Write the unit of mass used for atoms and its definition.

(N/A) The mass of an atom is very small compared to a kilogram $(kg)$,so the kilogram is not a convenient unit to measure such small quantities.
In nuclear physics,the unit used for atoms is the atomic mass unit $(amu)$,which is also denoted by the unified mass unit $(u)$.
Definition: The twelfth part of the mass of an unexcited carbon $(C^{12})$ atom is called $1$ $amu$,which is $1$ $u$.
The mass of an unexcited carbon atom is $1.992647 \times 10^{-26} \ kg$.
$1$ $u = \frac{\text{Mass of unexcited carbon atom}}{12} = \frac{1.992647 \times 10^{-26}}{12} \ kg = 0.166 \times 10^{-26} \ kg = 1.66 \times 10^{-27} \ kg$.
The atomic masses of various elements expressed in atomic mass unit $(u)$ are close to being integral multiples of the mass of a hydrogen atom. However,there are many striking exceptions to this rule.
Accurate measurement of atomic masses is carried out with mass spectrometers.