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Topic: Why are there various values for an element's AMU  (Read 7070 times)

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Offline tech99

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Why are there various values for an element's AMU
« on: August 01, 2008, 09:02:17 PM »
Example, I see H being 1.00794, 1.0079, and 1.007825.

Is there an "official" value or does it depend on usage?

Why do some have fewer decimals (Kr is 83.8 in one place, 83.798 in another)


Thanks from a "new" student

Offline enahs

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Re: Why are there various values for an element's AMU
« Reply #1 on: August 01, 2008, 11:21:46 PM »
It just depends on who is reporting it and to how many significant figures.

I would not really trust any that has more decimal places then the basic one from the NIST:
http://physics.nist.gov/PhysRefData/PerTable/index.html

Offline Mitch

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Re: Why are there various values for an element's AMU
« Reply #2 on: August 02, 2008, 03:54:43 AM »
The mass assumes Carbon-12 equals exactly 12.0000000000000000000000000000000amu. Thus it won't be exactly 1 for hydrogen.
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Offline Borek

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Re: Why are there various values for an element's AMU
« Reply #3 on: August 02, 2008, 03:58:32 AM »
Note, that molar mass of the element may depend on its source, so number of significant digits have to be treated with caution.
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Offline Borek

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Re: Why are there various values for an element's AMU
« Reply #4 on: August 02, 2008, 03:59:27 AM »
The mass assumes Carbon-12 equals exactly 12.0000000000000000000000000000000amu. Thus it won't be exactly 1 for hydrogen.

As I read the question that's not the problem, rather what is a correct mass of element as found in nature.
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Offline azmanam

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Re: Why are there various values for an element's AMU
« Reply #5 on: August 02, 2008, 11:18:54 AM »
Another thing to think about is that the molar mass is a weighted average of the relative abundance of isotopes.  Thus, even though Mitch correctly noted that we have set the mass of one atom of carbon-12 to 12.000... amu, you'll notice a periodic table for carbon reads 12.011.  That's because there is a bit of carbon-13 naturally, thus the accepted molar mass is the weighted average of all the carbon-12 and all the carbon-13.  Same for hydrogen. There is some naturally occurring deuterium and tritium, so the mass of hydrogen is not exactly 1.

So perhaps the mass is a bit different depending on where the relative abundance of the isotopes was measured?
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Offline Borek

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Re: Why are there various values for an element's AMU
« Reply #6 on: August 02, 2008, 11:43:36 AM »
So perhaps the mass is a bit different depending on where the relative abundance of the isotopes was measured?

That's exactly the problem. For most elements abundance is quite constant and doesn't depend on the source, but there are interesting exclusions to that rule; unfortunately, I can't remeber where I have read about it.
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Offline tech99

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Re: Why are there various values for an element's AMU
« Reply #7 on: August 02, 2008, 05:15:06 PM »
Thanks for your replies...

Are you saying that NIST is the recognized authority?
The NIST table shows Kr atomic weight is 83.798; the table on this forum says  83.80. Are they both correct?

If I need to solve a problem for Kr, which value should be used?
I didn't take chemistry in school. I imagine the teacher has everyone use the same table.

I get the impression that the amu calculations depend on percentages of known isotopes. Is that always being revised - the known isotopes?

 
   

Offline Borek

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Re: Why are there various values for an element's AMU
« Reply #8 on: August 02, 2008, 05:39:06 PM »
Are you saying that NIST is the recognized authority?

Definitely.

Quote
The NIST table shows Kr atomic weight is 83.798; the table on this forum says  83.80. Are they both correct?

If I would ever need that high accuracy, I would go for NIST data. 83.80 is just a rounded down value, thus is it hardly inorrect. Less accurate.

Quote
If I need to solve a problem for Kr, which value should be used?

In most cases even 84 will do.

Quote
I get the impression that the amu calculations depend on percentages of known isotopes. Is that always being revised - the known isotopes?

In theory it can be revised any time. In practice some changes happen now and then, but they are of almost no practical importance right now. For 99.99% of applications 4 significant figures (or +/-0.01% accuracy) are enough; this level of accuracy exists for probably at least 50 years.
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Offline enahs

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Re: Why are there various values for an element's AMU
« Reply #9 on: August 02, 2008, 06:18:25 PM »
Quote
That's exactly the problem. For most elements abundance is quite constant and doesn't depend on the source, but there are interesting exclusions to that rule; unfortunately, I can't remeber where I have read about it.


Any other solar system would have this problem for sure.

It is also very plausible that deep in the earth the ratio of isotopes we measure on the surface would be different, for certain elements more then others.

Offline enahs

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Re: Why are there various values for an element's AMU
« Reply #10 on: August 02, 2008, 06:21:28 PM »
Quote
The NIST table shows Kr atomic weight is 83.798; the table on this forum says  83.80. Are they both correct?

There is only 0.00238 % difference between these two numbers.

They are essentially the same. And I would be willing to wager whatever you are going to be calculating, you are using a formula that is simplified and not "truly" correct, and this small difference is insignificant compared to that; if you are actually trying to get the "true" answer. But again, with that you are still probably talking fractions of a percent difference.


Focus more on the concepts and the science, and not ultimate accuracy.




Offline Borek

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Re: Why are there various values for an element's AMU
« Reply #11 on: August 02, 2008, 06:28:17 PM »
Quote
That's exactly the problem. For most elements abundance is quite constant and doesn't depend on the source, but there are interesting exclusions to that rule; unfortunately, I can't remeber where I have read about it.


Any other solar system would have this problem for sure.

It is also very plausible that deep in the earth the ratio of isotopes we measure on the surface would be different, for certain elements more then others.

I seem to remember that for some element difference between amus for samples from different sources are in the range of 0.1 or something like that, and it was not one of the radioactive elements were sample age and history play an obvious role. But I can't remember the source of the information. Could be it was IUPAC or NIST site, or someone on CHEMED-L was referring to results posted there.
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Offline sjb

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Re: Why are there various values for an element's AMU
« Reply #12 on: August 03, 2008, 04:54:45 AM »
Greenwood and Earnshaw touch on this a bit, see my notes at http://www.chemicalforums.com/index.php?topic=23116.msg89039

S

Offline Borek

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Re: Why are there various values for an element's AMU
« Reply #13 on: August 03, 2008, 05:45:14 AM »
Boron it was, and 0.01 and not 0.1, my bad. But looks like this is the source I remember, thatk you.

There is another thing worth of checking - uranium from Oklo Fossil Reactors in Gabon; there was a kind of natural reactor there - and 235/238 ratio in these ores is different from that seen in other uranium ores.

From http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

Quote
In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF6 samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the 235U isotope. Normally the concentration is 0.7202%; these samples had only 0.7171% Рa significant difference. This discrepancy required explanation, as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes. Thus the French Commissariat à l'énergie atomique (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of the uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a 235U to 238U ratio as low as 0.440%. Subsequent examination of other isotopes showed similar anomalies, such as Nd and Ru as described in more detail below.
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