Atomic mass and atomic mass unit (amu) are related but slightly different concepts.
Atomic mass refers to the mass of an atom of a particular element, usually expressed in atomic mass units (amu) or unified atomic mass units (u). It represents the average mass of all the isotopes of an element, taking into account their natural abundance.
On the other hand, an atomic mass unit (amu) is a unit of mass used to express the relative masses of atoms and molecules. It is defined as 1/12th the mass of a carbon-12 atom, which is approximately equal to the mass of a proton or a neutron. Therefore, the amu is a convenient scale for comparing the masses of different atoms.
The atomic mass unit (amu) is based on the atomic mass scale, which assigns a mass of exactly 12 amu to the carbon-12 isotope. Using this scale, the masses of other elements are determined relative to carbon-12. For example, an atom of oxygen-16 is approximately 16 times heavier than an atom of hydrogen-1, both in terms of amu.
In summary, atomic mass refers to the mass of an atom of an element, while the atomic mass unit (amu) is a unit of mass used to express the relative masses of atoms and molecules. The amu is based on the atomic mass scale, which assigns a mass of exactly 12 amu to the carbon-12 isotope.