📚 A-Level Chemistry: Chemical Bonding and Structure | A-Level 化学：化学键与结构
Understanding how atoms combine and how particles interact is at the heart of A-Level Chemistry. This article reviews the main types of bonding, molecular shapes, and intermolecular forces, with emphasis on structure-property relationships.
理解原子如何结合以及粒子如何相互作用是 A-Level 化学的核心。本文回顾主要键型、分子形状和分子间作用力，重点关注结构与性质的关系。
1. Ionic Bonding | 离子键
Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom, producing oppositely charged ions. The electrostatic attraction between these cations and anions forms a giant ionic lattice.
当电子从金属原子转移到非金属原子时，会形成带相反电荷的离子，从而产生离子键。这些阳离子和阴离子之间的静电引力构成巨型离子晶格。
The lattice enthalpy of an ionic compound is the energy released when one mole of the solid is formed from its gaseous ions. More highly charged ions and smaller ionic radii give stronger ionic bonding.
离子化合物的晶格焓是指一摩尔固态化合物由其气态离子形成时释放的能量。离子电荷越高、半径越小，离子键越强。
For example, sodium chloride NaCl has a lattice of Na⁺ and Cl⁻ ions, while magnesium oxide MgO contains Mg²⁺ and O²⁻ ions. MgO has a much higher melting point than NaCl because the charges are greater and the ions are smaller.
例如，氯化钠 NaCl 由 Na⁺ 和 Cl⁻ 离子组成晶格，而氧化镁 MgO 含有 Mg²⁺ 和 O²⁻ 离子。MgO 的熔点远高于 NaCl，因为离子电荷更高且离子半径更小。
2. Covalent Bonding | 共价键
A covalent bond is a shared pair of electrons between two non-metal atoms. Atoms share electrons to achieve a full outer shell, usually an octet, except for hydrogen which needs only two electrons.
共价键是两个非金属原子之间共享的一对电子。原子通过共享电子以达到满外层，通常是八隅体，但氢只需要两个电子。
Covalent bonds can be single, double, or triple. For example, oxygen gas O₂ has a double bond, and nitrogen gas N₂ has a triple bond. Double and triple bonds are shorter and stronger than single bonds.
共价键可以是单键、双键或三键。例如，氧气 O₂ 有双键，氮气 N₂ 有三键。双键和三键比单键更短、更强。
A dative covalent bond, also called a coordinate bond, forms when one atom provides both electrons for the shared pair. This occurs, for instance, in the ammonium ion NH₄⁺ when NH₃ donates a lone pair to H⁺.
配位共价键，又称配位键，是指一个原子提供共享电子对中的两个电子。例如，在铵离子 NH₄⁺ 中，NH₃ 向 H⁺ 提供孤对电子。
3. Metallic Bonding | 金属键
Metallic bonding is the electrostatic attraction between a lattice of positive metal ions and a ‘sea’ of delocalised electrons. These delocalised electrons are free to move throughout the metal structure.
金属键是正金属离子晶格与离域电子 ‘海’ 之间的静电吸引。这些离域电子可以在金属结构中自由移动。
This model explains the high electrical and thermal conductivity of metals, as well as their malleability and ductility. The layers of metal ions can slide over each other without breaking the metallic bond.
该模型解释了金属的高导电性、导热性以及延展性和可塑性。金属离子层可以在不破坏金属键的情况下相互滑动。
The strength of metallic bonding depends on the charge of the metal ion and the number of delocalised electrons per atom. Transition metals often have strong metallic bonding and high melting points.
金属键的强度取决于金属离子的电荷和每个原子提供的离域电子数。过渡金属通常具有强金属键和高熔点。
4. 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. Fluorine is the most electronegative element with a value of 4.0 on the Pauling scale.
电负性是原子在共价键中吸引成键电子对的能力。氟是电负性最强的元素，鲍林标度值为 4.0。
If two atoms have a large electronegativity difference, the bond is polar covalent. A difference greater than about 1.7 usually indicates ionic character, although the boundary is not sharp.
如果两个原子的电负性差值很大，该键为极性共价键。差值大于约 1.7 通常表明具有离子性，但界限并不绝对。
Polar bonds can create a molecular dipole if the geometry does not cancel out the individual bond dipoles. For example, carbon dioxide CO₂ is linear and non-polar, while water H₂O is bent and polar.
如果分子几何形状没有抵消单个键的偶极矩，极性键会产生分子偶极。例如，二氧化碳 CO₂ 为直线形且非极性，而水 H₂O 为弯曲形且为极性。
5. Intermolecular Forces | 分子间作用力
Intermolecular forces are much weaker than ionic, covalent, or metallic bonds. They include London dispersion forces, permanent dipole-dipole forces, and hydrogen bonds.
分子间作用力远弱于离子键、共价键或金属键。它们包括伦敦色散力、永久偶极-偶极力和氢键。
London dispersion forces arise from temporary fluctuations in electron distribution and are present in all molecules. They increase with molecular size and surface area.
伦敦色散力源于电子分布的瞬时波动，存在于所有分子中。它们随分子大小和表面积增大而增强。
Hydrogen bonding occurs when hydrogen is bonded to nitrogen, oxygen, or fluorine. It is the strongest type of intermolecular force and explains the unusually high boiling point of water.
当氢与氮、氧或氟成键时，会产生氢键。它是最强的分子间作用力，解释了水异常高的沸点。
6. Shapes of Molecules (VSEPR) | 分子形状 (VSEPR)
The valence shell electron pair repulsion (VSEPR) theory predicts molecular shapes by assuming that electron pairs around a central atom repel each other and arrange themselves to minimise repulsion.
价层电子对互斥理论 (VSEPR) 通过假设中心原子周围的电子对相互排斥并排列成使排斥最小的方式来预测分子形状。
Common shapes include the following.
常见形状包括以下几种。
- Linear – 直线形: 2 electron pairs, bond angle 180°
- Trigonal planar – 平面三角形: 3 electron pairs, bond angle 120°
- Tetrahedral – 四面体形: 4 electron pairs, bond angle 109.5°
- Trigonal bipyramidal – 三角双锥形: 5 electron pairs, bond angles 90° and 120°
- Octahedral – 八面体形: 6 electron pairs, bond angle 90°
Lone pairs occupy more space than bonding pairs, so they compress bond angles. For example, water H₂O has two lone pairs on oxygen, reducing the H-O-H angle from 109.5° to about 104.5°.
孤对电子占据的空间比成键电子对更大，因此压缩键角。例如，水 H₂O 的氧上有两对孤对电子，使 H-O-H 键角从 109.5° 减小到约 104.5°。
7. Bond Enthalpy and Bond Length | 键焓与键长
Bond enthalpy is the energy required to break one mole of a covalent bond in the gaseous state, averaged over a range of compounds. It is always an endothermic process.
键焓是指在气态下断裂一摩尔共价键所需的能量，通常取一系列化合物的平均值。该过程总是吸热的。
ΔH = Σ(bond enthalpies of bonds broken) – Σ(bond enthalpies of bonds formed)
Reaction enthalpy can be estimated using the equation above. It works best for gaseous reactions and gives average values rather than exact experimental data.
可以利用上式估算反应焓变。该方法最适合气相反应，给出的是平均值而非精确的实验数据。
Bond length is the average distance between the nuclei of two bonded atoms. Shorter bonds are generally stronger because the bonding electrons are closer to both nuclei and the electrostatic attraction is greater.
键长是两个成键原子核之间的平均距离。较短的键通常更强，因为成键电子更靠近两个原子核，静电引力更大。
8. Giant Covalent Structures | 巨型共价结构
Some non-metal elements form giant covalent lattices in which atoms are held together by a network of strong covalent bonds throughout the entire structure. Diamond and graphite are two allotropes of carbon.
一些非金属元素形成巨型共价晶格，其中原子通过整个结构中强共价键网络连接。金刚石和石墨是碳的两种同素异形体。
In diamond, each carbon atom is bonded tetrahedrally to four other carbon atoms, giving a very hard structure with a high melting point. It does not conduct electricity because all electrons are localised in bonds.
在金刚石中，每个碳原子以四面体方式与另外四个碳原子成键，形成非常坚硬且熔点高的结构。它不导电，因为所有电子都定域在键中。
In graphite, each carbon atom is bonded to three others in flat layers. The fourth outer electron is delocalised between the layers, allowing graphite to conduct electricity and act as a lubricant.
在石墨中，每个碳原子与另外三个碳原子在平面层中成键
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