# Why is atomic mass unit/unified atomic mass defined against Carbon 12?

I have searched the web all over this question and I have not really found an answer to my satisfaction. From my understanding of the topic so far, I understand that reason why concept of unified atomic mass works better than providing absolute mass of protons/neutrons (in grams or kilograms) is because "u" provides between clearer presentation of relative masses among elements in whole numbers than the negative exponents would in regards to absolute mass. If this premise is true than why not define atomic mass unit in terms of multiple of protons and neutron? Like since proton and neutron are roughly equal in mass ~ 1.66* 10^-24g. Why not say 1u = mass of one proton and 1 neutron. Why does the definition of amu need to include 1/12 of carbon 12, because mass of 1/12 of carbon gives me mass of 1 proton 1 neutrons and this seems redundant to me?

But now if we accept the definition of amu as 1/12 of carbon 12? Why not include other elements like amu is 1/1 of Hydrogen atom, 1/4 of helium atom, 1/16 of oxygen atom, because all this would would give me same value of 1u= 1.66*10^-24g ? If anyone can give their input on this, that would be grea.

• Saying "roughly equal" doesn't cut it, it needs to be really, really, really, really, really precise. Apr 11 '21 at 0:36
• Why 12C? — see Why is the definition of the mole as it is? Apr 11 '21 at 0:37
• This will be a very long answer but the good news is that it has been addressed in detail in the International Journal of Mass Spectrometry and Ion Processes, 86 (1988) 1-19. EVOLUTION OF THE UNIFIED SCALE OF ATOMIC MASS, ‘*C =12u *. Apr 11 '21 at 2:12
• Why is/was the standard for a kilogram a metal cylinder under triple bell jars in Paris?
– Zhe
Apr 11 '21 at 3:11
• Be aware that the mass of nucleus does not depend only on the nucleon count, but also on total bounding energy. E.g. the mass of 4He is 0.7% less than of 2p+2n. I rather wonder, why was not chosen an element with a single isotope. Probably because new values would be more different from those based on 16O. Apr 11 '21 at 4:48