But a tiny fraction of the hydrogen atoms in water have an extra neutron in their nucleus.

但是水中的一小部分氢原子 在其原子核中有一个额外的中子。

Those extra neutrons are necessary for atoms with atomic numbers higher than thirty or so, as you can see in this chart relating the number of neutrons to the number of protons in a stable atom.

对于原子序数高于 30 左右的原子来说，这些额外的中子是必需的，正如您在这张图表中看到的，中子数与稳定原子中质子数的关系。