# Carbon-12 Standard

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When we figure out the relative masses of atoms and isotopes we find them relative to 1/12th the mass of a Carbon-12 atom. Can someone explain why? And when we say 'relative', what do we mean? Are we dividing by it?

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#2

Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000. Therefore on this scale, 1/12th of carbon-12's mass is exactly 1. So when you're working out Ar, and multiply by 1/12th of C-12, you multiply by exactly 1. It's really hard to explain sorry haha, but the point is you need an exact number of 1, and you can only get that by using the 1/12th of the 12.000amu of carbon.

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(Original post by

Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000. Therefore on this scale, 1/12th of carbon-12's mass is exactly 1. So when you're working out Ar, and multiply by 1/12th of C-12, you multiply by exactly 1. It's really hard to explain sorry haha, but the point is you need an exact number of 1, and you can only get that by using the 1/12th of the 12.000amu of carbon.

**RonnieRJ**)Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000. Therefore on this scale, 1/12th of carbon-12's mass is exactly 1. So when you're working out Ar, and multiply by 1/12th of C-12, you multiply by exactly 1. It's really hard to explain sorry haha, but the point is you need an exact number of 1, and you can only get that by using the 1/12th of the 12.000amu of carbon.

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#4

(Original post by

Hey, thanks for the explanation, I appreciate it. I can tell how it's difficult to grasp and explain to someone else. So essentially it's the practicality and convenience that comes with being able to attain a 1 when you get 1/12th the mass of a Carbon-12 atom.

**High Stakes**)Hey, thanks for the explanation, I appreciate it. I can tell how it's difficult to grasp and explain to someone else. So essentially it's the practicality and convenience that comes with being able to attain a 1 when you get 1/12th the mass of a Carbon-12 atom.

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#5

**RonnieRJ**)

Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000. Therefore on this scale, 1/12th of carbon-12's mass is exactly 1. So when you're working out Ar, and multiply by 1/12th of C-12, you multiply by exactly 1. It's really hard to explain sorry haha, but the point is you need an exact number of 1, and you can only get that by using the 1/12th of the 12.000amu of carbon.

__assigned__a mass of 12.0000 so that it can act as the standard reference to which everything else is compared.

Prior to using carbon, both hydrogen and oxygen were used.

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(Original post by

It is not that this specific isotope of carbon actually has a mass of exactly 12.0000, it is

Prior to using carbon, both hydrogen and oxygen were used.

**charco**)It is not that this specific isotope of carbon actually has a mass of exactly 12.0000, it is

__assigned__a mass of 12.0000 so that it can act as the standard reference to which everything else is compared.Prior to using carbon, both hydrogen and oxygen were used.

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#7

(Original post by

If that's the case, why did scientists use Carbon in particular? (I assume because Carbon is a solid and more convenient than a gas). And then why did they choose ti assign 12? They could have used a number like 10 and then used 1/10th the mass?

**High Stakes**)If that's the case, why did scientists use Carbon in particular? (I assume because Carbon is a solid and more convenient than a gas). And then why did they choose ti assign 12? They could have used a number like 10 and then used 1/10th the mass?

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(Original post by

Not sure about the first part but the second should be obvious.... how many nucleons are there in the carbon-12 nucleus!

**JMaydom**)Not sure about the first part but the second should be obvious.... how many nucleons are there in the carbon-12 nucleus!

"his specific isotope of carbon actually has a mass of exactly 12.0000, it is assigned a mass of 12.0000"

Or do they mean it's around 12 but not exactly 12 and they gave it 12 for simplicity's sake?

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#9

**High Stakes**)

If that's the case, why did scientists use Carbon in particular? (I assume because Carbon is a solid and more convenient than a gas). And then why did they choose ti assign 12? They could have used a number like 10 and then used 1/10th the mass?

By assigning a value of exactly 12 to a carbon-12 isotope this makes the relative mass of the smallest atom = 1 (Hydrogen-1).

The relative masses approximate to the number of nucleons in each isotope.

If scientists had assigned a value of 10 to carbon-12 then the relative mass of a 1-H isotope would become 0.8333.

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(Original post by

In order to make the scale as representative and convenient as possible.

By assigning a value of exactly 12 to a carbon-12 isotope this makes the relative mass of the smallest atom = 1 (Hydrogen-1).

The relative masses approximate to the number of nucleons in each isotope.

If scientists had assigned a value of 10 to carbon-12 then the relative mass of a 1-H isotope would become 0.8333.

**charco**)In order to make the scale as representative and convenient as possible.

By assigning a value of exactly 12 to a carbon-12 isotope this makes the relative mass of the smallest atom = 1 (Hydrogen-1).

The relative masses approximate to the number of nucleons in each isotope.

If scientists had assigned a value of 10 to carbon-12 then the relative mass of a 1-H isotope would become 0.8333.

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#11

(Original post by

I mean why not use another element, assign it with a mass of 10 and then use 1/10? Why did we use Carbon-12 in particular?

**High Stakes**)I mean why not use another element, assign it with a mass of 10 and then use 1/10? Why did we use Carbon-12 in particular?

Just read through the history and development of the concept on wiki

https://en.wikipedia.org/wiki/Relative_atomic_mass

Here is the actual explanation: http://www.iupac.org/publications/ci.../1_holden.html

IV. The Atomic Weights Scale

The atomic weights scale of H = 1 was originally used by Dalton and (except for Berzelius' time) had been used for approximately 100 years when the ACS and the German committees began reporting their tables. Lothar Meyer and Seubert had published on the hydrogen scale, but Ostwald and Brauner strongly urged the adoption of the O = 16 scale. Clarke reported his table on both scales, while the German committee used the O = 16 scale exclusively and argued for its adoption. In October 1899, the German committee asked the international delegates if O = 16 should be fixed as the future standard. Of the 49 replies, 40 favored oxygen only seven favored hydrogen with two accepting either or both As a result, the first international table was published on the O = 16 scale. A vigorous protest against the decision was made by university chemistry teachers in Germany and by the committee for the decennial revision of the United States Parmacopoeia among others, who preferred the H = 1 standard. In the third report of the German Atomic Weights Commission, the results of another vote indicated 106 chemists in favor of H = 1 and 78 chemists in favor of O = 16. At the time, doubt was expressed as to whether a majority opinion could ever be accepted as final in such theoretical matters. As a result, the smaller ICAW continued publishing the annual tables on both scales until a consensus could be reached. This practice brought a reply in strong opposition from Ostwald .

The proponents for the H = 1 scale, such as Clarke, argued that it had the advantage of being Dalton's standard and it was the most natural basis for atomic weights because hydrogen is the lightest atom known and it was also the standard for gaseous densities. Teachers also argued that it is easily intelligible to beginners, whereas the oxygen standard was more difficult to explain.

The proponents for the oxygen scale argued that oxygen was the experimental reference standard. Every atomic weight was related to oxygen either directly or indirectly. Hydrogen was only a nominal unit with the actual determination of atomic weight referred to hydrogen through the ratio H/O. Every time that this fundamental base ratio, H/O, would be redetermined (see e.g., William Noyes' contribution59), the entire atomic weights table must be changed. Thus, this apparent historically conservative approach for the standard implied a wide-spread and radical change to all of the data. It thus became a debate between the teacher (theory) and the laboratory chemist (practice).

Beginning with the 1906 report, the ICAW used the O = 16 scale following a new survey of the larger committee. The final count was thirty one votes for O = 16, two votes for H = 1, nine votes for both scales and seventeen abstaining. Thus, the scale was settled for some thirty years, except for a brief discussion in 1920 on going back to the hydrogen scale. Beginning in the 1930s, when the neutron was discovered and the structure of nuclei was accepted to be a combination of protons and neutrons, H = 1 became a near impossible choice as a reference for atomic weights. The atomic number of heavy elements would not represent the number of nuclides in the nucleus in an H = 1 scale.

In 1929, the discovery of the two oxygen isotopes, 17O and 18O by Giauque and Johnston60 led to a situation in which the chemist's scale of O = 16 differed from the physicist's scale of 16O = 16. When Dole reported the variation in oxygen's atomic weight value in water versus air, this implied a variation in the isotopic composition of oxygen and the two scales took on a small but a variable difference. The ICAW briefly discussed the atomic weight standard in their 1932 report, where they considered 1H = 1, 4He = 4, 16O = 16 and O = 16 before choosing to follow Aston, who argued that the two scales satisfied everyone's requirement.

The variable scale difference was of great concern to Wichers and for a number of years he attempted to have the ICAW fix the difference between the two scales by definition. This would effectively define the isotopic composition of oxygen to be a particular value in nature. Failing with this solution, he solicited proposals for an alternate scale which would be acceptable to both the physics community as well as to the chemists worldwide.

In April 1957 at the bar in the Hotel Krasnapolski in Amsterdam, Nier suggested to Mattauch that the 12C = 12 mass scale be adopted because of carbon's use as a secondary standard in mass spectrometry . Also, 12C = 12 implied acceptable relative changes in the atomic weight scale, i.e., 42 parts-per-million (ppm) compared to 275 ppm for the 16O = 16 scale (which would not acceptable to chemists). Enthusiastically, Mattauch made a worldwide effort in the late 1950s to publicize the 12C = 12 scale and obtain the physicist's approval, while Wichers obtained the chemist's approval. (Mattauch was so anxious to tell the world of Nier's suggestion of the 12C mass scale as the resolution of the atomic weight dispute between chemists and physicists that he rushed off to the IUPAC General Assembly in Paris without his passport. After phoning Nier at the Max Planck Institute in Mainz, Mattauch had to spend the night at Trier on the German-French border, while he waited until Nier retrieved his passport from the Institute before he could continue on to the Commission meeting and unveil the solution ). Following the approval of the International Union of Pure and Applied Physics (IUPAP) General Assembly at Ottawa, Canada in 1960 and the IUPAC General Assembly at Montreal, Canada in 1961, the atomic weights were officially given on the 12C = 12 scale for the first time in the 1961 report . Mattauch and his colleagues combined data on direct nuclidic mass measurements with data on measured binding energies and beta decay energies derived from the masses to produce a consistent least squares fit of all nuclidic masses. This mass data was combined with the isotopic compositions to provide atomic weight values used in that 1961 Atomic Weight report.

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(Original post by

Carbon-12 is common, stable and easily measured in a mass spectrometer.

Just read through the history and development of the concept on wiki

https://en.wikipedia.org/wiki/Relative_atomic_mass

**charco**)Carbon-12 is common, stable and easily measured in a mass spectrometer.

Just read through the history and development of the concept on wiki

https://en.wikipedia.org/wiki/Relative_atomic_mass

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#13

(Original post by

I see. I'm grateful for your help, thank you.

**High Stakes**)I see. I'm grateful for your help, thank you.

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#14

(Original post by

But they said they assigned the mass of 12 rather than measured it. In the post i quotes earlier?

"his specific isotope of carbon actually has a mass of exactly 12.0000, it is assigned a mass of 12.0000"

Or do they mean it's around 12 but not exactly 12 and they gave it 12 for simplicity's sake?

**High Stakes**)But they said they assigned the mass of 12 rather than measured it. In the post i quotes earlier?

"his specific isotope of carbon actually has a mass of exactly 12.0000, it is assigned a mass of 12.0000"

Or do they mean it's around 12 but not exactly 12 and they gave it 12 for simplicity's sake?

You have decided to use a C12 atom as the reference. It has 12 nucleons in it's nucleus. Why did they assign it as 12.000 rather than 1.000?

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#15

**charco**)

It is not that this specific isotope of carbon actually has a mass of exactly 12.0000, it is

__assigned__a mass of 12.0000 so that it can act as the standard reference to which everything else is compared.

Prior to using carbon, both hydrogen and oxygen were used.

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#16

(Original post by

Yes, read again, I said 12.0000amu (or just u) which is the scale they used.

**RonnieRJ**)Yes, read again, I said 12.0000amu (or just u) which is the scale they used.

It's a trivial point, but one with potential confusion.

You said "Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000."

You did not say that it had been assigned this mass. This could lead people into thinking that carbon-12 has this magical number by coincidence.

It is important to know that carbon-12 is the reference used today and for that reason is given a simple number.

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#17

(Original post by

I have read again.

It's a trivial point, but one with potential confusion.

You said "Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000."

You did not say that it had been assigned this mass. This could lead people into thinking that carbon-12 has this magical number by coincidence.

It is important to know that carbon-12 is the reference used today and for that reason is given a simple number.

**charco**)I have read again.

It's a trivial point, but one with potential confusion.

You said "Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000."

You did not say that it had been assigned this mass. This could lead people into thinking that carbon-12 has this magical number by coincidence.

It is important to know that carbon-12 is the reference used today and for that reason is given a simple number.

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#18

(Original post by

My wording is self-explanatory. By saying "unified atomic unit" I imply it is just a unit of mass, not the actual mass, and I'm sure op understood

**RonnieRJ**)My wording is self-explanatory. By saying "unified atomic unit" I imply it is just a unit of mass, not the actual mass, and I'm sure op understood

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#19

(Original post by

I didn't understand what you were trying to say and I already knew the answer.

**BJack**)I didn't understand what you were trying to say and I already knew the answer.

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(Original post by

Maybe you should retake English GCSE then

**RonnieRJ**)Maybe you should retake English GCSE then

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