# Atomic Mass

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One unified atomic mass unit (u) is one-twelfth the mass of the carbon-12 atom.

\begin{aligned} \, u & = \frac{1}{12} \left( \frac{mass \, of \, 1 \, mole \, of \, carbon \, – 12}{avogadro \, ‘s \, number} \right) \\ & = \frac{1}{12} \left( \frac{0.012 \, kg}{6.02 \times 10^{23}} \right) \\ & = 1.6605 \times 10^{-27} \, kg \end{aligned}

The relative atomic mass (mr) of an atom is the ratio of the mass of the atom to the unified atomic mass unit.

$m_{r} = \frac{mass \, of \, atom}{\frac{1}{12} \, the \, mass \, of \, a \, carbon \, – 12 \, atom}$

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