For an atom of an element the average mass of the atom is compared to   $\frac{1}{12}$ th of the mass of one C – 12 atom is

The atomic mass of an element is taken as the no. of times of $\text{1}/{\text{12}}^{th}$ of the mass of C-12 isotope.

Ex: Reactive Atomic mass of oxygen is  $\frac{1}{12}\times 12\text{\hspace{0.17em}amu}\times \text{16 =16 amu}$.