You are Here: Home

# Why do scientists always say 'relative atomic mass' when talking about a.... watch

1. (Original post by morecambebay)
The atomic weight isnt exactly 12 . The relative is.
Atomic weight and relative atomic mass are synonymous, and are both defined by the IUPAC as (to quote them):

"An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C."

Atomic mass (sans relative) is something different slightly, I think, although I wouldn't swear to that having not studied it in a while.
2. Noooooo that's the AVERAGE MASS of all of the carbon isotopes.

There are three naturally occurring isotopes:

12C
13C
14C

In other words, a naturally occurring sample of carbon has all three isotopes (in different amounts)

The internationally agreed reference is 12C so logically the average mass of a carbon atom is going to be a little higher... no?
3. (Original post by Miss Understood)
single atom or isotope?

If carbon - 12 has a mass of exactly 12. Why do scientists say carbon-12's mass is relative instead of exact.

Also, why do scientist refer to the word 'relative atomic mass' when they are just talking about 1 isotope/atom. Surely they should say the 'exact'.

Maybe I should just accept it....for now.
Its a measurement of atomic mass units.

In the periodic table for the mass of elements they write them in whole numbers.
The units for these whole numbers is the atomic scale unit 'u' which is used as writing down the mass of each element in Kg is quite messy.

1u = is roughly 1.66×10^ -27 Kg

We use atomic mass units as its easier to work with whole numbers than to work with multiples of 1.66x10^-27's

That isn't the true value of 1u, 1u is some hugely long number which we round to 2 significant figures to make our lives a lot easier, obviously this produces a small rounding error.

Carbon 12's mass is 12u roughly but it is not exactly that, there's a lot of rounding of numbers involved which eventually gives you some degree of error so they say carbon 12- mass is relative.

A proton/nuetron doesn't exactly = to 1u (is actually a tiny bit bigger)
So carbon 12 doesn't really have 12u mass it actually has a slightly bigger mass but we round it to make the numbers easier

Hope that helps, I'm coming from a Physics perspective, I have never taking A level chemistry and I don't plan too :P but mass numbers play quite a big role in nuclear energy/binding energies and particle physics so this should all be relevent.
4. (Original post by charco)
The mass compared to a carbon 12 isotope (which has an internationally agreed mass of 12 .0000000000 atomic mass units)

Okay, so how does the relative atomic mass help decipher the the mass number/no of electrons/protons of elements?

Why does Carbon - 12 have an exact mass of 12? How did they figure can from?

If Carbon -12 wasn't relative but exact would the mass in Kg?
5. I am loving this thread just for the fact it has got completely derailed and nobody even knows what the initial question was anymore.
6. (Original post by Phil1541)
Carbon 12's mass is 12u roughly but it is not exactly that, there's a lot of rounding of numbers involved which eventually gives you some degree of error so they say carbon 12- mass is relative.
.
This is simply not true.

It is the reference.

it is ridiculous to say that the reference is inaccurate. It makes a mockery of the idea of a reference.

True we can't measure anything with 100% accuracy, but that is another issue.

Philosophically a reference has to be 100% accurate.
7. (Original post by Miss Understood)
Okay, so how does the relative atomic mass help decipher the the mass number/no of electrons/protons of elements?
It doesn't, but then again it was never intended to do so...

(Original post by Miss Understood)
Why does Carbon - 12 have an exact mass of 12? How did they figure can from?
It was chosen by the higher beings in the IUPAC

Originally 'they' (or their predecessors) chose 16oxygen ( = 16.000) ...

and then 'those' that came after chose 1hydrogen ( = 1.000)..

but finally 'they' decided that a better choice would be carbon-12 for the simply reason that it's more common and can be determined very easily in a mass spectrometer.

(Original post by Miss Understood)

If Carbon -12 wasn't relative but exact would the mass in Kg?
???
8. (Original post by charco)
This is simply not true.

It is the reference.

it is ridiculous to say that the reference is inaccurate. It makes a mockery of the idea of a reference.

True we can't measure anything with 100% accuracy, but that is another issue.

Philosophically a reference has to be 100% accurate.
No I'm right....

Proton = 1.67x10-27kg
Is the same value for a nuetron.

Thats already a slight error due to rounding of numbers,

We say carbon 12 is equal of 6 nuetrons + 6 protons and we don't count electons as there mass is insignificant.

So already we know that carbon 12 has a larger mass than 12u, as 'u' = 1.66x10-27 while protons and nuetrons have masses of 1.67x10-27. (were not even including electrons masses here)

Its accurate enough but when you're working with mass energy equivelances you need to try and be as precise as possible as a small difference in mass gives a large difference in energy, but this is going into the realm of Physics and is a little off topic

Were both technically right as you never really need to work in such precise detail but I thought it would be useful for the OP to know
9. (Original post by Miss Understood)
Okay, so how does the relative atomic mass help decipher the the mass number/no of electrons/protons of elements?

Why does Carbon - 12 have an exact mass of 12? How did they figure can from?

If Carbon -12 wasn't relative but exact would the mass in Kg?
I feel a bit stupid asking this at this stage of the debate but what level of answer do you need - KS3, GCSE or A Level etc?
10. (Original post by Phil1541)
No I'm right....

Proton = 1.67x10-27kg
Is the same value for a nuetron.

Thats already a slight error due to rounding of numbers,

We say carbon 12 is equal of 6 nuetrons + 6 protons and we don't count electons as there mass is insignificant.

So already we know that carbon 12 has a larger mass than 12u, as 'u' = 1.66x10-27 while protons and nuetrons have masses of 1.67x10-27. (were not even including electrons masses here)
What you are saying is totally correct. the ACTUAL MASSES of the particles that make up the nucleus are not exact.

BUT, that is nothing to do with the concept of relative mass, which does not measure absolute values, it measures relative values.

On any relative scale the actual reference (particle mass, length, time etc) MUST be an integral value ..

.. unless defined otherwise.
11. (Original post by charco)
What you are saying is totally correct. the ACTUAL MASSES of the particles that make up the nucleus are not exact.

BUT, that is nothing to do with the concept of relative mass, which does not measure absolute values, it measures relative values.

On any relative scale the actual reference (particle mass, length, time etc) MUST be an integral value ..

.. unless defined otherwise.
Touche

I thought I'd got the relative idea wrong cheers for pointing that out to me
12. (Original post by js374)
I feel a bit stupid asking this at this stage of the debate but what level of answer do you need - KS3, GCSE or A Level etc?
A level. Everyone has confused me. I feel like I don't know anything anymore.

I basically wanted someone to explain what the AQA textbook says.

Relative atomic masses are measured on a scale on which the mass of an atom of Carbon-12 is exactly 12. No other isotope has a relative atomic mass which is exactly a whole number. This is because protons and neutrons do not have a relative masses of exactly 1

High resolution spectrometers can measure the masses of atoms to serval decimal places. This allows us to identify elements by the exact mass of their atoms, that apart from carbon 12 whose relative atomic mass is exactly 12, are not exactly whole numbers.

From the way the book has worded it, it seems to mean that the relative atomic mass of just one carbon-12 atom is exactly 12. I was wondering why they said relative atomic mass instead of exact mass.
What do they mean by 'exactly 12'?
Is the relative atomic mass, the average mass of isotopes of the same element?
How did scientists work out the mass number/atomic number?

I want to get an A in chemistry, at this rate my only hope is a U.
13. (Original post by Miss Understood)
single atom or isotope?

If carbon - 12 has a mass of exactly 12. Why do scientists say carbon-12's mass is relative instead of exact.

Also, why do scientist refer to the word 'relative atomic mass' when they are just talking about 1 isotope/atom. Surely they should say the 'exact'.

Maybe I should just accept it....for now.
They just say relative atomic mass because it's not really measured to any unit, you have to use a constant to turn it into grams or whatever, so that number is relative to the other atoms/molecules.

Btw, carbon isn't exactly 12, they round it up to make it easier.
14. (Original post by Miss Understood)
A level. Everyone has confused me. I feel like I don't know anything anymore.

I basically wanted someone to explain what the AQA textbook says.

Relative atomic masses are measured on a scale on which the mass of an atom of Carbon-12 is exactly 12. No other isotope has a relative atomic mass which is exactly a whole number. This is because protons and neutrons do not have a relative masses of exactly 1

High resolution spectrometers can measure the masses of atoms to serval decimal places. This allows us to identify elements by the exact mass of their atoms, that apart from carbon 12 whose relative atomic mass is exactly 12, are not exactly whole numbers.

From the way the book has worded it, it seems to mean that the relative atomic mass of just one carbon-12 atom is exactly 12. I was wondering why they said relative atomic mass instead of exact mass.
What do they mean by 'exactly 12'?
Is the relative atomic mass, the average mass of isotopes of the same element?
How did scientists work out the mass number/atomic number?

I want to get an A in chemistry, at this rate my only hope is a U.
When we use the phrase "relative atomic mass" this implies that the units we are using are "atomic mass units". Saying that the "exact mass" is 12, is meaningless because we haven't specified any units. Is it 12 kg, 12 stone, 12 Mini Coopers? 'Relative' doesn't mean it's not exact, it simply means that we are using atomic mass units.

As has been said, the definition of the atomic mass unit is 1/12 the mass of a Carbon-12 atom. That's why we talk about "relative" atomic mass, all we mean is the mass relative to 1/12 the mass of Carbon-12. So by definition, the mass of Carbon-12 is exactly 12.

Hope that helps.
15. You're getting a lot of conflicting information and a lot of it isn't right...

I'd just read this TBH http://en.wikipedia.org/wiki/Relative_atomic_mass
16. (Original post by Phil1541)
No I'm right....

Proton = 1.67x10-27kg
Is the same value for a nuetron.

Thats already a slight error due to rounding of numbers,

We say carbon 12 is equal of 6 nuetrons + 6 protons and we don't count electons as there mass is insignificant.

So already we know that carbon 12 has a larger mass than 12u, as 'u' = 1.66x10-27 while protons and nuetrons have masses of 1.67x10-27. (were not even including electrons masses here)

Its accurate enough but when you're working with mass energy equivelances you need to try and be as precise as possible as a small difference in mass gives a large difference in energy, but this is going into the realm of Physics and is a little off topic

Were both technically right as you never really need to work in such precise detail but I thought it would be useful for the OP to know
Going off topic a bit..., so OP please ignore:

Unless I misunderstand you (which is possible, it's very late!), I don't think this is right. The rest masses of protons and neutrons in an atom don't add up the rest mass of the atom because of the mass-energy equivalence you talk about. Work is done in binding the nucleons together, i.e potential energy of the system is lost. This is exhibited as loss of mass, hence the mass of bound nucleons is less than the sum of the individual nucleons. It is not, I believe, due to rounding errors in the definition of the atomic mass unit.
17. (Original post by Phil1541)
No I'm right....

Proton = 1.67x10-27kg
Is the same value for a nuetron.

Thats already a slight error due to rounding of numbers,

We say carbon 12 is equal of 6 nuetrons + 6 protons and we don't count electons as there mass is insignificant.

So already we know that carbon 12 has a larger mass than 12u, as 'u' = 1.66x10-27 while protons and nuetrons have masses of 1.67x10-27. (were not even including electrons masses here)

Its accurate enough but when you're working with mass energy equivelances you need to try and be as precise as possible as a small difference in mass gives a large difference in energy, but this is going into the realm of Physics and is a little off topic

Were both technically right as you never really need to work in such precise detail but I thought it would be useful for the OP to know
A carbon 12 atom weighs less then the invividual protons, neutrons and electrons that it is made from.

I'm sure Its exactly 12u. Well I was until I read this thread...

EDIT: Just checked my Physics notes. Just consider the Physics of it for a second.

Assume that the mass of Carbon-12 is 12.0000000u

A mass of an electron is 0.000549u
so the mass of 6 electrons is 0.033u

Therefore the mass of a carbon 12 nucleus is 12u - 0.033u = 11.9967u

Now consider the individual nucleons.
Mass of Proton = 1.00726u
Mass of Neutron = 1.00866u

So 6 of each = 12.0956u which is more then a carbon 12 nucleus as you quite rightly said.

However, the binding energy of Carbon-12 is -0.989u (in mass, not energy) which is exactly the binding energy of carbon-12 (look it up)

hence, carbon 12 is exactly 12u.

All the other RAMs are taken from this. For example Hydrogen is given as 1u, 1 twelth of the mass of carbon-12, it isn't however because of the difference in binding energies. The rest of the elements are not exact numbers of 'u's.
18. (Original post by Mbob)
When we use the phrase "relative atomic mass" this implies that the units we are using are "atomic mass units". Saying that the "exact mass" is 12, is meaningless because we haven't specified any units. Is it 12 kg, 12 stone, 12 Mini Coopers? 'Relative' doesn't mean it's not exact, it simply means that we are using atomic mass units.

As has been said, the definition of the atomic mass unit is 1/12 the mass of a Carbon-12 atom. That's why we talk about "relative" atomic mass, all we mean is the mass relative to 1/12 the mass of Carbon-12. So by definition, the mass of Carbon-12 is exactly 12.

Hope that helps.
Thanks, just to clarify a few things:

Why is Carbon-12 the sole measurement of relative atomic mass and not any other element?

How did scientist work out the relative atomic mass of every element and how did that soon correlate to working out the quantities of protons/neutrons in an atom?
19. (Original post by Joinedup)
You're getting a lot of conflicting information and a lot of it isn't right...

I'd just read this TBH http://en.wikipedia.org/wiki/Relative_atomic_mass
Most decent response so far.

Just to rap things up. Why does my AQA textbook say Relative atomic mass is defined on a scale on which C-12 is exactly 12?

How did chemists know that relative atomic mass can help find atomic no/mass no.
20. (Original post by Miss Understood)
A level. Everyone has confused me. I feel like I don't know anything anymore.

I basically wanted someone to explain what the AQA textbook says.

Relative atomic masses are measured on a scale on which the mass of an atom of Carbon-12 is exactly 12. No other isotope has a relative atomic mass which is exactly a whole number. This is because protons and neutrons do not have a relative masses of exactly 1

High resolution spectrometers can measure the masses of atoms to serval decimal places. This allows us to identify elements by the exact mass of their atoms, that apart from carbon 12 whose relative atomic mass is exactly 12, are not exactly whole numbers.

From the way the book has worded it, it seems to mean that the relative atomic mass of just one carbon-12 atom is exactly 12. I was wondering why they said relative atomic mass instead of exact mass.
What do they mean by 'exactly 12'?
Is the relative atomic mass, the average mass of isotopes of the same element?
How did scientists work out the mass number/atomic number?

I want to get an A in chemistry, at this rate my only hope is a U.
You win the award for clearest post of this thread. If you will forgive me for saying it, I think you may have got confused because the initial question you asked was a little ambiguous so the answers were fairly divergent.

The phrase relative is used because all of the masses used are 'relative' to the mass of carbon-12. The exact mass of an atom of carbon-12 would be ~1.396x10^-17 g. This is clearly a very tiny number and is a pain to work with so it is much simpler to use the relative system. Take water as an example, the relative mass is ~18* so this is ~18* times heavier than 1/12 of a carbon atom. The weight of everything is calculated relative to the carbon atom weight. Exactly 12 just means that there are no decimal places and it has not been rounded.

Relative atomic mass take into account isotopic differences. So while carbon-12 has a RAM of 12, carbon does not and has a RAM of 12.011 to take account of the isotopes.

How scientists worked out the numbers was a long programme of research by different scientists. Lavoisier, Dalton, Avagadro and Mendeleev are some of the more important people if you want to read up on it.

I hope that is clearer.

* I use "~" because it is not exactly 18 and but i can't be bothered to work it out/look it up but I also don't want Charco or Phil jumping down my throat about how it is not a whole number

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 4, 2011
The home of Results and Clearing

### 2,411

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Keele University
Sun, 19 Aug '18
2. University of Melbourne
Sun, 19 Aug '18
3. Sheffield Hallam University