The mass of an atom is very small compared to a kilogram, kilogram is not a very convenient unit to measure such small quantities.

In nuclear physics, the unit of atoms is taken as amu means atomic mass unit, it is also denoted by unified mass unit $(u).$

Definition : The twelfth part of the mass of unexcited carbon $\left(C^{12}\right)$ atom is called $1$ amu that is $1 \mathrm{u}$.

The mass of unexcited carbon atom is $1.992647 \times 10^{-26} \mathrm{~kg}$

$1\mathrm{u}=\frac{\text { Mass of unexcited carbon atom }}{12}$ $=\frac{1.992647 \times 10^{-26}}{12} \mathrm{~kg}$ $=0.166 \times 10^{-26} \mathrm{~kg}$ $1 \mathrm{u}=1.66 \times 10^{-27} \mathrm{~kg}$

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 a mass spectrometers.