BromicAcid - 16-12-2010 at 18:30
Haven't seen this posted here but starting soon periodic tables will include a range for atomic weight for 10 of our favorite elements. I am thinking
this might be a pain in the butt for people like me that like to calculate out my molecular weights to the umpteenth decimal place since I will now
have a range to work with.
http://www.scientificamerican.com/article.cfm?id=mass-migrat...
psychokinetic - 16-12-2010 at 22:35
Damn scientific progress. 
TheOrbit - 16-12-2010 at 22:38
oh no that will be a severe headache to all of us
bquirky - 16-12-2010 at 23:51
I haven't been able to get a good grasp on what this is about.
Is the issue that the mass of the neutrons and protons fluctuating ?
or is it as the linked artical says and an acnolagement that nature abhors a perfectionist and a 'pure' sample of something will contain a few
different isotopes in varying amounts?
if the mass of the particals is changing that whould be (to me at least) quite impressive
woelen - 17-12-2010 at 00:21
No, masses are not fluctuating, this news actually does not really surprise me.
As an example I take chlorine. It has two isotopes, 35Cl and 37Cl. Both have a certain mass, which can be determined with high precision. In nature,
however, we always have mixes of both isotopes. And what they have found now, is that these mixes are not the same on every place on earth. The ratio
of 35Cl : 37Cl is not constant throughout the whole of the earth, but there are small variations. A sample takes on place X may have a somewhat higher
relative amount of 37Cl than a sample taken on place Y on earth. So, the average atomic mass of the sample from place X is somewhat higher than the
sample from place Y.
This problem already is known for a longer time for uranium and lithium. From both of these elements we artificially separate isotopes and the effect
of this on global distribution of the isotopes is rather striking. Especially for the lithium this has become a nuisance, because the uncertainty in
atomic mass is so large that stoichiometrically precise reactions become more and more cumbersome to be carried out at high precision.
smuv - 17-12-2010 at 00:35
This actually has real world consequences, I one worked at a nuclear power plant, we got a shipment of boric acid (used as neutron moderator) from a
different location than we usually do. It was more potent than had ever been observed in the (looong) life of the plant, we attributed this to a
higher Boron-10 composition than our other source.
EDIT: "Boron-10 content may be as low as 19.1% and as
high as 20.3% in natural samples" (wikipedia)
[Edited on 12-17-2010 by smuv]
blogfast25 - 17-12-2010 at 13:04
Not a big problem if your best scale is 0.001 g, I'd imagine 
psychokinetic - 18-12-2010 at 00:22
A fluctuation wouldn't surprise me, really. Just because we have always measured something at a certain measurement doesn't mean they all are or
always will be.
Saerynide - 26-12-2010 at 00:19
What??? So Cl is no longer 335.453 as I have engrained in my memory from my youngin' days? Darn scientists!
garage chemist - 26-12-2010 at 04:29
The fluctuation in the atomic weights of elements from chemical suppliers is most often caused by isotope separating companies selling off "depleted"
elements from which a wanted isotope has been partially removed. This is true not only for the generally known example of uranium, but also for
lithium and boron. Lithium-6 is needed for breeding tritium and for hydrogen bombs.
Boron-10 is needed in the form of enriched BF3 gas for neutron counter tubes.
The depleted Li and B are perfectly suitable for all non-nuclear applications, but their atomic weight is significantly different from the natural
substance. If exact stochiometry is wanted, determining the isotope composition of your material is the only solution.