Unified atomic mass unit (amu) is defined as $1/12$ of the mass of a Carbon-12. I want to know is this amu equal to $\pu{1.99e-23 g}$ and why do we use amu which is $1/12$ the mass of carbon? Why use "amu"?
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$\begingroup$ What's easiest to compare, 12 and 15.999, or 1.993E-23 and 2.657E-23? $\endgroup$– OrangeDogCommented Jul 5, 2017 at 13:49
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$\begingroup$ chemistry.stackexchange.com/questions/32243/… $\endgroup$– MithoronCommented Jul 5, 2017 at 13:58
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1 Answer
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$$ \pu{1 u} = \pu{1.660539040(20) \cdot 10^{-27} kg} $$
The u (amu is the old unit name) is $1/12$ of the weight of an $^{12}\ce C$ atom.
The way the u is chosen ensures that all core and atom masses are multiples of 1($\pm $0.1) u. The u is a count of nucleons in an atom.
Until the 60s, chemists used $1/16$ of the mass of a average atom in a oxygen-isotope-mixture and physicist used $1/16$ of the mass of a $^{16}\ce O$ atom.
It's really just about choosing what is the best to measure and have a consistent result with. So you would chose an element with a very consistent isotope ratio (isotope-pure $^{12}\ce C$ e.g.) and one where you can measure weights easily.