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Atomic Weight Not So Constant 147

Posted by kdawson
from the thulium-and-thalium dept.
DangerousBeauty writes "Yahoo has a Canadian Press story up about new changes to the periodic table of elements concerning the weights of specific elements — it seems that the weights fluctuate based on where they are found in nature. Quoting: '"People are probably comfortable with having a single value for the atomic weight, but that is not the reality for our natural world," says University of Calgary associate professor Michael Wieser.' He is is secretary of the International Union of Pure and Applied Chemistry's Commission on Isotopic Abundances and Weights."
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Atomic Weight Not So Constant

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  • by Anonymous Coward

    American atoms are fat.

    • by tchdab1 (164848)

      It's global warming, it causes everything to rise.

  • by Fluffeh (1273756) on Wednesday December 15, 2010 @06:30AM (#34558632)
    Link to actual article is:
    link [iupac.org]
    • by chichilalescu (1647065) on Wednesday December 15, 2010 @06:39AM (#34558700) Homepage Journal

      Yes. thank you for that link. Anybody with minimal knowledge of how atomic weights are computed (i.e. a weighted average of the atomic masses of the various isotopes) can guess that if the concentrations of isotopes are different in different samples, the "atomic weight" will be different.
      I went and read the famous abstract anyway. quote: "This fundamental change in the presentation of the atomic weights represents an important advance in our knowledge of the natural world and underscores the significance and contributions of chemistry to the well-being of humankind in the International Year of Chemistry 2011."
      This article is just about the results of some measurements. ok, useful measurements, but NOT an important advance in our knowledge of the natural world.

      • by JustOK (667959)

        the value of the findings vary depending on the observer.

      • The important advance is in the presentation of weighted atomic masses as ranges with context, rather than the wet chemistry behind the changed numbers. At the very least, presenting a range of masses reminds us to think about the sources of the atoms analysed and variations in their collective attributes.

      • And it really isn't even news. The fact that isotope ratios vary (and thus average atomic weight/mass) depending on the source/location is fundamental to things like carbon dating.

        Atomic weights based on accepted isotope distributions have always been somewhat approximate. That accepted weights would be revised should have been expected.
        • by Rich0 (548339)

          Yup - this is also one of the reasons why the mass of a liter of water/etc was avoided as the basis for the kilogram. It is almost impossible to obtain water of any particular isotopic composition, and it varies around the world. Water is also non-ideal for other reasons as well which I won't get into...

      • by jc42 (318812)

        Yeah; my first reaction was "WTF? Archaeologists have been using variations in isotope ratios for decades as a way of learning about the diet of the people and other critters who left remains." This works (sort of) because different plant species have different isotope ratios for many of their constituent elements. Isotope ratios are also used to do things like locate the source of plant, mineral and metallic objects. Anyone who knows about this understands that the published "atomic weight" numbers are

      • by Coren22 (1625475)

        I was a little surprised by the summary, as I don't recall ever seeing atomic weight on the periodic table. The only number I recall seeing as a constant was the only constant that can be used; the number of protons.

        http://en.wikipedia.org/wiki/Periodic_table [wikipedia.org]

      • by c++0xFF (1758032)

        I've always been bothered by the fact that no standard deviation is ever given. Of course, there's not much room on there for that info on a periodic table, but I don't even see it when the full properties are listed.

        TFA presents the value as a range ([1.007 84; 1.008 11] for Hydrogen). Why not say 1.00798 +/- 0.014%. IANAC (chemist), but that seems to give the same information, but in a way that's more natural to the way people will use the value. Unless I know what the correct average weight is where

    • by Interoperable (1651953) on Wednesday December 15, 2010 @07:51AM (#34559092)

      Thanks. The article makes it clear that the major change here is that the way in which atomic weights will be presented is changing. It's not just that they're being updated to reflect a more complete measurement, it's that atomic weights will now be represented as a range of possible values rather than a single value. It's not every day that the way in which information is presented in the periodic table changes.

      • by ari_j (90255)
        Actually, from the Slashdot front page you can tell everything you need to know about the differences between the article and the summary: It was posted by kdawson, and therefore the differences are fundamental in nature.
        • by osu-neko (2604)

          Actually, from the Slashdot front page you can tell everything you need to know about the differences between the article and the summary: It was posted by kdawson, and therefore the differences are fundamental in nature.

          Has it ever been explained why kdawson posts usually have titles that are flat-out contradicted by the article in question? Perhaps another study is in order...

      • Re: (Score:2, Informative)

        by Anonymous Coward

        But the range of possible values for the atomic weight is already represented by the number of decimal places displayed. For example, compare the the number of decimal places for F (18.9984032) and Pb (207.2).

        • by vegiVamp (518171)

          How does that work ? Does that mean Pb can vary between 207.20 and 207.29... ? How about when the weight of an element could vary between, say, 200.45 and 200.47 ?

  • I don't get it (Score:4, Insightful)

    by FTWinston (1332785) on Wednesday December 15, 2010 @06:33AM (#34558656) Homepage

    Isotopes exist, right. And by definition, different isotopes of the same element have different mass. I'd take it as a given that the distribution of certain isotopes are different in different places.

    But what is this article actually saying? The atomic mass number is meant to be the universal average ... now they may have got that slightly wrong, but why exactly do we need a range of universal averages for each isotope? That's surely some sort of misnomer.

    • Re:I don't get it (Score:5, Informative)

      by Sockatume (732728) on Wednesday December 15, 2010 @06:36AM (#34558682)

      They're recalculating the average atomic weight, the one on the periodic table, based on the abundances of the different isotopes in nature. If you're trying to calculate the mass of, say, 300,000 molecules of something, you use the average atomic weight and don't try to figure out what isotope each atom is.

      • by ChipMonk (711367)
        So you're saying they're using the atomic weight of some hypothetical "typical" atom, even though that "typical" atom never actually, you know, occurs in nature.
      • by shaitand (626655)

        The atomic weight has always been a weighted average that isn't new. What is new is that they are no longer going to print that average as the singular atomic weight. Now they are going to print a range because the abundance of a given isotope varies based on where a sample is gathered.

    • What I do not get is, of course weight will be different in nature. Weight is dependant on acceleration due to gravity and mass. An atom would weigh more on Earth than it would on the moon.

      I think these chemists mean 'atomic mass'? I'm an engineer so correct me if I'm wrong.

      • Re:I don't get it (Score:5, Informative)

        by Marcika (1003625) on Wednesday December 15, 2010 @06:56AM (#34558792)

        What I do not get is, of course weight will be different in nature. Weight is dependant on acceleration due to gravity and mass. An atom would weigh more on Earth than it would on the moon.

        I think these chemists mean 'atomic mass'? I'm an engineer so correct me if I'm wrong.

        Atomic weight is a dimensionless quantity (ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12).

        I think the convention in chemistry is to call the absolute mass of an isotope (in kg or whatever) "atomic mass", and to call its relative mass (dimensionless) "atomic weight".

        • Re:I don't get it (Score:5, Informative)

          by PeterKraus (1244558) <peter.kraus@member.fsf.org> on Wednesday December 15, 2010 @07:25AM (#34558938) Homepage

          Atomic mass is the precise mass of a given isotope, in AMU (symbol u) or Daltons (Da), where 1u = 1Da = 1/12 of a mass of one C12 atom.

          Atomic weight, or relative atomic mass, is the abundancy-weighted average of atomic masses of isotopes. I'm not sure about the unit, but I guess using u or Da might not be to far off.

          For general purposes we use molar mass (or specifically relative molar mass), which is the mass of a mole of atoms (or molecules or whatever). In case of atoms, it's the atomic weight times avogadro constant. Unit g/mol, (the biochemists tend to use Daltons, 1 Da = 1g/mol, confuzzlingly enough).

          IAACh, but I'd guess the folk in IUPAC would get it right, so if I'm contradicting them, I'm wrong.

          • by jo_ham (604554)

            Well, IUPAC can't be right all the time - they standardised on "Sulfur" for goodness sake! ;)

            I just got used to hydrogen being 1.0079, now I'm going to have to memorise the table all over again.

            • they standardised on "Sulfur".

              What's that? :P

              (Makes you wonder why there's no fenolfthalein either, eh?)

      • What I do not get is, of course weight will be different in nature. Weight is dependant on acceleration due to gravity and mass. An atom would weigh more on Earth than it would on the moon.

        I think these chemists mean 'atomic mass'? I'm an engineer so correct me if I'm wrong.

        You aren't alone in that opinion - there is some controversy [wikipedia.org] over the name, simply because it is *not* a "weight" in any sense of the word.

        The most popular suggested replacement is "relative atomic mass" (the base unit is 1/12 the mass of a carbon 12 atom), but even that is somewhat misleading since it's actually intended to be relative to the average atomic mass of a sample of the element as found "in nature".

        The change is a result of them realizing that that there is actually some variation in the proport

        • by pclminion (145572)

          it is *not* a "weight" in any sense of the word.

          Yes it is. It is a weight in the sense meant by the term "atomic weight."

          But if you really want change for change's sake (it's not like this leads to confusion among chemists), let's call it atomic mojo. How about that?

      • by vlm (69642)

        Weight is dependant on acceleration due to gravity and mass. An atom would weigh more on Earth than it would on the moon.

        Not relevant. Theres nothing chemists love more than STP standard temperature and pressure. Extending that to "we're going to define all our weights as being in a 9.81 m/s2 grav field" is to be expected from that crowd (which I was/are almost a part of)

        Wait until you learn about the various gas laws, and start posting to slashdot that they are all wrong because a mole of gas "on the moon" would take up a heck of a lot more than 22.4 liters.

    • This isn't news, we have known for a long time that there are different isotope ratios from different sources. This basically means that the atomic weight of say carbon, when looking really closely, is sample dependant, meaning different samples will have differing atomic weights because they have differing ratios of isotopes.
    • by LWATCDR (28044)

      Well from reading the link what I am getting is that they are changing how it is listed and calculated. It does make sense in away since the variablity in atomic weight of hydrogen is much higher than say uranium when looking at it as a percentages. So they are going to show Hydrogen as a range instead of a fixed number.

  • Isotopes (Score:5, Insightful)

    by MichaelSmith (789609) on Wednesday December 15, 2010 @06:33AM (#34558658) Homepage Journal

    Atomic weight is calculated based on the number of isotopes of any given element. A handful have only one isotope and therefore a stable atomic weight, but most elements have more than one isotope, carbon 12, 13 or 14, for example.

    Makes much more sense than weights fluctuate based on where they are found in nature. Its why centrifuges can be used to separate uranium 235 from uranium 238.

    • weights fluctuate based on where they are found in nature

      But it's the truth. When we discovered some silicon on the moon, man, did that have a different weight.

      • by aliquis (678370)

        Next up on Brainiac:

        Silicone breast, will they float or will they sink?

        And are they really the best thing to grab hold of in case of office flood thanks to global warming?

        • Next up on Brainiac:

          Silicone breast, will they float or will they sink?

          And are they really the best thing to grab hold of in case of office flood thanks to global warming?

          Well, for the record, silicone is typically slightly denser than water [answers.com] so that'd make for a less-than-excellent flotation device.

          And, as a strange aside, newly-filled saline implants sometimes have a bit of air in them, and they can audibly slosh, which is a bit weird.

  • by bhaak1 (219906) <bhaak@gmx.net> on Wednesday December 15, 2010 @06:34AM (#34558666) Homepage

    The Atomic Weight is only an average of the isotopes found in nature divided by some constant mass unit.

    How could they be constant if "they vary from sample to sample" [wikipedia.org] as even Wikipedia knows?

    Somebody seemed to have failed his physics or chemistry classes.

    • Somebody seemed to have failed his physics or chemistry classes

      That's a little harsh. Yes, it's been known for quite some time that average atomic weights vary from sample to sample, and the information content of the paper may not seem fundamentally novel. However, this is a paper where scientists are recommending a change in IUPAC's policy. For these standards boards, this is a fundamental issue. Think of it as similar to the "Pluto is/isn't a planet" debate. It seems like it's just semantics they're arguing (and I'm inclined to think that, in either case, it i

      • by bhaak1 (219906)

        Somebody seemed to have failed his physics or chemistry classes

        That's a little harsh. Yes, it's been known for quite some time that average atomic weights vary from sample to sample, and the information content of the paper may not seem fundamentally novel. However, this is a paper where scientists are recommending a change in IUPAC's policy. For these standards boards, this is a fundamental issue.

        I didn't really mean the scientists in question. But as is usual nowadays, ./ headlines and summaries are often quite incorrect.

        Even though the stories are seen by several people before the hit the front page.

        Think of it as similar to the "Pluto is/isn't a planet" debate. It seems like it's just semantics they're arguing (and I'm inclined to think that, in either case, it is. Funny story: I had to correct some kid in a museum recently because he was telling his little brother that Pluto no longer existed.), but passions can still become pretty inflamed.

        Anyway, just thought I'd try to put the whole thing in perspective.

        :-) [xkcd.com]

  • Meh (Score:4, Insightful)

    by Zironic (1112127) on Wednesday December 15, 2010 @06:34AM (#34558668)

    Looking at the title of the story I thought it would be something funky, but the entire story is just that they want to make the periodic table slightly more accurate for atoms that have isotopes. Everyone that has gone through high school chemistry should already know that that for unstable elements the table reference is an average at best.

    This story is basically "ZOMG, it turns out that the weight of my mac and cheese isn't constant because the ratio of cheese to mac can vary!!!"

    • by jo_ham (604554)

      Not just unstable elements. Plenty of stable elements have more than one, sometimes multiple isotopes. boron, chlorine, bromine, tin to name just a couple off the top of my head.

  • by Anonymous Coward on Wednesday December 15, 2010 @06:36AM (#34558680)

    The scientific paper can be found here [doi.org].

    In Section 1.1 the weight is defined as the weighted mean over all the isotopes. Caesium 135 still has atomic mass 134.9059770(11) and caesium 137 still has mass 136.9070895(5); the way in which the relative abundances of isotopes is measured - that is all.

  • Given that the relative abundance of isotopes is quite variable, you might say that this development was periodictable.
  • Wait let me guess.. IUPACHCIAW, right??
  • Well duh [wikipedia.org].
  • The IUPAC definition[1] of atomic weight is: An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C. The definition deliberately specifies "An atomic weight...", as an element will have different atomic weights depending on the source. For example, boron from Turkey has a lower atomic weight than boron from California, because of its different isotopic composition.[6][7] Nevertheless,
  • But other than some intergalactic engineers, and of course astronomers, I don't think that any engineer will care. The earth was so properly blended when it was made that it's safe to assume that the isotope mixtures are a constant.

    • The earth was so properly blended when it was made that it's safe to assume that the isotope mixtures are a constant.

      Your assumption is actually wrong and that's why the values are updated. For instance, if you get a metal, from different mines around the world, the relative abundances of each of the isotopes vary slightly and this leads to different atomic weights for the same metal. This is why the atomic weights are updated. You can read about it in wikipedia [wikipedia.org].

      Indeed, this won't matter much (even for a chemist), but this is not a problem just for intergalactic engineers (and please do remember the fact that it is the In

  • Next they'll be telling us their weight depends on their energy too.

  • Elements have isotopes, different isotopes have different atomic weights, the proportion of isotopes present differs from sample to sample, the standard periodic table reports an average atomic weight that may or may not be appropriate to the sample you're considering at the moment. Way to report the scientific news of 70 years ago.

  • Hasnt this always been known that even different mines from the same countries produce isotope "fingerprints" that let the 3 letter agencies identify where nuke material come from? This is simply making it more obvious to those who use the reference for the range of values found so far ?

  • ...are that it's proved to be a completely inappropriate way of measuring the age of a sample, particularly for older samples.

    In fact for any sample over 2000 years old the errors are absolute.

    So in fact, this is big, big news.

    • Um...no. Why would you think that? Do you have any citations for this?

      We know that the isotope concentration of C14 changes in the atmosphere, and we think we know why. That's why the dates are calibrated against other ways of estimation (dendrochronology, ice cores, varves, etc). We also know that C14 is not uniformly distributed because there are carbon sinks (oceans, rivers from ice melt, etc.), so that is taken into account as well. The referenced paper has no bearing on this at all, and radioca
    • by smellsofbikes (890263) on Wednesday December 15, 2010 @12:51PM (#34562350) Journal

      ...are that it's proved to be a completely inappropriate way of measuring the age of a sample, particularly for older samples.

      In fact for any sample over 2000 years old the errors are absolute.

      So in fact, this is big, big news.

      I'd be curious to see where it's been "proved" to be an inappropriate way of age measuring, since carbon-14 dating closely correlates with tree ring data [wikipedia.org] out to 26,000 years back, using the INTCAL04 data group, which is internationally recognized as valid, and likewise it correlates well with deep ocean sediments, coral, cave rock formations, and other sources [wikipedia.org], all of which give similar age data to radiocarbon dating, which is currently using the INTCAL09 data for correction, that is internationally recognized as valid out to 50,000 years [qub.ac.uk]. So, if there's a problem with radiocarbon dating, the same problem is also affecting how fast sediments accumulate, coral grow, and stalactites form, and I've never heard of anyone suggesting anything that can affect all those, at the same time, and alter them all in a proportional manner. If you've any suggestions for something that could do that, I'd love to hear about it.

  • This isn't a big shock. In nature, there is a process called isotope fractionation. The idea is simple, in biological systems lighter isotopes react faster. Also with something like the evaporation of water, the heavy water will naturally be discriminated against. This means that in plants, carbon 13 and especially 14 do not react as quickly as carbon 12. The plants as a result contain less carbon 13 and 14 than you would otherwise expect. We usually measure this depletion against a worldwide limestone sta
  • The pope has been found to be catholic, birds fly, fish swim and bears defecate in woods.

    This sounded like it might be a fundamental change in something big, but it isn't. As many have already said, anyone with a passing knowledge of chemistry - even misremembered over 19 years like mine - is aware of the underlying reasons and the implications!

    It's presented as sensational but it's really not news in any way, shape or form.

    • by khr (708262)

      The pope has been found to be catholic, birds fly, fish swim and bears defecate in woods.

      quote>

      While I don't know about the pope, but there's birds that swim, fish that fly, bears that defecate outside the woods and other things that defecate in the woods...

    • The pope has been found to be catholic, birds fly, fish swim and bears defecate in woods.

      I'm an ostrich you insensitive clod.

  • I thought it was obvious: the mass of a given atom is constant, but measure its weight in different locations (equator, north pole, the moon) and you get different results. Of course, I'm just a simple physicist.
  • by khr (708262)

    "People are probably comfortable with having a single value for the atomic weight, but that is not the reality for our natural world"

    They didn't consult Karl Rove [wikipedia.org] for this, did they?

  • Trying to discuss theoretical physics with a skilled theoretical physicist is like playing card with someone who plays by changing the rules when they are losing. So long as you're not betting much on it, what the heck. And when they go all in, well, we know they must not be playing with their own money.

  • You heathens just keep screwing things up worse and worse. Everybody knows that there is only one true periodic table: Fire Earth Air Water [jumbojoke.com]

  • The Oklo natural nuclear reactors http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor [wikipedia.org] were discovered on the basis of isotope ratio deficiency
  • Many periodic tables even state it is the average mass of all samples. Any element with common isotopes is going to vary depending on where it is found; the same goes for carbon, which is how carbon dating works. Even largely stable elements that do not have a gas form could vary in extreme conditions.
  • I feel dumber for having clicked that link:

    Newfoundland comedian wha?

    Bane of high school students everywhere huh?

    Stupid jokey crap that never ends?!

    Shit, if I want god-knows-how-many-paragraphs of lead-in followed by a bunch of handwavy bullshit seemingly intended for people that enjoy feeling smart without having to think or understand things, I'd get a subscription to People magazine.

    While the underlying news is definitely for nerds, THIS was not the link we're looking for.

  • Just because it's measured one way in one place or time...

"Mach was the greatest intellectual fraud in the last ten years." "What about X?" "I said `intellectual'." ;login, 9/1990

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