The term "average atomic mass" is needed to represent the weighted average mass of all the naturally occurring isotopes of an element. Most elements exist as a mixture of isotopes, which are atoms of the same element with different numbers of neutrons. These isotopes have different masses, but they occur in different proportions in nature.
The average atomic mass takes into account the mass of each isotope and its relative abundance in nature. It provides a more accurate and representative value of the mass of an element compared to using the mass of a single isotope. The average atomic mass is usually expressed in atomic mass units (amu) or unified atomic mass units (u).
Knowing the average atomic mass is important for various applications in chemistry and physics. It is used, for example, to calculate stoichiometry in chemical reactions, determine the molar mass of a substance, and interpret mass spectrometry data. The average atomic mass also plays a role in understanding the periodic table, as elements are ordered based on their atomic numbers and atomic masses.