[Vicstar]

Hi. um - i know this might sound dumb but...

why was carbon-12 chosen to be the standard of the mole?

was it because physicists and chemists argued a lot a comprised for carbon? or is there another reason? Do you know any good websites for this sort of information?

[Arkcon]

Pretty arbitrarily really, at the start, it was hydrogen =1, pretty much because it was the first element in the periodic table.  I figure the switch to carbon was just as arbitrary.  Probably an example of a bad choice would be sulfur.  As I heard it, it's isotope distribution is different, depending on where in the earth it's taken from. 

[ARGOS++]

Dear Vicstar;

Sorry!,   ─    But the “Mole Concept” has nothing to do with  C⁽¹²⁾ !

The “Atomic Mass” has to do with C⁽¹²⁾ and is standardised this way.
For that you may read on:     "Atomic Mass”

For the ““Mole Concept” you may read on:   "Mole”
─    and on:                   "Avogadro Constant”

I hope it may help.

Good Luck!
                    ARGOS⁺⁺

[sjb]

Pretty arbitrarily really, at the start, it was hydrogen =1, pretty much because it was the first element in the periodic table.  I figure the switch to carbon was just as arbitrary.  Probably an example of a bad choice would be sulfur.  As I heard it, it's isotope distribution is different, depending on where in the earth it's taken from. 

Even things like hydrogen have some isotopic variance, if I recall correctly - have more details I think in http://www.amazon.co.uk/Chemistry-Elements-Earnshaw/dp/0750633654, amongst others. I'll have a look tonight and see if what's said can be easily précised.

S

[ARGOS++]

Dear sjb;

Of course! ,  that Hydrogen has Isotopes, most people should know.

They are called (the most known):  Deuterium (“Heavy Hydrogen”, 0.015 %), and Tritium ( < 0.001 %).

Apropos Standard:  ¹⁶O and ^[16]O were in the meantime, side by side, the used Definition for Physics and Chemistry, generating two Periodic Tables.

Good Luck!
                    ARGOS⁺⁺

[sjb]

Dear sjb;

Of course! ,  that Hydrogen has Isotopes, most people should know.

They are called (the most known):  Deuterium (“Heavy Hydrogen”, 0.015 %), and Tritium ( < 0.001 %).

I agree with what you've said, but I didn't mean that. The issue I vaguely remember is that even in some "natural" sources of hydrogen, the deuterium (and tritium) content is different enough compared to the figures you've just given to potentially cause some issues with (not very) precise calculations.

S

[ARGOS++]

Dear sjb;

In case I have misunderstood you:   Sorry about that!

But your last Statement about the variation of the ratio is much more than only hard to believe, because:
Nearly all Scientists think/believe that Deuterium was built during the “Big Bang”, and it is one of the most stable Isotopes.
The “only” known consuming processes, except the “man’s hand”, for Deuterium are the fusion and fission in stars.
On the other hand, if there would be a “place“ where Deuterium is enriched, a little bit, somebody must earn Millions and Millions of “Dollars”, and that could not be hide for long.

But it could be possible you remember the fact that the ratio since the “Big Bang” was not stabile and is often used in the astronomy as in:  "The Deuterium Puzzle”.

If someone of the “Nuclear Chemistry Forum”, maybe Mitch, can spend us more accurate information about?


Good Luck!
                    ARGOS⁺⁺

[Arkcon]

Dear sjb;

Of course! ,  that Hydrogen has Isotopes, most people should know.

They are called (the most known):  Deuterium (“Heavy Hydrogen”, 0.015 %), and Tritium ( < 0.001 %).

I agree with what you've said, but I didn't mean that. The issue I vaguely remember is that even in some "natural" sources of hydrogen.

Maybe I meant the same thing, and maybe I didn't.    What I'd heard was the isotopic distribution of sulfur differed greatly from different mines on the Earth -- specifically, sulfur mined in the US had a different distribution of the 4 isotopes than sulfur mined elsewhere.  'Course, now I can't find a reference for that point easily, at least, it's not on Wikipedia.

'Course I don't know how academic the problem of isotope distribution is.  I figure, the oceans would throughly mix the Hydrogen, Deuterium, and Tritium well enough to make any sample the same, but sjb appears to differ.  We all know that a sample of Carbon is going to have different amounts of ¹⁴C, depending on whether the sample is atmospheric, an recently living sample or an ancient one, but we still use it as the standard, so my initial blurb about sulfur may be meaningless.

[ARGOS++]

Dear Arkcon;

Sorry!,   ─  But I still believe that sjb and I were only discussing about the Isotopes of Hydrogen.

Of course that most other (I’m not able to know all!) Isotope ratios are varying depending of their natural sources.
But Deuterium is a “big” different case, because its natural source.
For Tritium the case looks very different, because its radio activity.
And that’s all I was talking about.   All other you are talking about looks ok for me.

Good Luck!
                    ARGOS⁺⁺

[sjb]

I think I may have misunderstood, or hijacked the thread, so sorry about that.

Essentially the molecular weight of hydrogen itself does not vary, but what can vary is the apparent molecular weight derived from the source of the elements in question. Here are the notes from the book.

(Edit 2008-02-22: notes removed due to concerns over readability)

(Edit 2008-02-23: pdf notes on text added)

[Borek]

If you can post them again - but with less agressive compression - that'll be great. It is hardly readable right now due to compression artefacts. PM me if you need assistance.

[AWK]

This copy comes from Chemistry of the Elements 
By: Greenwood, N.N.; Earnshaw, A.
Elsevier (1997 ?)
Chapter 1 - Origin of elements

[sjb]

Arrgh, thought I had cited the reference, but re-looking at the post it didn't go. So to rectify that:

Chemistry of the Elements, NN Greenwood and A Earnshaw, Butterworth Heinemann, 2nd edn., 1997, pp 16-18

I'll have another go at scanning next time I'm near the scanner - was more concerned about the size of the file so may try typing it up and pdf'ing or similar (several bits in the text will not "translate" well to forum software, and LaTeX is not a language I speak well). For the while I'll remove the images.

(Edit 2008-02-23: have now added a pdf of notes from the text, rather than the original)

[AWK]

This chapter was free on www.knowel.com. May be still it is free. (free registration may be needed)

[sjb]

This chapter was free on www.knowel.com. May be still it is free. (free registration may be needed)

Did you mean http://www.knovel.com? If so, then yes the chapter is (I think, do not have a username/password combo) available from http://www.knovel.com/web/portal/basic_search/display?_EXT_KNOVEL_DISPLAY_bookid=402. Makes the minor niggle at the back of my mind about copyright a little easier to handle I suppose. Good practise to get the note taking skills running again though

S