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

1. 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.
2. (Original post by morecambebay)
Carbon 12 doesnt have a mass of exactly 12.

Lets say for the sake of example that a hydrogen weighs 1.289989434, and a carbon weighs 12.084858
When you are using them to compare the weights of other things, it is more accurate to use carbon as a comparison because the decimal is smaller.
Okay, so why does everyone keep saying Carbon-12 is exactly 12 and that no other isotope has a relative atomic mass (<--the term 'relative atomic mass' being used again') which is exactly a whole number because protons and neutrons do not have a relative atomic mass of exactly 1.
3. (Original post by Miss Understood)
Okay, so why does everyone keep saying Carbon-12 is exactly 12 and that no other isotope has a relative atomic mass (<--the term 'relative atomic mass' being used again') which is exactly a whole number because protons and neutrons do not have a relative atomic mass of exactly 1.
Everyone like who? it isnt exact, it is just the one whose decimal causes the least trouble.

Edit: Hold on, the relative atomic mass will be exactly 12 because if you divide the exact number by 12 (to get the comparison) then see how many times it fits into the origonal, you will get 12. The actual mass isnt though.
4. (Original post by Miss Understood)
Okay, so why does everyone keep saying Carbon-12 is exactly 12 and that no other isotope has a relative atomic mass (<--the term 'relative atomic mass' being used again') which is exactly a whole number because protons and neutrons do not have a relative atomic mass of exactly 1.
It is the reference. By definition it has to be a whole number integer (read my other post)
5. (Original post by morecambebay)
Everyone like who? it isnt exact, it is just the one whose decimal causes the least trouble.
My AQA Chemistry textbook. If carbon isn't slap bang exact, they should really say so should they.

6. (Original post by Miss Understood)
X
(Original post by morecambebay)
X

This conversation is very confusing as neither of you are using units for your numbers. Carbon-12 is exactly 12 what?!? Apples? Oranges? Lebanese dancing girls? You get the idea!

I assume you mean that Carbon-12 is 12g/mol.

Where the hell did "1.289989434" come from?
7. (Original post by Miss Understood)
My AQA Chemistry textbook. If carbon isn't slap bang exact, they should really say so should they.

AQA are nutters - screw them

they make all sorts of mistakes
8. (Original post by js374)
This conversation is very confusing as neither of you are using units for your numbers. Carbon-12 is exactly 12 what?!? Apples? Oranges? Lebanese dancing girls? You get the idea!

I assume you mean that Carbon-12 is 12g/mol.

Where the hell did "1.289989434" come from?
it has no units...

Ar of carbon is 12. No units.
9. relative to 1/12th the weight of carbon twelve which is near enough 1. People can round slightly because the numbers we are talking about here are minuscule.
10. (Original post by js374)
This conversation is very confusing as neither of you are using units for your numbers. Carbon-12 is exactly 12 what?!? Apples? Oranges? Lebanese dancing girls? You get the idea!

I assume you mean that Carbon-12 is 12g/mol.

Where the hell did "1.289989434" come from?
^^^^FAIL^^^^
11. (Original post by fwed1)
relative to 1/12th the weight of carbon twelve which is near enough 1. People can round slightly because the numbers we are talking about here are minuscule.
It's not "near enough 12" it's EXACTLY 12.0000000 (as many dp (not rude) as you wish to go !!!!)

It's the reference ....

IT IS ASSIGNED A VALUE OF EXACTLY 12.00000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000
12. (Original post by js374)
This conversation is very confusing as neither of you are using units for your numbers. Carbon-12 is exactly 12 what?!? Apples? Oranges? Lebanese dancing girls? You get the idea!

I assume you mean that Carbon-12 is 12g/mol.

Where the hell did "1.289989434" come from?
I was pointing out why carbon is used instead of hydrogen, and I said 'for the sake of example'.
13. (Original post by charco)
It's not "near enough 12" it's EXACTLY 12.0000000 (as many dp (not rude) as you wish to go !!!!)

It's the reference ....

IT IS ASSIGNED A VALUE OF EXACTLY 12.00000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000
I thought it was but the OP suggested different lol, i'm not very good at chemistry. If it is exactly 12 (which makes sense) i don't see the problem/point of this thread.
14. I'm confused....
15. (Original post by fwed1)
I thought it was but the OP suggested different lol, i'm not very good at chemistry. If it is exactly 12 (which makes sense) i don't see the problem/point of this thread.
Don' worry. the OP just needs to get things a little clearer in his/her mind..
16. (Original post by morecambebay)
Everyone like who? it isnt exact, it is just the one whose decimal causes the least trouble.

Edit: Hold on, the relative atomic mass will be exactly 12 because if you divide the exact number by 12 (to get the comparison) then see how many times it fits into the origonal, you will get 12. The actual mass isnt though.
Carbon 12 is what's used to define it, so carbon is exact. A mole is literally defined as the number of atoms in 12g of carbon 12. All other relative atomic weights (which is what the relative atomic mass is - atomic mass is a different thing) are defined relative to carbon 12.
17. (Original post by Miss Understood)
I'm confused....
The exact (atomic) weight of carbon is 12 point something. when you take this 12 point something and divide it by 12 to get a comparison (relative) amount...that amount will fit into 12 point something exactly 12 times.

I dont know what you are neg repping. ^ This is right.
18. (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.
It's relative to the mass of a single carbon-12 atom.
19. (Original post by Concept186)
Carbon 12 is what's used to define it, so carbon is exact. A mole is literally defined as the number of atoms in 12g of carbon 12. All other relative atomic weights (which is what the relative atomic mass is - atomic mass is a different thing) are defined relative to carbon 12.
Did I not write that in the bit you quoted?
20. (Original post by dentistry1)
^^^^FAIL^^^^
I think that is a little harsh. I must admit I am now unsure as to what the OP is asking. Initially I interpreted the question one way and gave an answer on that aspect. It would appear that other people have interpreted her question differently.

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