(a) Atomic weight of an element is the ratio of the average mass of a chemical element’s atoms to one-twelfth the mass of an atom of the isotope carbon-12, which is accepted as the standard. (b) Atomic weights can be referred to as relative numbers as they are based on a standard agreed on throughout the world. Many years ago, the standard agreed on was the mass of oxygen atom. Later on the standard was changed to be the atom of the carbon-12 isotope.
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