Why do we use carbon-12 (or any element) for relative atomic mass?

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Tarn Williamson
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I understand that the mass of carbon-12 has been defined 12 u. However, protons and neutrons have a relative mass of 1 each and there are 6 protons and 6 neutrons in a carbon-12 atom which makes a relative atomic mass of 12. So why do we need to compare? Oxygen has 8 protons and 8 neutrons, a relative atomic mass of 16. Why is it compared to carbon-12 to find its relative atomic mass? I just don't see why elements are compared when we can work out their relative atomic masses by using the number of protons and electrons? Am i completely missing something? I am sorry if i have not explained myself well.
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The Polymath
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(Original post by Tarn Williamson)
I understand that the mass of carbon-12 has been defined 12 u. However, protons and neutrons have a relative mass of 1 each and there are 6 protons and 6 neutrons in a carbon-12 atom which makes a relative atomic mass of 12. So why do we need to compare? Oxygen has 8 protons and 8 neutrons, a relative atomic mass of 16. Why is it compared to carbon-12 to find its relative atomic mass? I just don't see why elements are compared when we can work out their relative atomic masses by using the number of protons and electrons? Am i completely missing something? I am sorry if i have not explained myself well.
I also want to know, so if anyone knows could you quote me? Ta
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Cubic
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(Original post by The Polymath)
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(Original post by Tarn Williamson)
However, protons and neutrons have a relative mass of 1 each
I think it's because they have slightly different masses.
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natninja
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(Original post by Tarn Williamson)
I understand that the mass of carbon-12 has been defined 12 u. However, protons and neutrons have a relative mass of 1 each and there are 6 protons and 6 neutrons in a carbon-12 atom which makes a relative atomic mass of 12. So why do we need to compare? Oxygen has 8 protons and 8 neutrons, a relative atomic mass of 16. Why is it compared to carbon-12 to find its relative atomic mass? I just don't see why elements are compared when we can work out their relative atomic masses by using the number of protons and electrons? Am i completely missing something? I am sorry if i have not explained myself well.

(Original post by The Polymath)
I also want to know, so if anyone knows could you quote me? Ta
First you have to realize that neither a proton nor a neutron have a mass of 1u (both are very slightly heavier) and when a nucleus binds it becomes more stable and therefore it must lose energy. At this level mass and energy are interchangeable so the mass of the nucleus is less than that of it's constituent parts. Carbon is used as it is pretty much exactly 12u for the nucleus. Oxygen doesn't actually have an atomic mass of 16, if you look at a periodic table with masses to several decimal places you will see this. I hope that's sufficient.
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Si'sDoesn'tMatter
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According to the internets (Wiki and wiki.answers) it was changed to carbon-12 from oxygen-16. The reason being that although oxygen-16 was used, some people had difficulties as a result of there being multiple common isotopes of Oxygen (naturally occuring, and oxygen-16). I assume carbon-12 was the next most common, easily available element/isotope.

Hope that helps.
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Tarn Williamson
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Thanks for the replies guys So everything is literally compared to carbon-12? Surely there are other ways to work out the relative atomic masses? I accept that carbon-12 is given the relative atomic mass of 12 but other elements such as iron is given 56. I am seeing a link between the number of protons + neutrons and the relative atomic mass. The only way I can think of that makes it impossible to judge it on just these two particles is that something else has to be taken into account. Also i know that some elements have a relative atomic mass with a decimal but isn't this just because it is a weighted mean of all the isotopes?
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Cubic
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(Original post by Tarn Williamson)
Thanks for the replies guys So everything is literally compared to carbon-12? Surely there are other ways to work out the relative atomic masses? I accept that carbon-12 is given the relative atomic mass of 12 but other elements such as iron is given 56. I am seeing a link between the number of protons + neutrons and the relative atomic mass. The only way I can think of that makes it impossible to judge it on just these two particles is that something else has to be taken into account. Also i know that some elements have a relative atomic mass with a decimal but isn't this just because it is a weighted mean of all the isotopes?
Relative atomic mass is the avarage mass of an atom of an element divided by 1/12th the mass of an atom of Carbon 12.

The avarage mass of an atom of an element takes into account the number of protons and nuetrons, for all isotopes, and the abundancy of these isotopes.

Certain isotopes are more common than others and therefore their number of protons and nuetron contribute more towards the avarage than less common isotopes.

For example, if in a sample of boron, approximately 80% of the atoms are boron-11 and 20% are boron-10.

((80x11)+(20x10))/100 = 10.8

10.8 would be the AR based on this sample.
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Killjoy-
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(Original post by Tarn Williamson)
I understand that the mass of carbon-12 has been defined 12 u. However, protons and neutrons have a relative mass of 1 each and there are 6 protons and 6 neutrons in a carbon-12 atom which makes a relative atomic mass of 12. So why do we need to compare? Oxygen has 8 protons and 8 neutrons, a relative atomic mass of 16. Why is it compared to carbon-12 to find its relative atomic mass? I just don't see why elements are compared when we can work out their relative atomic masses by using the number of protons and electrons? Am i completely missing something? I am sorry if i have not explained myself well.
(Original post by The Polymath)
I also want to know, so if anyone knows could you quote me? Ta
From my understanding:

Relative atomic mass - the mass of a single atom of an element is experimentally measured using a mass spectrometer and then multiplied by the Avogadro number so that you get the actual mass of one mole.

Avogadro number is defined as 12g/ mass of one atom of C-12.
So relative atomic masses are relative in that they are all compared to C-12 by the Avogadro number.

Remember that neutrons and protons differ in mass by just over 0.1%(1).
There is also the issue of 'lost-mass' and binding energies (2).

1u is defined as 1/12th of the mass of an atom carbon-12.
You can give the mass of a neutron and a proton as a multiple of u.
You can say that the mass of 1 mole of H gas is about 1/12th the mass of 1 mole of carbon-12. Why you can do this is clear when you think about the number of nucleons.

Also, you need to take into account isotopic abundances when determining relative isotopic mass.(3)
On the periodic table all three of the things mentioned above are accounted for.

When it comes to compounds you just sum RAMs to get relative molecular mass or relative formula masses. Of course, this doesn't account for binding energy.

I hope what I've said above is helpful / makes sense
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username1107833
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(Original post by natninja)
First you have to realize that neither a proton nor a neutron have a mass of 1u (both are very slightly heavier) and when a nucleus binds it becomes more stable and therefore it must lose energy. At this level mass and energy are interchangeable so the mass of the nucleus is less than that of it's constituent parts. Carbon is used as it is pretty much exactly 12u for the nucleus. Oxygen doesn't actually have an atomic mass of 16, if you look at a periodic table with masses to several decimal places you will see this. I hope that's sufficient.
So the reason carbon-12 is used is because the actual mass of the nucleus is almost the same as multiplying the masses of the number of protons and neutrons it contains? All masses are therefore relative to carbon-12 - but is this just an estimate, so it isn't their exact atomic mass?? :confused:
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natninja
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(Original post by theCreator)
So the reason carbon-12 is used is because the actual mass of the nucleus is almost the same as multiplying the masses of the number of protons and neutrons it contains? All masses are therefore relative to carbon-12 - but is this just an estimate, so it isn't their exact atomic mass?? :confused:
It's arbitrary, an atomic mass unit is defined as a twelfth of the mass of C-12 much like the definition of the metre based on the speed of light or the absolute temperature scale
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username1107833
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(Original post by natninja)
It's arbitrary, an atomic mass unit is defined as a twelfth of the mass of C-12 much like the definition of the metre based on the speed of light or the absolute temperature scale
So the atomic masses in the periodic table are in fact the actual masses? It's a strange analogy but is it like saying that string is 2 metres long relative to the speed of light?
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natninja
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(Original post by theCreator)
So the atomic masses in the periodic table are in fact the actual masses? It's a strange analogy but is it like saying that string is 2 metres long relative to the speed of light?
Sort of - you need to multiply by a conversion constant (the number of moles) to get them out in units of mass because on the periodic table it is in grammes/mole.

It's not a strange analogy at all. A metre is defined (since 1983) as 'the distance travelled by light in 1/299792458 of a second.' With a second defined as 1/86400 of a mean solar day. (See where I'm coming from - all our systems of units come from measurement of a physical property and then arbitrarily defining everything else relative to that.)
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RoyalBlue7
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Well I think is a convention.

Once for my one of my school tests at IGCSE I defines RAM as the average mass of an atom of an element relative to the mass of a hydrogen atom (I wrote the isotope's name I guess, shame I can't remember it). My reasoning was that one twelfth of the C-12 isotope was the same as that hydrogen nucleus.

I guess its just a convention. WIkipedia says " The choice of carbon-12 was made to minimise further divergence with prior literature.[4] The new and current unit was referred to as the "unified atomic mass unit" u.[6] and given a new symbol "u," which replaced the now deprecated "amu" that had been connected to the old oxygen-based system."

So they didn't want to disagree. Science was natural philosophy but you need conventions.
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username1107833
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(Original post by natninja)
Sort of - you need to multiply by a conversion constant (the number of moles) to get them out in units of mass because on the periodic table it is in grammes/mole.

It's not a strange analogy at all. A metre is defined (since 1983) as 'the distance travelled by light in 1/299792458 of a second.' With a second defined as 1/86400 of a mean solar day. (See where I'm coming from - all our systems of units come from measurement of a physical property and then arbitrarily defining everything else relative to that.)
Ahh I finally get it! Thanks
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Siyar
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They have not clarified at all the question asked.They have clarified the factor why carbon has been chosen instead of oxygen.why we compare if already we know the no of neurtons and protons are masses.and we know neutrons and protons of all elements.In other way how we practically calculate /compare atomic mass with carbon.I also dont know the answer
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ZomgCeramicMoab
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There are multiple isotopes of carbon.
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charco
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(Original post by Siyar)
They have not clarified at all the question asked.They have clarified the factor why carbon has been chosen instead of oxygen.why we compare if already we know the no of neurtons and protons are masses.and we know neutrons and protons of all elements.In other way how we practically calculate /compare atomic mass with carbon.I also dont know the answer
Mass spectrometry can measure the actual masses of atoms to many decimal places.
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