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# why isn't the weight of hydrogen the sum of its parts?

Posted on 2010-01-13
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Hi

If you measure protons and neutrons and electrons in atomic mass units the weights are:
protons = 1.007277
neutrons = 1.008665
electrons =  0.000548

Normally atoms are less than the sum of their parts because of binding energy in the nucleus. Hydrogen has no binding energy in the nucleus so it should be the sum of its parts. Its actually higher at  1.00794 (1.007277  +  0.000548  = 1.007825)

Why is this?

thanks
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Question by:andieje
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Expert Comment

ID: 26302736
If you got the weight of Hydrogen from a periodic table, that weight is from both 1H and 2H (deuterium). This makes the atomic mass of naturally ocurring Hydrogen slightly higher than the 1 you might expect. Also, for some reason, the isotopic mass of 1H is larger than 1 (but closer that the mixture of 1H and 2H).
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Expert Comment

ID: 26302759
Also, you were right about weights in binding energies, according to http://en.wikipedia.org/wiki/Atomic_mass_unit in your other question :)
Quantum-level stuff gets weird!
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Author Comment

ID: 26302798
Hi

This is just the weight of H1 and not the relative atomic mass of H1 and H2

So the question still stands i believe :)
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Accepted Solution

Discusfish earned 1000 total points
ID: 26302846
Actually,
normal H standard atomic mass (a mixture of 1H and 2H) = 1.00794
1H = 1.00782503207
You'll see 1H is very close to being 1. The discrepancy between 1H and 12C creeps in because of the binding energy.
http://en.wikipedia.org/wiki/Isotopes_of_hydrogen#Table
(The isotope number should come before the element, not after, and be in superscript if possible).

Good luck getting through the tricky effects of quantum scale stuff on real world things to your 18-year-old.... :)
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LVL 6

Expert Comment

ID: 26302878
I'm now very intrigued by this... Wish I still knew some postgrad chem people :/
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LVL 6

Expert Comment

ID: 26302900
According to http://www.school-for-champions.com/chemistry/atomic_weight.htm
electron mass isn't taken into account, which would go some way towards this discrepancy, perhaps?
Also, the mass defect of the binding energy is not linear. 12C is where everything is defined from - everything else will therefore wander from this idealised mass.
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LVL 6

Expert Comment

ID: 26302910
I assume then that in 1H, no binding energy defect == slightly heavier than you'd expect.
http://www.school-for-champions.com/science/atomic_weight.htm is also quite interesting.
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Author Comment

ID: 26305091
1H should be the some of is constituent parts, not heavier than the sum of its parts.
If electrons aren't taken into account then the problem is 'worse' not better because 1H is heavier without taking the electron weight into account.
1H should be 1.007277 if weight only includes electrons
1h should be  (1.007277  +  0.000548  = 1.007825) if weight is electron + proton

Where did you get the figures in this post from:
13/01/10 12:32 PM, ID: 26302846

If they are correct then 1H is the sum of its parts and there is no question. My figure for  1.00794 must be wrong
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Author Comment

ID: 26305186
My data was incorrect. Never trust anything on the web!
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Author Closing Comment

ID: 31676603
although this did not provide the answer I am awarding points because the data in this post allowed me to work out the answer
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