Why Does a Unified Atomic Mass Unit Differ from Calculated Values?

AI Thread Summary
The unified atomic mass unit is defined as 1/12 of the mass of a carbon-12 isotope, equating to 1.66 x 10^-27 kg, but calculations using Avogadro's constant can yield slightly different results due to rounding and measurement uncertainties. The mass difference between protons and neutrons arises from their quark composition; protons contain two up quarks and one down quark, while neutrons have two down quarks and one up quark, with down quarks being more massive. A calculation for the mass of a carbon-12 atom using 0.012/6.02 x 10^23 may lead to discrepancies in results, highlighting potential errors in calculation or interpretation. The discussion emphasizes the complexities of atomic mass definitions and the fundamental differences in particle composition. Understanding these concepts is crucial for accurate atomic mass calculations.
al_201314
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Hi guys

I understand that the unified atomic mass unit is defined as 1/12 of the mass of a carbon-12 isotope and has a value of 1.66 X 10^-27kg. Why is this value slightly different with when I use the calculation 0.012/6.02 X10^23?

Why in theory, is the mass of a protn different from a neutron?

Thanks!
 
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oh dear I realized what I missed out.. but I'd appreciate any help regarding the difference in mass of a proton and a neutron.

I have one more question, given the avagadro's constant, I was told to find the mass of an atom of carbon-12. I took 0.012/6.02X10^23 which gives me 1.99 X10^-26kg but the answer I was given was ^-25. Was there a mistake on my part or the answer is wrong?

Thanks!
 
Last edited:
al_201314 said:
Why in theory, is the mass of a protn different from a neutron?

The simple "hand-waving" answer is that protons and neutrons contain different quarks which have different masses. A proton is composed of two up quarks and a down quark. Wheras neutrons consist of two down quarks and an up quark. The exact mass of these quarks are unknow but it is commonly accepted that a down quark is more massive than an up quark.

~H
 

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