Atomic weight is a measure of the average mass of an atom of an element, expressed in atomic mass units (amu). It takes into account the relative abundance of different isotopes of the element, which are atoms with the same number of protons but different numbers of neutrons. For example, the atomic weight of carbon is approximately 12.01 amu, reflecting the presence of both carbon-12 and carbon-13 isotopes. The atomic weight is important in chemistry because it helps predict how elements will react with one another. It is used in calculations involving molecular weight and stoichiometry, which is the study of the quantities of reactants and products in chemical reactions. Understanding atomic weight allows scientists to better comprehend the behavior of elements in various chemical processes.