Question

atomic mass unit is____.

Answer 1

1 atomic mass unit (amu), or simply 1 unified atomic mass unit (u), is defined as one twelfth ($\frac{1}{12}$) of the mass of an atom of carbon-12.
The value, in terms of kilograms, is approximately:
 $1 \text{ amu (or u)} \approx 1.660539040(20) \times 10^{-27} \text{ kg}$  
This unit is used to express the weights of atoms and molecules.