Isotopic (fractional) abundance is the natural proportion, expressed as a fraction, in which a particular isotope of an element occurs. These abundances are needed to calculate an element's atomic weight, since that value is the average of the isotope masses weighted by abundance. For example, chlorine occurs mainly as two isotopes whose abundances combine to give its non-integer atomic weight.

同位素丰度是某一元素的特定同位素在自然界中出现的比例,以分数表示。计算元素的原子量需要这些丰度,因为原子量是各同位素质量按丰度加权的平均值。例如,氯主要以两种同位素存在,其丰度共同决定了它非整数的原子量。