One atomic mass unit is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom. Carbon-12 is the carbon isotope that has six protons and six neutrons.
A single atom's atomic mass is just its total mass, which is usually given in atomic mass units, or amu. An atom of carbon with six neutrons, carbon-12, has an atomic mass of 12 amu by definition.
An atomic mass unit (abbreviated as AMU or amu) is defined as 1/12 the mass of a carbon-12 atom. The nucleus of the carbon-12 (C-12) atom contains six protons and six neutrons. One AMU is the average of the proton and neutron rest masses, in approximate words.
This equates to about 1.67377 x 10 -27 kilogram (kg) or 1.67377 x 10 -24 gram (g). In AMU, an atom's mass is generally equal to the nucleus' total amount of protons and neutrons.
The AMU is used to express and differentiate between various isotopes of elements by expressing their relative masses. Thus, uranium-235 (U-235) has an AMU of roughly 235, but uranium-238 (U-238) has a little higher AMU. The disparity is due to the fact that U-238, the most common naturally occurring uranium isotope, contains three more neutrons than U-235, which has been utilized in nuclear reactors and atomic bombs.