英語 English：Isotope
日語 Japanese：同位体 (どういたい)
定義 Definition：Isotopes are atoms that have the same atomic number but different numbers of neutrons. For example, hydrogen and deuterium are isotopes. Hydrogen has one neutron and deuterium has two neutrons. To write down isotopes in an empirical formula, hydrogen is written as $^1H$, and deuterium is written as $^2H$.
Isotopes are further divided into two categories: stable isotopes, which have stable nuclei, and radioactive isotopes, which have unstable nuclei.
体系 System：物質 (Substance) ＞ 単体 (Simple substance) ＞ 原子 (Atom) ＞ 原子核 (Atomic nucleus) ＞ 同位体 (Isotope)