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# Why is carbon-12 used as the standard measurment for the mass of atoms? watch

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1. Why is carbon-12 used as the standard measurment for the mass of atoms?
2. In the student monologue, the Ar of C-12 is an incredible 12.0000 something, but anyway, what matters is that it's the most perfect number, as most elements are messed up and dripping with loads of decimals... So, when you divide C12's 12.0000 by 12, you get a perfect 1.

A perfect 1 can't be achieved with almost all other elements.

What I feel that all you have to know, is that the Ar is just the bug number next to the element. But for the exam's sake, learn the actual definition: The average mass of an atom OVER 1/12th the mass of 1 atom of C-12.
3. i was JUST thinking this, good thinking janet!
4. (Original post by janet9)
In the student monologue, the Ar of C-12 is an incredible 12.0000 something, but anyway, what matters is that it's the most perfect number, as most elements are messed up and dripping with loads of decimals... So, when you divide C12's 12.0000 by 12, you get a perfect 1.

A perfect 1 can't be achieved with almost all other elements.

What I feel that all you have to know, is that the Ar is just the bug number next to the element. But for the exam's sake, learn the actual definition: The average mass of an atom OVER 1/12th the mass of 1 atom of C-12.
The Ar of C-12 is perfectly 12.0000 it's because C-12 is used as the standards and other elements are relative to C-12.

I would think that C-12 is choosen as the standard as it is one of the most stable atom/ most common/ first discovered or some reason like that.
5. It used to be hydrogen but physicists would use the relative atomic mass of hydrogen whereas chemists used H=1. This lead to inconsistencies so they said bugger it and chose carbon.
6. I think it was because it was easier to measure it's absolute mass being a solid opposed to the gaseous elements hydrogen and oxygen used before it.
7. As EVS says it's a practical thing - it's easier to do. For this reason it was adopted in 1961 by IUPAC as the international standard

The first mass tables were made using hydrogen as the standard (but they were all wrong as they were based on the supposition that water had the formula OH) by Dalton about 200 years ago.

Then oxygen was used as teh reference - very common, but unfortunately a mixture of isotopes whose % abundance varies according to source.

Then 16O was used.

Then with the advent of accurate mass spectrometers and the convenience of using volatile carbon compounds it was decided to adopt carbon as the international standard.
8. (Original post by dumbgeek)
Why is carbon-12 used as the standard measurment for the mass of atoms?
because an atom of carbon is exactly 12 on the scale and it is in a few relative mass definitions , so pretty important

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