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

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.

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.