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."

Comments

You know the cliché

[Anonymous Coward]

American atoms are fat.

Re:You know the cliché

[camperslo]

The calories matter, but very little fructose can be used by the body directly compared to glucose. It gets converted to fat. If you're pretty active, you can burn that fat. But for things like providing the energy your brain needs, it isn't metabolized well. You can actually feel short on energy at the same time you're converting fructose to fat. The liver has to work harder to process it too. Most of the time what people perceive as energy from a soda is actually the caffeine.
And with fructose, your brain is less apt to tell you you're full, so gulping down large amounts of fructose is way too easy. It's junk calories that can contribute to fat without doing anything useful.
It's a waste of money. If you tried to live on 2000 calories of fructose a day you may not get fat but it wouldn't take long to be sickly.

Obesity in the U.S. has become much more of a problem since we started subsidizing corn in the Reagan era. The very high yield variety that is profitable (with subsidy) is very low protein and not suitable for eating. It does get fed to cows since it fattens them quickly, resulting in a more sickly and fat-loaded beef supply. It's a very unhealty diet for cows which would kill them if they weren't pumped full of antibiotics. It degrades our food supply in more ways than are obvious at a glance.

http://www.huffingtonpost.com/dr-mercola/sugar-may-be-bad-but-this_b_463655.html

Actual Link to Document

[Fluffeh]

Link to actual article is:
link

Re:Actual Link to Document

[chichilalescu]

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.

Re:Actual Link to Document

[Interoperable]

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.

Re:Actual Link to Document

[NeverVotedBush]

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.

Re:Actual Link to Document

[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).

Re:Actual Link to Document

[osu-neko]

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...

I don't get it

[FTWinston]

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

[Sockatume]

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.

Re:I don't get it

[SimonTheSoundMan]

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

[Marcika]

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

[Lloyd_Bryant]

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 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 proportions of the different isotopes in samples found "in nature", so instead of a single fixed value, it requires a range that the average should fall in, based on observed variations in those proportions.

Re:I don't get it

[shaitand]

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.

Isotopes

[MichaelSmith]

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.

Re:Isotopes

[aliquis]

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?

Atomic Weights were never constant

[bhaak1]

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.

Re:Atomic Weights were never constant

[ciderbrew]

Is the level of bitch a constant?

Meh

[Zironic]

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!!!"

Masses not changing; only ratio of isotopes.

[Anonymous Coward]

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.

Comment removed

[account_deleted]

Comment removed based on user account deletion

Re:Natural constants?

[vlm]

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.

Also, those same calculations show that some things, like proton mass, speed of light, gravitational constant, a couple others, have to have remained constant within a very large number of decimal places in order for old stuff to have changed the same way new stuff changes. More decimal places that we usually have sig figs to measure stuff, so by sig figs rules, have to treat them as constant, its not just an "easier" thing.

For your average chemical engineer bucket chemist, small changes in atomic weight are going to be statistical noise.

Wrong assumption

[Arrepiadd]

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).

Not that surprising

[raovq]

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.

Re:Natural constants?

[andrewagill]

Huh. Your argument is well-reasoned and compelling. I guess I'll have to go back to the permittivity and permeability of free space (the product of which is the inverse of the square of the speed of light in a vacuum), Coulomb's constant, the gravitational constant, the Stefan-Boltzmann constant, Planck's constant...

Re:The consequences for Carbon Dating...

[smellsofbikes]

...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 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, 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. 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.