currently, researchers at the University of Calgary in physics, environmental science, chemistry and geoscience are exploiting variations in atomic weights to elucidate the origins of meteorites, to determine sources of pollutants to air and water, and to study the fate of injected carbon dioxide in geological media.

This fundamental change in the presentation of the atomic weights is based upon work between 1985 and 2010 supported by IUPAC, the University of Calgary and other contributing Commission members and institutions.

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User comments : 11

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Yellowdart
5 / 5 (1) Dec 15, 2010
It's not that much of a challenge really. It's like learning geometry and then learning calculus only to find out the specifics of why the circumfrence of a circle is derivative of its area.

For most learning purposes, theyll stick with the original, without worrying about the intervals.
_ilbud
5 / 5 (2) Dec 15, 2010
My theory of a lego universe is crumbling around me.
lexington
5 / 5 (1) Dec 15, 2010
So why do atoms have different weights sometimes?
Ober
5 / 5 (3) Dec 15, 2010
Because of isotopes. Some atoms have an extra NEUTRON, which does not change the chemistry of the atom, but does increase the atoms weight. Thus in a sample of atoms, a few will be heavier, thus lifting the AVERAGE weight of the sample.
WABI
2.3 / 5 (3) Dec 15, 2010
I must say that the proper term that should be used here is "atomic mass", and not "atomic weight". Weight is a force, while mass is the measurement of inertia. You are enhancing an already strong misconception.

After all, Hydrogen doesn't lose its inertia by one-sixth when it's on the moon.
DamienS
3 / 5 (4) Dec 16, 2010
I must say that the proper term that should be used here is "atomic mass", and not "atomic weight". Weight is a force, while mass is the measurement of inertia. You are enhancing an already strong misconception.

You clearly don't understand scientific nomenclature.

Atomic weight is a dimensionless ratio of the average mass of atoms of a particular element to 1/12 of the mass of an atom of carbon12. Unlike atomic masses (the masses of individual atoms), atomic weights are not physical constants and can vary from sample to sample. Atomic weight is also known by the term relative atomic mass. Nevertheless, anyone within the scientific community knows the definition of atomic weight.
plasticpower
not rated yet Dec 16, 2010
I was always a bit curious what keeps the neutrons and protons bunched together so tightly that splitting them apart causes a massive release of energy. It doesn't make much sense to me why protons, all positively charged, don't repulse each other. And yet here we are..
PJK
5 / 5 (2) Dec 16, 2010

You clearly don't understand scientific nomenclature.


I think the point was that atomic weight is a misnomer.
Sure, it might be the standard nomenclature, but the quantity in question is a mass, not a weight.
PJK
4 / 5 (1) Dec 16, 2010
I was always a bit curious what keeps the neutrons and protons bunched together so tightly that splitting them apart causes a massive release of energy. It doesn't make much sense to me why protons, all positively charged, don't repulse each other. And yet here we are..


It's because the electromagnetic interaction isn't the only one at work. There's also the strong interaction. The strong "attractiveness" (which can be thought of as neutral pion exchange) overcomes the EM "repulsiveness".
DamienS
1.5 / 5 (2) Dec 16, 2010
I think the point was that atomic weight is a misnomer.
Sure, it might be the standard nomenclature, but the quantity in question is a mass, not a weight.

I don't think that that was the point, bur in any case, there is a grain of truth in the nomenclature as historically atomic weights really were measured by weighing (through gravimetric analysis).
BillBerger
not rated yet Dec 16, 2010
Does anybody think that this might confuse people by changing atomic mass to a range? Mostly atomic mass is used to calculate molar mass which is used to figure out how many moles of a substance you have, such in analytical or organic chemistry. I gather in more highly sensitive experiments the error may seem more prevalent, but surely people at that level would already know that atom mass can vary.