The SI units used for the masses of atomic particles are kilograms (kg) or grams (g). However, due to the incredibly small mass of atomic particles, it is more convenient to use atomic mass units (amu) or unified atomic mass units (u).

1 amu or 1 u is defined as exactly 1/12th of the mass of a carbon-12 atom. This means that a carbon-12 atom has a mass of exactly 12 amu or 12 u.

The masses of other atomic particles, such as protons, neutrons, and electrons, are expressed in terms of amu or u relative to the mass of a carbon-12 atom. For example, a proton has a mass of approximately 1.0073 amu or 1.0073 u, and an electron has a mass of approximately 0.00054858 amu or 0.00054858 u.

Using amu or u allows scientists to express the masses of atomic particles in convenient and manageable units, given their extremely small size.