# why “amu” is used as a unit in relative atomic mass? [duplicate]

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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"?

## marked as duplicate by Todd Minehardt, paracetamol, porphyrin, Wildcat, NotEvans.Jul 5 '17 at 18:29

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## 1 Answer

$$\pu{1 u} = \pu{1.660539040(20) \cdot 10^{-27} kg}$$

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.