Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: habbababba on January 26, 2015, 02:40:57 PM
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Hi!
I would like to get a clarification on the following:
The atomic mass (I do not want to use the term relative yet) is determined using the carbon - 12 scale in which carbon - 12 is arbitrarily set to have a mass of exactly 12. From this definition, it follows that all masses of all atoms must be relative, of course, to carbon - 12. I get it. This is where the term relative stems from. Good!
Now my first question is: is there such thing as an absolute mass of an atom? If not, does that mean that a carbon - 12 atom doesn't actually necessarily have 6 protons and 6 neutrons? Or, are these numbers (6 protons and 6 neutrons) arbitrarily assigned to a carbon - 12 atom?
Another thing: using the same scale, we set the mass of a proton to be equal to 1 u and that of a neutron to be equal also to 1 u. HOWEVER, The rest mass of a proton is 1.007276 u, and the rest mass of a neutron is 1.008665 u. How were these numbers determined? Are they relative to something? If yes, to what? 2 different values for the mass of the same subatomic particle would mean that there are 2 different mass scales?
Your contribution for clarification is appreciated.
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Hi!
I would like to get a clarification on the following:
The atomic mass (I do not want to use the term relative yet) is determined using the carbon - 12 scale in which carbon - 12 is arbitrarily set to have a mass of exactly 12. From this definition, it follows that all masses of all atoms must be relative, of course, to carbon - 12. I get it. This is where the term relative stems from. Good!
Now my first question is: is there such thing as an absolute mass of an atom? If not, does that mean that a carbon - 12 atom doesn't actually necessarily have 6 protons and 6 neutrons? Or, are these numbers (6 protons and 6 neutrons) arbitrarily assigned to a carbon - 12 atom?
Yes, it so happens that there are an Avogrado's number of carbon-12 atoms in a mass of 12 grams. 6 protons in a single atom will always mean carbon, and an additional 6 neutrons will specify the carbon-12 isotope.
Another thing: using the same scale, we set the mass of a proton to be equal to 1 u and that of a neutron to be equal also to 1 u. HOWEVER, The rest mass of a proton is 1.007276 u, and the rest mass of a neutron is 1.008665 u. How were these numbers determined? Are they relative to something? If yes, to what? 2 different values for the mass of the same subatomic particle would mean that there are 2 different mass scales?
No, these are measured to the same scale. There are two things at play here.
- There are also electrons in carbon-12
- Look into binding energy
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Thanks for the reply.
One more question: I understand that the number 12 is arbitrarily assigned to carbon - 12. However, why 12? Is there a special reason behind the numerical value of this number? I mean why not 10?
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@sjb said
6 protons in a single atom will always mean carbon, and an additional 6 neutrons will specify the carbon-12 isotope.
Further reading
http://www.livescience.com/37206-atom-definition.html
http://en.wikipedia.org/wiki/Isotopes_of_carbon
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I am a bit confused. every unit system is arbitrary...? and all physical quantities are given in an arbitrary unit system. it's just the question how well-defined and reproducible the refrences are.
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it's just the question how well-defined and reproducible the refrences are.
Can you please expand on that?
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e.g. what's a second? it's an absolutely arbitrary amount of time. how do you want do define it? see here http://physics.nist.gov/cuu/Units/second.html
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I understand that the number 12 is arbitrarily assigned to carbon - 12. However, why 12? Is there a special reason behind the numerical value of this number? I mean why not 10?
No, 12 is not arbitrary. The nucleus of an atom of carbon 12 contains exactly 6 protons and 6 neutrons, and all isotopes of carbon contain exactly 6 protons. They are discrete particles, and their quantity is what makes carbon carbon and is ultimately responsible for all of it's chemical properties.
What is somewhat arbitrary, and might be causing confusion, was the decision to base Avogadro's number on the number of atoms of carbon in 12 grams of carbon.
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@calmsea == just to do some stirring of the pot
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The atomic mass ... is there such thing as an absolute mass of an atom... {and so on}
Stirring more
gram, stone, pound, ounce, dram, grain, pennyweight, carat, mite, doite, scruple, talent, mina, shekel, pim, beka, gerah
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No, 12 is not arbitrary. The nucleus of an atom of carbon 12 contains exactly 6 protons and 6 neutrons, and all isotopes of carbon contain exactly 6 protons.
First off, how is the number of protons and neutrons in a nucleus experimentally determined? From what you said, it sounds to me like there is such thing as absolute mass of an atom.
If so, then why make all the masses relative to carbon - 12? Why not figure out the masses of all the other atoms by also counting their protons and neutrons in their nuclei? (If, of course, such a way is possible).
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http://en.wikipedia.org/wiki/Atomic_mass
http://en.wikipedia.org/wiki/Atomic_mass_unit
http://en.wikipedia.org/wiki/Molecular_mass
May be of interest.
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Also this may be of interest
http://www.sizes.com/units/atomic_mass_unit.htm
History of the atomic mass unit
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1. Mass of an atom doesn't equal sum of masses of its components, google "mass deficit".
2. It is much more convenient to express mass of an atom as 23.0 amu, than as 3.82×10-27 kg (even if these are perfectly equivalent).
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1. Mass of an atom doesn't equal sum of masses of its components, google "mass deficit".
2. It is much more convenient to express mass of an atom as 23.0 amu, than as 3.82×10-27 kg (even if these are perfectly equivalent).
Thanks for the input. I get all that. My only query is why '12'? Why not 10? Is there a specific convenience behind choosing the number 12 as the standard?
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From the links provided so far in this thread one would guess they wanted something that exists in nature and is stable. Carbon 10 has a half life less than 20 seconds.
And, another link says there was a discussion about using Oxygen 16, but Carbon 12 won out.
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From the links provided so far in this thread one would guess they wanted something that exists in nature and is stable. Carbon 10 has a half life less than 20 seconds.
And, another link says there was a discussion about using Oxygen 16, but Carbon 12 won out.
From the links we can also deduce that the number 12 is assigned by definition or, in other words, it's arbitrarily chosen (more on that later). So the point here is that carbon-12 is not '12' unless we assign that for it and as a result of applying the previous premise, carbon-10 will turn out to have a mass equal to 10 relative to carbon-12.
From the last link you had posted though, I've come to another conclusion and it may explain why the number 12 was chosen as the standard rather than another number. "Making the amu one-twelfth the mass of a carbon-12 nucleus, however, would lead to only a 42 parts per million change, which seemed within reason." The number 12 did not cause a significant change in the calculations held by chemists and so it appears to me that the number 12 after all does have a specific significance and isn't simply thrown completely at random into the discussion. Am I wrong about this?
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12 is a number of nucleons in C-12 nucleus, which definitely was part of the decision. But most of additional arguments behind were simply practical - to make calculations easier.