For instance, the mass of a neutral helium atom is 4.002603 unified atomic mass units.

一个稳定的氦原子核的质量是4。002603u。

That means that the nucleus of a helium atom has 0.030377 unified atomic mass units less mass than its component parts.

这意味着氦原子核的质量比其组成部分少 0.030377 个统一原子质量单位。

So the products of this reaction always have less total mass – as measured in unified atomic mass units – than the parent nucleus has.

因此，该反应的产物总质量（以统一原子质量单位衡量）总是小于母核。

But two neutrons and two protons – the component parts of a helium atom – taken together have a mass of 4.032980 unified atomic mass units.

但是两个中子和两个质子——氦原子的组成部分——加在一起的质量为 4.032980 个统一原子质量单位。

To quantify the mass of a nucleus, we use the unified atomic mass unit, written just as a small u, with a single neutral carbon-12 atom equaling exactly twelve unified atomic mass units.

为了量化原子核的质量，我们使用统一的原子质量单位，写成小写的 u，其中一个中性碳 12 原子恰好等于十二个统一的原子质量单位。