离子键Ionic bonding

钠与氯反应时，钠原子失去最外层的 1 个电子成为 Na⁺（2,8），氯原子得到 1 个电子成为 Cl⁻（2,8,8），两者都达到稀有气体的稳定结构。电子转移可以用电子式（点叉图）表示。离子化合物的化学式反映阴阳离子数之比，使正负电荷总数相等，如 MgCl₂、Al₂O₃。

When sodium reacts with chlorine, each sodium atom loses its one outer electron to become Na⁺ (2,8) and each chlorine atom gains one to become Cl⁻ (2,8,8), both reaching noble-gas configurations. The transfer is shown with dot-and-cross diagrams. The formula of an ionic compound gives the ratio of ions that balances the charges, as in MgCl₂ and Al₂O₃.

离子化合物中每个离子被大量带相反电荷的离子包围，如 NaCl 晶体中每个 Na⁺ 周围有 6 个 Cl⁻。熔化需要克服晶格中强烈的静电作用，所以熔点高（NaCl 为 801 °C）。固态时离子不能移动，不导电；熔融或溶于水后离子可以自由移动而导电。晶体受力时同种电荷的离子层错位相对而相互排斥，所以质脆。

In an ionic compound each ion is surrounded by many ions of opposite charge; in NaCl each Na⁺ has six Cl⁻ neighbours. Melting must overcome the strong electrostatic forces throughout the lattice, so melting points are high (801 °C for NaCl). Solid ionic compounds do not conduct because the ions cannot move; when molten or dissolved in water the ions are free to move and they conduct. A blow displaces layers so like charges meet and repel, so the crystals are brittle.

离子所带电荷越多、离子半径越小，静电吸引越强，晶格能越大，熔点越高。例如 MgO（Mg²⁺、O²⁻）的熔点约 2850 °C，远高于 NaCl。实际上纯粹的离子键并不存在：阳离子会使阴离子的电子云变形（极化），使键带有一定的共价性，阳离子越小、电荷越高，阴离子越大，极化越明显。

The higher the charges and the smaller the ions, the stronger the attraction, the larger the lattice energy and the higher the melting point: MgO (Mg²⁺, O²⁻) melts at about 2850 °C, far above NaCl. Purely ionic bonding does not really exist: a cation distorts (polarises) the anion's electron cloud, giving the bond some covalent character, which is greater for small, highly charged cations and large anions.