Changes to standard atomic weight values

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http://www.eurekalert.org/pub_releases/2010-12/uoc-awo121510.php

Atomic weights of 10 elements on periodic table about to make an historic change
Researchers from around the world compile more reliable data that will help science and industry



IMAGE: 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...
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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 - hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine 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 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, 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.

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 sulfur. 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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The year 2011 has been designated as the International Year of Chemistry. The IYC is an official United Nations International Year, proclaimed at the UN as a result of the initiative of IUPAC and UNESCO. IUPAC will feature the change in the standard atomic weights table as part of associated IYC activities.

As I use atomic weights in my job frequently, I can't say I'm going to be thrilled about having to use "ranges of values", particularly as some of the calculations are for fiscal transfer.
 
It's been a long time since I took Chemistry (20+ years), so maybe I'm misunderstanding this:

Atomic weight represents the mass of an assemblage of 6.02(10^23) atoms of a particular element (or isotope). All the atoms in the assemblage would have the same number of protons, electrons, and neutrons.

If that's correct, then I don't understand how an element can have a different atomic weight depending on where it is found. I mean, they're not talking about different isotopes of the same element, right?

Where does the extra mass come from? It can't be from different neutrons, or we'd be talking isotopes.
 
It's been a long time since I took Chemistry (20+ years), so maybe I'm misunderstanding this:

Atomic weight represents the mass of an assemblage of 6.02(10^23) atoms of a particular element (or isotope). All the atoms in the assemblage would have the same number of protons, electrons, and neutrons.

If that's correct, then I don't understand how an element can have a different atomic weight depending on where it is found. I mean, they're not talking about different isotopes of the same element, right?

Where does the extra mass come from? It can't be from different neutrons, or we'd be talking isotopes.

You're talking Avogadro's number when you bring up that figure.

What's really important about atomic weight, whether it measures 1000 atoms or a single atom, of the same type is it indicates the number of protons and neutrons in the atom. And then the protons tells us the balancing number of electrons.

I'm kind of curious about the story too though. I though an bona fide isotope was off by 1 atomic mass unit, not a fractional unit like the story mentions. And I'm really curious what this means at quantum level if the circumstances of an atom influence its atomic mass.

I mean what does it mean that synthetic human hormone has a fractionally different atomic mass versus one produced in the human?
 
I mean what does it mean that synthetic human hormone has a fractionally different atomic mass versus one produced in the human?

I interpreted that as an isotopic difference. Living tissue takes up carbon in a different manner than a synthetically derived molecule does, so the ratios of c-12 to c-14 would be different.

But I'm probably wrong...
 
I interpreted that as an isotopic difference. Living tissue takes up carbon in a different manner than a synthetically derived molecule does, so the ratios of c-12 to c-14 would be different.

But I'm probably wrong...

You're not wrong. That is exactly the issue, that the isotope distribution can be differemt.

I'm kind of curious about the story too though. I though an bona fide isotope was off by 1 atomic mass unit, not a fractional unit like the story mentions. And I'm really curious what this means at quantum level if the circumstances of an atom influence its atomic mass.

The atomic weight in question here is not the mass of an individual atom or isotope, but an average over the different isotopes weighted by their abundance in nature. And this abundance can vary depending on the source, e.g. atmospheric carbon vs. fossil carbon. So depending on the source, the atomic weight can be different because the isotope composition is different.

On the quantum level, nothing changes at all. Every isotope has still the same mass as before (this is not like the issue of the shrinked proton earlier this year). It is just the weighted average over all isotopes of an element that is updated. So for us, who do quantum physics with atoms, nothing changes at all, because we're using just one isotope anyway.


As I use atomic weights in my job frequently, I can't say I'm going to be thrilled about having to use "ranges of values", particularly as some of the calculations are for fiscal transfer.

At least the value does not pretend to be more accurate than it actually is. But I understand that it might be more cumbersome to pick a value form a given range, instead of just having one value, even if the latter might be wrong.
 
Ah I think I see......so they're essentially measuring the percent composition of say C14 versus C12 in a particular organic molecule? But then changing the periodic table is kind of sophistic then? Or is it that they're saying that on average, only X% of all carbon is C14 versus C12 so the Atomic Mass Unit of Carbon should be adjust a fractional percent to acknowledge that?
 
Or is it that they're saying that on average, only X% of all carbon is C14 versus C12 so the Atomic Mass Unit of Carbon should be adjust a fractional percent to acknowledge that?

This (if you also include C-13). Previously the global abundance of the isotopes was used to calculate one value that was displayed in the periodic table. The change discussed in the OP tries to address the problem that depending on the source of the element, the distribution can vary, resulting in a slightly different atomic weight on average.
 
The change discussed in the OP tries to address the problem that depending on the source of the element, the distribution can vary, resulting in a slightly different atomic weight on average.

Ah, now I understand. The article really could have been worded better for my tastes. I can see how this makes some sense, then. The way I had initially interpreted it, it seemed as if some atoms of C-12 would have a different mass than other atoms C-12. This didn't make sense to me.
 
The new table, outlined in a report released this month, will express atomic weights of 10 elements - hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium - in a new manner that will reflect more accurately how these elements are found in nature.

This is great news! For a long time, I have suspected I didn't dose the thallium correctly in my supervisor's coffee, but with the new information that should be a problem of the past :goodjob:
 
I guess you rather mean valium :D.

http://www.eurekalert.org/pub_releases/2010-12/uoc-awo121510.php



As I use atomic weights in my job frequently, I can't say I'm going to be thrilled about having to use "ranges of values", particularly as some of the calculations are for fiscal transfer.

I don't think that will have an effect, unless someones working at the CERN or in a geochemistral field. Most people will just use the mean value, like for everything else.
 
I don't think that will have an effect, unless someones working at the CERN or in a geochemistral field. Most people will just use the mean value, like for everything else.
Until the Lawyers get involved.

Ideally it will just require that standards for custody transfer metering be modified to use a set value.
 
Well technically not even two protons have the same mass, it depends heavily on they're environment and surrounding fields (E-Mag and more complicated kinds).

In this case "technically" means "so small, that there is no way we can remotely detect evidence of it anytime soon". For all intents and purposes, even if you are doing highest-precision spectroscopy, all protons and all atoms of the same isotope have the same mass.

And no, quantum mechanics does not magically get rid of the experimental uncertainty. At the end you just get a probability distribution of probability distributions.
 
So, does my bum look fat, according to Quantum Mechanics?
 
As long as the atomic weight of cobalt remains 58,93 all is right with the world.
 
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