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Atomic Weight Not So Constant
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."
American atoms are fat.
Link to actual article is:
link
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I don't think there are any constants in nature. We humans just like to perceive them as such, so it makes our calculations a whole lot easier.
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.
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.
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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" as even Wikipedia knows?
Somebody seemed to have failed his physics or chemistry classes.
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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!!!"
The scientific paper can be found here.
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.
This is absolutely to be expected, but the title brings visions of detectable changes in mass of a particular isotope, which would be earth shattering.
Given that the relative abundance of isotopes is quite variable, you might say that this development was periodictable.
Set your phasers on "funky"!
Wait let me guess.. IUPACHCIAW, right??
Well duh.
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, given the cost and difficulty of isotope analysis, it is usual to use the tabulated values of standard atomic weights which are ubiquitous in chemical laboratories. http://en.wikipedia.org/wiki/Atomic_weight
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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.
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.
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.
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 International Union of Pure and Applied Chemistry that is updating these not a bunch of astronomers working at/for NASA).
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.
They should be measuring lean muscle mass.
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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 standard, when values carbon 13 can be up to 30% less for oils and fuels which have been processed by plants and microbes. We can actually use the depletion value to determine if an oil is 'ready' to be extracted and its quality or find what plant (or type of plant) a sample may have come from. While this is a very well documented and understood phenomenon, it has safely been ignored up until now. In test tubes the isotopes react more or less the same and most chemists don't give it a second thought. Any real work that needs very precise and accurate masses can utilize equipment like high res mass spec, which can tell you the amount of each isotope present. This paper proposes using an interval to cover this fractionation that can occur, making the point that you cannot assume the ratio of carbon 12 to carbon 13 (or any other element) is constant throughout the universe, which changes the average molecular mass.
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.
Lots of chemical reactions can significantly fractionate isotopes, although the effect is typically small (of the order of a few tens of parts per thousand at most) and often negligible for normal chemistry. The effects are larger in light elements because the relative differences in mass between isotopes are bigger e.g. deuterium is twice as heavy as normal hydrogen. For heavier elements like iron, fractionation effects still exist, but are orders of magnitude smaller.
One example is evaporation of water: 16O evaporates more easily than 18O. Thus the "average" atomic weight of oxygen in seawater is higher than that of rainwater. The magnitude of the fractionation effects depends on the isotopes, but also on temperature and reaction kinetics, so there may be a lot of useful information in these small differences.
People (especially geochemists) have been doing work using these effects for decades, making use of the differences in isotopic ratios through space and time to investigate earth and biological processes. Try Googling stable isotope geochemistry..
However, having an "average" value on periodic tables is still useful - for most bulk-compositional calculations such an approximation is entirely appropriate.
Cheers
J
I learned back around 1970 that it was already standard practice to identify the source of metal samples (e.g. in captured weapons) by measuring the ratios of the stable isotopes - there was some index somewhere of which eastern European mines yielded which isotope ratios. Archaeologists have been tracking the movement of metals and jewels by looking at isotope ratios to determine where the sample came from. Someone even found that the ratios of stable iron isotopes differ between young adult men and women - to name only a very few uses of isotope ratios.
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.
Escher was the first MC and Giger invented the HR department.
"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 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.
deleting the extra space after periods so i can stay relevant, yeah.
You heathens just keep screwing things up worse and worse. Everybody knows that there is only one true periodic table: Fire Earth Air Water
The Oklo natural nuclear reactors http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor were discovered on the basis of isotope ratio deficiency
Typical of the hyping of science news. I remember reading this about lead a long time ago. In fact the isotope ratios of lead are used to characterize bullets for forensic analysis. At work we had to account for the actual isotope ratio of Lithium 6 vs Lithium 7 in calculating the purity of a lithium hydroxide sample. We believe the lithium was waste from lithium that had been isotopically enriched for nuclear weapons.
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.
Great Intellect...
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.
Of course they are not constant. I do not see the point of even posting this "big new discovery". In periodic table values ARE averaged out of many isotopes/states you find in nature. Blah..
Just because it's measured one way in one place or time...
Why is this news? Is it just that we are choosing a different weighted average? Big deal.
The partially explained hypothesis in that article is mildly interesting, however, as with much of Huffington Post's reporting on health issues, there is no indication of data to support the theory. It is just a single Doctor offering personal observations and fitting it to a theory.
Using Occum's razor perhaps the most likely answer is that corn subsidies helped make the average price of calories cheaper at a time when average incomes were increasing. Companies also are motivated by profits to sell foods that are less filling per dollar. This resulted in increased caloric intake by most people and was accompanied by shift to a more sedentary lifestyle. So caloric intake increased and metabolic caloric expenditure decreased on average. The burden of proof lies with the claim that fructose of equal caloric value will lead to increase in weight compared to other calories. So far the evidence is lacking.
I've seen evidence that the increase in corn in the diets of livestock has changed the composition of fat ingested by people in the USA. This composition change may contribute to cardiovascular and other diseases but not to weight gain.
As Mark Haub recently demonstrated, the evidence for calorie control as the route toward weight loss is quite convincing. However, overall health is much more complicated than, though certainly influenced by, maintaining ones weight.