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

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.

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.

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.

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.

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

[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".

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[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.