What is the definition of atomic mass?

Atomic mass is defined as the average mass of an element's isotopes, weighted by their natural abundance. This means it takes into account the different isotopes of an element, which have varying numbers of neutrons, and reflects how common each isotope is in nature.

This average is expressed in atomic mass units (amu) and allows scientists to compare the mass of different atoms effectively. While one might think of atomic mass in terms of its relationship to protons and neutrons, simply counting these particles does not provide a complete or accurate representation of an element's atomic mass because it does not factor in isotope distribution. By using the average mass, we can more precisely understand the element's behavior in chemical reactions and the formation of compounds.

Therefore, the correct answer encompasses both the concept of averaging and the isotopic nature of elements, which distinguishes it from alternative definitions that only consider the total number of particles or give a specific weight, which lacks the nuance of isotopic abundance.