Atomic and nuclear masses

Nuclear masses, expressed in kilograms, are inconveniently tiny numbers (of order $10^{-25}$ kg or smaller), so it is far more practical to use a specially defined unit called the atomic mass unit (u). One atomic mass unit is defined as exactly $1/12$ of the mass of a single atom of the most abundant naturally-occurring isotope of carbon, $^{12}_{6}C$:

In this unit, the standard particle and atom masses are: mass of neutron $=1.008665$ u, mass of proton $=1.007276$ u, mass of hydrogen atom $=1.007825$ u, and by definition the mass of $^{12}_{6}C$ is exactly $12$ u. It is important to note that the values usually quoted are atomic masses (nucleus plus its full complement of orbiting electrons), not bare nuclear masses; to get the mass of the nucleus alone, the mass of the atom's $Z$ electrons must be subtracted from the atomic mass. …