Every element has a unique atomic weight because the atomic weight, also known as the atomic mass, is determined by the composition of isotopes within that element.
Isotopes are atoms of the same element that have different numbers of neutrons in their atomic nuclei. While isotopes of an element share the same number of protons (which defines the element), their differing numbers of neutrons result in variations in atomic mass.
The atomic weight of an element is calculated as the weighted average of the masses of all its naturally occurring isotopes, taking into account their relative abundances in a given sample. The calculation involves multiplying the mass of each isotope by its abundance and summing the contributions of all isotopes.
For example, let's consider carbon, which has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 is the most abundant isotope, carbon-13 is less abundant, and carbon-14 is the least abundant. The atomic weight of carbon is calculated by taking into account the masses and abundances of these isotopes.
The atomic weight of an element is usually reported in atomic mass units (u) or grams per mole (g/mol). It provides a standard measure of the average mass of all atoms of that element, accounting for the different isotopes present.
The unique atomic weight of each element allows scientists to precisely quantify the mass of substances in chemical reactions, determine stoichiometry (the ratio of elements in compounds), and conduct various analytical and theoretical calculations in chemistry and physics. It is an essential characteristic that helps distinguish elements and allows for accurate scientific calculations and measurements.