英語 English：Radioisotope
日語 Japanese：放射性同位体 (ほうしゃせいどういたい)
定義 Definition：A radioisotope is one of the isotopes emitting radiation.
For example, the uranium atom has a radioisotope called uranium 235 ( $^{235}U$ ), which emits radiation.
Uranium 235 has a large number of neutrons, which increases its energy and makes its nucleus unstable. When the nucleus of uranium 235 decays, it emits radiation.
記号 Symbol：RI
類義 Synonym：Radioactive isotope Radionuclide
対義 Antonym：Stable isotope
体系 System：物質 (Substance) ＞ 単体 (Simple substance) ＞ 原子 (Atom) ＞ 原子核 (Atomic nucleus) ＞ 同位体 (Isotope) ＞ 安定同位体 (Stable isotope) ＋ 放射性同位体 (Radioisotope)