Isotope: One of two or more species of atoms of a chemical element having nuclei with the same number of protons but different Numbers of neutrons.

同位素：一种化学元素的两个或多个类型，它们的原子核具有相同的质子数，而中子数却不同。

youdao

So, we're going to start with talking about bonding, and any time we have a chemical bond, basically what we're talking about is having two atoms where the arrangement of their nuclei and their electrons are such that the bonded atoms results in a lower energythan for the separate atoms.

那么，下面我们将从成键开始讲起，无论什么时候我们有一个化学键，基本上我们所讨论的,都是如何安排两个原子的原子核的位置,与电子的位置使得成键的两个原子,最终比分开时的能量更低。

麻省理工公开课 - 化学原理课程节选