Why is Carbon-12 used as the measurement of relative masses?

[Count Duckula]

Hi everyone,

This is really annoying me, so, I am aware of the story why oxygen was replaced with carbon-12, (because chemists were using natural occurring oxygen which was a mix of oxygen-16,17,18 and therefor was an avg?, but physicists wanted pure, isolated oxygen-16 so there was two tables with two different atomic masses). But, why was carbon-12 agreed to be the new element to which all other elements masses would be compared to?
-Is it because carbon-12 is the most abundant isotope (I think 98.9%?) therefore both parties (chemists, physicists) can use natural occurring carbon and get an accurate measurement without using an isolated carbon atom?

Also, is how the discovery of how many nucleons there was in an atom, by comparing masses?

 

[D H]

There were two oxygen standards. Physicists wanted a standard based on a specific isotope; a ¹⁶O atom had an atomic mass of 16 per the physicists' oxygen standard. Chemists wanted a standard based on something they could weigh; a mole of oxygen atoms had a mass of 16 grams per the chemists' oxygen standard. Attempts to get physicists and chemists to agree went nowhere. Making ¹²C the basis for a new standard was a political compromise that satisfied both sides. Physicists got their way by making the standard based on a specific isotope. Chemists got their way in the sense that the new carbon standard changed the definition of the mole (and hence atomic mass) by but a tiny, tiny amount.

 

[mfb]

Also, is how the discovery of how many nucleons there was in an atom, by comparing masses?

That is a very practical way, indeed.

I would guess that C12 was chosen to avoid oxygen in the reference, and to get a new value which is close to the old one (otherwise gold would be better, it has just a single natural isotope).

In 1-3 years, the current definition might get replaced by a fixed value of the avogadro constant.

 

[Count Duckula]

Hmm, but what is the need for comparison at all, if 1.67x10^-27 is agreed to be 1U of relative mass, and you know the atomic mass of an atom, that is the mass, right?

Sorry, I think I'm confusing myself by over thinking this! :(

Maybe C is used simply because it's easier to weigh as its a solid rather than the O and H used before it.

 

[mfb]

Hmm, but what is the need for comparison at all, if 1.67x10^-27 is agreed to be 1U of relative mass, and you know the atomic mass of an atom, that is the mass, right?

That (some kg value) is not the definition of 1u - but it might become the new definition soon.
It is not easy to measure the avogadro constant, comparisons between different isotopes are easier.

Maybe C is used simply because it's easier to weigh as its a solid rather than the O and H used before it.

For masses of single atoms, this is not so important.

 

[D H]

Hmm, but what is the need for comparison at all, if 1.67x10^-27 is agreed to be 1U of relative mass, and you know the atomic mass of an atom, that is the mass, right?

If all you care about is three decimal places of accuracy, all you need to know is the atomic mass. There's a problem here. Physicists and chemists often measure things to much more than three places of accuracy. For example, per the ¹²C=12 standard, an ¹⁶O atom has an atomic mass of 15.9949146196, not 16.