What is the definition of atomic mass?

Answer 1

Atomic mass is defined as the weighted average mass of an atom's isotopes relative to 1/12 the mass of a carbon-12 atom. It is usually expressed in atomic mass units (amu). This value takes into account the relative abundance of each isotope of an element as well as their individual masses, providing a measure that reflects the average mass of an atom of that element as found in nature.