定义Definition
离子是带电荷的原子或原子团，由原子或分子得到或失去电子形成。失去电子形成带正电的阳离子，得到电子形成带负电的阴离子。离子化合物由阴、阳离子通过静电吸引结合而成，其水溶液和熔融态能导电。
An ion is a charged atom or group of atoms, formed when an atom or molecule gains or loses electrons. Losing electrons gives a positive cation and gaining electrons a negative anion. Ionic compounds are held together by the electrostatic attraction between oppositely charged ions and conduct electricity when molten or dissolved in water.
单原子离子与多原子离子Simple and polyatomic ions
单原子离子如 Na⁺、Mg²⁺、Cl⁻、O²⁻；多原子离子由几个原子共价结合后整体带电，如 NH₄⁺、OH⁻、SO₄²⁻、NO₃⁻、CO₃²⁻。主族元素形成离子时通常达到稀有气体的电子构型。
Simple ions include Na⁺, Mg²⁺, Cl⁻ and O²⁻; polyatomic ions are groups of covalently bonded atoms carrying an overall charge, such as NH₄⁺, OH⁻, SO₄²⁻, NO₃⁻ and CO₃²⁻. Main-group elements usually form ions with a noble-gas configuration.
离子的大小Ionic size
阳离子比对应原子小（失去外层电子，核电荷对剩余电子吸引增强），阴离子比对应原子大（增加的电子间排斥增大）。等电子离子中核电荷越大半径越小：O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺。
Cations are smaller than their atoms (the outer shell is lost and the remaining electrons are held more tightly), and anions are larger (extra electrons repel). In an isoelectronic series, higher nuclear charge means a smaller ion: O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺.
年表Timeline
- 1834
法拉第与休厄尔商定使用 ion、cation、anion 等术语
Faraday, with Whewell, introduces the terms ion, cation and anion