📚 Chemical Bonding: Ionic, Covalent, Metallic and Intermolecular Forces | 化学键：离子键、共价键、金属键与分子间作用力
Chemical bonding explains why atoms combine, how they arrange in space, and why materials have such different melting points, conductivities, and solubilities. This unit focuses on the three strong bonding types—ionic, covalent, and metallic—and the weaker intermolecular forces that control physical properties such as boiling point.
化学键解释了原子为何相互结合、如何在空间中排列，以及为什么不同材料的熔点、导电性和溶解性差异巨大。本单元重点学习三种强化学键——离子键、共价键和金属键——以及控制沸点等物理性质的较弱分子间作用力。
1. Why Atoms Form Bonds | 原子为何形成化学键
Atoms form bonds to achieve a more stable electronic arrangement, usually by filling their outer shell to resemble a noble gas. This is the basis of the octet rule: many atoms tend to gain, lose, or share electrons until they have eight electrons in their valence shell.
原子形成化学键是为了获得更稳定的电子排布，通常是填满最外层，达到稀有气体的结构。这就是八隅律的基础：许多原子倾向于得到、失去或共享电子，直到价层拥有八个电子。
Bond formation is exothermic because the bonded state has lower potential energy than the separated atoms. Energy is released when bonds form, and energy must be supplied to break them.
成键过程放热，因为成键状态比分离原子的势能更低。形成化学键时释放能量，打破化学键则需要吸收能量。
However, the octet rule is not universal. Species such as BF₃ and SF₆ contain atoms with six or twelve valence electrons, so the rule is best treated as a useful guide rather than a strict law.
但八隅律并非普遍适用。BF₃ 和 SF₆ 等物种中，中心原子分别只有六个或十二个价电子，因此该规则最好视为有用指导，而非严格定律。
2. Ionic Bonding and Lattice Energy | 离子键与晶格能
Ionic bonding is the electrostatic attraction between oppositely charged ions. It usually forms between metals and non-metals: metals lose electrons to form cations, while non-metals gain electrons to form anions.
离子键是带相反电荷离子之间的静电吸引力。它通常形成于金属与非金属之间：金属失去电子形成阳离子，非金属得到电子形成阴离子。
Solid ionic compounds exist as giant ionic lattices, where each ion is surrounded by ions of opposite charge. The lattice enthalpy is the energy released when one mole of a solid ionic lattice is formed from its gaseous ions.
固态离子化合物以巨型离子晶格存在，每个离子都被带相反电荷的离子包围。晶格焓是由气态离子形成一摩尔固态离子晶格时释放的能量。
Lattice enthalpy becomes more exothermic as ionic charge increases and ionic radius decreases. This is because a greater charge density brings the ions closer together and strengthens the electrostatic attraction.
离子电荷越高、离子半径越小，晶格焓越负。这是因为更高的电荷密度使离子更紧密地靠近，增强了静电吸引力。
Ionic substances have high melting points, are brittle, and do not conduct when solid. They conduct electricity when molten or dissolved in water because the ions become free to move.
离子化合物熔点高、质地脆，固态时不导电。熔融或溶于水后能够导电，因为离子可以自由移动。
3. Covalent Bonding: Single, Double and Triple Bonds | 共价键：单键、双键与三键
A covalent bond is a shared pair of electrons between two non-metal atoms. Each atom usually contributes one electron to the shared pair, giving both atoms greater stability.
共价键是两个非金属原子之间共享的一对电子。每个原子通常贡献一个电子形成共享电子对，从而使两个原子都更稳定。
Single bonds share one electron pair, double bonds share two pairs, and triple bonds share three pairs. As the number of shared pairs increases, the bond becomes shorter and stronger.
单键共享一对电子，双键共享两对电子，三键共享三对电子。随着共享电子对数增加，键长变短、键能增大。
ctrons in a shared pair. Examples include the N→B bond in NH₃BF₃ and the O→H bond in H₃O⁺.
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