定义Definition
相对同位素质量是某一同位素一个原子的质量与碳-12 原子质量的 1/12 之比，没有单位。它非常接近该同位素的质量数，但一般不是整数，如 ³⁵Cl 为 34.969，³⁷Cl 为 36.966。
Relative isotopic mass is the mass of one atom of an isotope divided by one-twelfth of the mass of a carbon-12 atom; it has no unit. It is very close to the isotope's mass number but usually not a whole number: 34.969 for ³⁵Cl and 36.966 for ³⁷Cl.
与相对原子质量的区别Not the same as relative atomic mass
相对同位素质量针对单一同位素；相对原子质量是元素各同位素相对同位素质量按丰度的加权平均值。例如 Ar(Cl) ≈ 34.969 × 0.758 + 36.966 × 0.242 ≈ 35.45。
Relative isotopic mass refers to one isotope; relative atomic mass is the abundance-weighted average of an element's isotopes. For chlorine, Ar ≈ 34.969 × 0.758 + 36.966 × 0.242 ≈ 35.45.
为什么不是整数Why it is not a whole number
质子和中子的质量都略大于 1 u，而原子核形成时会损失一部分质量（质量亏损，对应结合能），所以除碳-12 外，相对同位素质量都不是整数。它可用质谱精确测定。
Protons and neutrons each have a mass slightly above 1 u, and some mass is lost when a nucleus forms (the mass defect, corresponding to the binding energy), so except for carbon-12 the values are not whole numbers. They are measured precisely by mass spectrometry.