ATOMIC WEIGHT

ratio of the average mass of a chemical element's atoms to some standard. Since 1961 the standard unit of atomic mass has been 1/12 the mass of an atom of the isotope carbon-12 (an isotope is one of two or more species of atoms of the same chemical element that have different atomic masses). The atomic weight of carbon is 12.011, the average that reflects the typical ratio of natural abundances of its isotopes. (See Table showing the chemical elements and their atomic weights. The concept of atomic weight is fundamental to chemistry, because most chemical reactions take place in accordance with simple numerical relationships among atoms. Since it is almost always impossible to count the atoms involved directly, chemists measure reactants and products by weighing and reach their conclusions through calculations involving atomic weights. The quest to determine the atomic weights of elements occupied the greatest chemists of the 19th and early 20th centuries. Their careful experimental work became the key to chemical science and technology. Reliable values for atomic weights serve an important purpose in a quite different way, when chemical commodities are bought and sold on the basis of the content of one or more specified constituents. The ores of expensive metals such as chromium or tantalum and the industrial chemical soda ash are examples. The content of the specified constituent must be determined by quantitative analysis. The computed worth of the material depends on the atomic-weight values used in the calculations.