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## Atomic Weight Not So Constant147

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

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

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.

• #### Isotopes (Score:5, Insightful)

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.

• #### Atomic Weights were never constant (Score:5, Insightful)

<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.

• #### Meh (Score:4, Insightful)

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!!!"

• #### Re:Natural constants? (Score:5, Insightful)

on Wednesday December 15, 2010 @08:14AM (#34559178)

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.

• #### The consequences for Carbon Dating... (Score:1, Insightful)

on Wednesday December 15, 2010 @08:46AM (#34559348)

...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.

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