质量亏损Mass defect

核子结合成原子核时放出能量，体系能量降低，按 E = mc² 质量也相应减少。计算时常用原子质量（含电子），电子质量在两边抵消。例如铀-235 裂变中，产物总质量比反应物少约 0.2 u，即约 0.09% 的质量转化为约 200 MeV 能量。化学反应也有质量变化，但只有约 10⁻¹⁰ 量级，无法直接测量。

When nucleons bind into a nucleus they release energy, so the system's energy and, by E = mc², its mass both decrease. Calculations usually use atomic masses including electrons, which cancel on both sides. In uranium-235 fission the products weigh about 0.2 u less than the reactants: roughly 0.09% of the mass becomes about 200 MeV. Chemical reactions change mass too, but only by about one part in 10¹⁰, too little to measure directly.

1905 年爱因斯坦在狭义相对论中导出质能等价关系。1919 年起阿斯顿用自制的质谱仪精确测量同位素质量，提出“整数规则”，并发现实际质量与整数存在微小偏差，提出“装填分数”概念，获 1922 年诺贝尔化学奖。1932 年考克饶夫和沃尔顿用加速器轰击锂核，首次实验验证了核反应中的质能关系。

Einstein derived mass–energy equivalence in special relativity in 1905. From 1919 Aston measured isotope masses precisely with his mass spectrograph, formulated the whole-number rule and found small deviations from whole numbers, which he expressed as the packing fraction; he received the 1922 Nobel Prize in Chemistry. In 1932 Cockcroft and Walton split lithium nuclei with an accelerator, the first experimental check of mass–energy equivalence in a nuclear reaction.

质量亏损（mass defect）有时也被用来指原子质量与质量数之差，后者在核数据表中更规范的名称是“质量过剩”（mass excess），以能量单位 keV 或 MeV 表示，¹²C 的质量过剩按定义为零。阅读文献时应注意区分。

Mass defect is sometimes also used for the difference between an atomic mass and its mass number; in nuclear data tables that quantity is properly called the mass excess, given in keV or MeV, and is zero for ¹²C by definition. Keep the two apart when reading the literature.