📚 Chemical Bonding | 化学键
Chemical bonding is the force that holds atoms together in compounds. Understanding bonding is essential to explaining the properties and behaviour of substances. This article covers ionic, covalent, metallic bonding, electronegativity, molecular shapes, intermolecular forces, and bond energies, aligned with the Cambridge A-Level Chemistry syllabus.
化学键是将原子结合在化合物中的力。理解化学键对于解释物质的性质和行为至关重要。本文涵盖了离子键、共价键、金属键、电负性、分子形状、分子间作用力和键能等内容，符合剑桥A-Level化学大纲。
1. Introduction to Chemical Bonding | 化学键简介
Atoms bond to achieve a more stable electronic configuration, often resembling that of a noble gas. The three main types of strong chemical bonds are ionic, covalent, and metallic. In addition, intermolecular forces are weaker interactions between molecules.
原子通过成键来达到更稳定的电子构型，通常类似于稀有气体的构型。三种主要的强化学键是离子键、共价键和金属键。此外，分子间作用力是分子之间较弱的相互作用。
2. Ionic Bonding | 离子键
tive anions. These oppositely charged ions are held together by strong electrostatic forces in a giant ionic lattice. For example, in sodium chloride, Na → Na⁺ + e⁻ and Cl + e⁻ → Cl⁻. The resulting lattice has a high melting point, and it conducts electricity when molten or dissolved in water because the ions are free to move.
离子键发生在金属和非金属之间。电子从金属转移到非金属，形成正阳离子和负阴离子。这些带相反电荷的离子通过强烈的静电引力聚集在一起，形成巨大的离子晶格。例如，在氯化钠中，Na → Na⁺ + e⁻，Cl + e⁻ → Cl⁻。形成的晶格具有高熔点，并在熔融或溶于水时导电，因为离子可以自由移动。
3. Covalent Bonding | 共价键
he bond, achieving a full outer shell. A single bond shares one pair of electrons (e.g., H– ble bond shares two pairs (e.g., O=O in O₂), and a triple bond shares three pairs (e.g., N≡ lar forces or giant covalent structures (macromolecules) like diamond and silicon dioxide.
共价键涉及非金属原子之间共享电子对。每个原子为键提供一个电子，从而实现满最外层。单键共享一对电子（如H₂中的H–H），双键共享两对电子（如O₂中的O=O），三键共享三对电子（如N₂中的N≡N）。分子可以是具有弱分子间作用力的简单分子结构，也可以是像金刚石和二氧化硅这样的巨型共价结构（大分子）。
4. Metallic Bonding | 金属键
Metallic bonding is the electrostatic attraction between positive metal ions and a 'sea' of delocalised electrons. These delocalised electrons can move throughout the metal lattice, which explains metals' electrical and thermal conductivity. The layers of ions can slide over each other, making metals malleable and ductile. The strength of metallic bonding depends on the charge of the cation and the number of delocalised electrons; thus, magnesium has stronger bonding than sodium.
金属键是正金属离子与离域电子"海"之间的静电引力。这些离域电子可以在金属晶格中移动，这解释了金属的导电性和导热性。离子层可以相互滑动，使金属具有延展性和可塑性。金属键的强度取决于阳离子的电荷和离域电子的数量；因此，镁的键合比钠更强。
5. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. Pauling scale values increase across a period and up a group. When atoms of different electronegativity form a bond, the electron density shifts towards the more electronegative atom, creating a polar covalent bond. Partial charges are shown as δ⁺ and δ⁻. For instance, in HCl, chlorine is more electronegative and bears δ⁻ while hydrogen bears δ⁺. A large difference in electronegativity (>1.7) typically leads to ionic bonding rather than covalent.
电负性是原子在共价键中吸引键合电子对的能力。鲍林标度的数值在同周期从左到右增大，在同族中向上增大。当电负性不同的原子形成键时，电子密度偏向电负性更强的原子，形成极性共价键。部分电荷用δ⁺和δ⁻表示。例如，在HCl中，氯电负性更大，带δ⁻，而氢带δ⁺。电负性差很大（>1.7）通常导致离子键而不是共价键。
6. Shapes of Molecules: VSEPR Theory | 分子形状：价层电子对互斥理论
The Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom repel each other and arrange themselves as far apart as possible. The shape of a molecule is determined by the number of bonding pairs and lone pairs. Common shapes include linear (2 bonding pairs, 0 lone pairs, angle 180°), trigonal planar (3,0,120°), tetrahedral (4,0,109.5°), trigonal pyramidal (3,1,107°), bent (2,2,104.5°), trigonal bipyramidal (5,0,90° and 120°), and octahedral (6,0,90°). Below is a summary table:
价层电子对互斥理论（VSEPR）指出，中心原子周围的电子对相互排斥，尽可能远离。分子的形状由键合电子对和孤对电子对的数量决定。常见形状包括直线形（2键对，0孤对，键角180°）、平面三角形（3,0,120°）、四面体形（4,0,109.5°）、三角锥形（3,1,107°）、弯曲形（2,2,104.5°）、三角双锥形（5,0,90°和120°）和八面体形（6,0,90°）。下面是一张总结表：
| Total electron pairs | Bonding pairs | Lone pairs | Shape | Bond angle | Example |
|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° | CO₂ |
| 3 | 3 | 0 | Trigonal planar | 120° | BF₃ |
| 3 | 2 | 1 | Bent | <120° | SO₂ |
| 4 | 4 | 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 3 | 1 | Trigonal pyramidal | 107° | NH₃ |
| 4 | 2 | 2 | Bent | 104.5° | H₂O |
| 5 | 5 | 0 | Trigonal bipyramidal | 90°, 120° | PCl₅ |
| 6 | 6 | 0 | Octahedral | 90° | SF₆ |
上表总结了常见分子形状对应的电子对总数、键合对、孤对、形状名称、键角和例子。注意孤对电子的排斥力更强，会使键角略小于理想值。
7. Intermolecular Forces: Van der Waals Forces | 分子间作用力：范德华力
Van der Waals forces are weak intermolecular attractions. There are two main types: instantaneous dipole–induced dipole (London dispersion forces) and permanent dipole–permanent dipole interactions. London forces exist between all molecules and increase with molecular size (number of electrons), leading to higher boiling points. Permanent dipole forces occur between polar molecules, such as HCl, and are stronger than London forces only when molecules have similar size. The strength of these forces affects physical properties like boiling point and solubility.
范德华力是弱的分子间引力。主要有两种类型：瞬时偶极-诱导偶极力（伦敦色散力）和永久偶极-永久偶极相互作用。伦敦力存在于所有分子之间，并随着分子大小（电子数）的增加而增强，导致沸点更高。永久偶极力存在于极性分子之间，如HCl，在分子大小相似时比伦敦力强。这些力的强度影响沸点和溶解度等物理性质。
8. Hydrogen Bonding | 氢键
Hydrogen bonding is a special type of permanent dipole–dipole interaction, stronger than ordinary van der Waals forces. It occurs when hydrogen is covalently bonded to a highly electronegative atom (N, O, or F) and is attracted to a lone pair on another N, O, or F atom. Water (H₂O), ammonia (NH₃), and hydrogen fluoride (HF) exhibit hydrogen bonding. This force explains the relatively high boiling points of these compounds and the unusual properties of water, such as ice being less dense than liquid water.
氢键是一种特殊的永久偶极-偶极相互作用，比普通的范德华力更强。当氢与高电负性原子（N、O或F）共价键合，并受到另一个N、O或F原子上孤对电子的吸引时，就形成氢键。水（H₂O）、氨（NH₃）和氟化氢（HF）都存在氢键。这种作用力解释了这些化合物相对较高的沸点，以及水的奇特性质，比如冰的密度比液态水小。
9. Properties Related to Bonding and Structure | 与键合和结构相关的性质
The type of bonding and structure determines the physical properties of a substance. Ionic compounds are hard, brittle, have high melting points, and conduct electricity only when molten or aqueous. Simple molecular substances have low melting/boiling points
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