📚 A-Level WJEC Chemistry: Chemical Bonding | 化学键考点精讲
Chemical bonding explains how atoms are held together in elements and compounds. Understanding the different types of bonding—ionic, covalent, and metallic—is essential for predicting the physical and chemical properties of substances. This revision guide covers all key topics required for the WJEC A-Level Chemistry specification, including shapes of molecules, electronegativity, and intermolecular forces, with clear definitions and exam-relevant details.
化学键解释了元素和化合物中原子如何结合在一起。理解离子键、共价键和金属键等不同类型的化学键，对于预测物质的物理和化学性质至关重要。本精讲覆盖WJEC A-Level化学大纲要求的所有核心主题，包括分子形状、电负性、分子间作用力等，并提供清晰的定义和与考试密切相关的细节。
1. Introduction to Chemical Bonding | 化学键概述
Atoms tend to achieve a more stable electron arrangement, often by attaining a full outer shell. Noble gases are stable because they have complete s and p subshells. When atoms combine, they either transfer or share electrons, leading to ionic or covalent bonding. A third type, metallic bonding, involves delocalised electrons in a lattice of metal cations.
原子倾向于获得更稳定的电子排布，通常通过达到全满的外层电子层来实现。稀有气体之所以稳定，是因为它们具有全满的s和p亚层。当原子结合时，它们要么转移电子，要么共享电子，从而形成离子键或共价键。第三种类型是金属键，涉及金属阳离子晶格中的离域电子。
2. Ionic Bonding | 离子键
Ionic bonding is the electrostatic attraction between positive and negative ions. It forms when a metal atom transfers one or more electrons to a non-metal atom. The metal becomes a cation, the non-metal becomes an anion, and both achieve a noble gas electron configuration. For example, in NaCl, sodium loses one electron to form Na⁺, and chlorine gains one to form Cl⁻.
离子键是阳离子和阴离子之间的静电吸引。当金属原子将一个或多个电子转移给非金属原子时形成离子键。金属变成阳离子，非金属变成阴离子，两者都达到稀有气体的电子构型。例如，在NaCl中，钠失去一个电子形成Na⁺，氯获得一个电子形成Cl⁻。
Ionic compounds exist as giant ionic lattices, where ions are packed in a regular, repeating pattern. The strength of the ionic bond depends on the charges of the ions and the distance between them. Smaller, highly charged ions produce stronger electrostatic forces, leading to higher melting points. For example, MgO (Mg²⁺ and O²⁻) has a much higher melting point than NaCl.
离子化合物以巨型离子晶格形式存在，其中离子以规则的重复模式排列。离子键的强度取决于离子的电荷和它们之间的距离。较小、带电荷高的离子产生更强的静电力，从而导致更高的熔点。例如，MgO（Mg²⁺ 和 O²⁻）的熔点远高于NaCl。
When writing formulas for ionic compounds, the total positive charge must balance the total negative charge. Polyatomic ions such as SO₄²⁻, NO₃⁻, and NH₄⁺ must be handled correctly. For instance, ammonium sulfate is (NH₄)₂SO₄, showing two NH₄⁺ ions for one SO₄²⁻ ion.
书写离子化合物的化学式时，总正电荷必须与总负电荷平衡。多原子离子，如SO₄²⁻、NO₃⁻ 和 NH₄⁺，必须正确处理。例如，硫酸铵的化学式为(NH₄)₂SO₄，表示每个SO₄²⁻ 离子对应两个NH₄⁺ 离子。
3. Covalent Bonding | 共价键
Covalent bonding is the sharing of one or more pairs of electrons between two non-metal atoms. The shared electron pair is attracted to the nuclei of both atoms, holding them together. A single covalent bond involves one shared pair (e.g., H-H, Cl-Cl). A double bond has two shared pairs (O=O), and a triple bond has three (N≡N).
共价键是两个非金属原子之间共享一对或多对电子。共享的电子对同时被两个原子的原子核吸引，从而将它们结合在一起。单键涉及一对共享电子（如 H-H、Cl-Cl），双键有两对共享电子（O=O），三键有三对（N≡N）。
Many non-metal elements and compounds exist as simple covalent molecules with fixed numbers of atoms. Examples include H₂O, CO₂, and NH₃. These molecules have strong covalent bonds inside but only weak intermolecular forces between molecules, which explains their low melting and boiling points. In contrast, giant covalent structures like diamond and silicon dioxide have an extended network of strong covalent bonds, making them very hard and high-melting.
许多非金属元素和化合物以含有固定数目原子的简单共价分子形式存在，例如H₂O、CO₂ 和NH₃。这些分子内部有很强的共价键，但分子间只有弱的分子间作用力，这就解释了它们具有低熔点和低沸点的原因。相比之下，巨型共价结构（如金刚石和二氧化硅）具有延伸的强共价键网络，因此它们极硬且熔点极高。
In Lewis dot-cross diagrams, only the outer shell electrons are shown. Dots and crosses distinguish electrons from different atoms. For molecules with multiple bonds, the number of shared pairs is drawn accordingly.
在路易斯点叉图中，只显示最外层电子。点和叉用来区分来自不同原子的电子。对于含多重键的分子，要相应地画出共享电子对的数量。
4. Dative Covalent (Coordinate) Bonding | 配位共价键
A dative covalent bond, or coordinate bond, is a covalent bond where both electrons in the shared pair come from the same atom. Once formed, it is identical to an ordinary covalent bond. It occurs when an atom with a lone pair donates that pair to an electron-deficient atom or ion.
配位共价键，又称配位键，是一种共价键，其中共享电子对的两个电子都来自同一个原子。一旦形成，它就与普通的共价键完全相同。当一个带有孤对电子的原子将该电子对提供给缺电子的原子或离子时，就会形成配位键。
Common examples include the ammonium ion NH₄⁺, where the nitrogen atom of ammonia donates its lone pair to a H⁺ ion. Similarly, the hydronium ion H₃O⁺ forms when water donates a lone pair to H⁺. Aluminium chloride dimer Al₂Cl₆ also contains dative bonds: each aluminium atom accepts a lone pair from a chlorine atom on the neighbouring AlCl₃ unit.
常见的例子包括铵离子NH₄⁺，其中氨分子中的氮原子将其孤对电子提供给H⁺ 离子。类似地，水合氢离子H₃O⁺ 是水分子将孤对电子提供给H⁺ 而形成的。二聚氯化铝Al₂Cl₆ 也含有配位键：每个铝原子从相邻AlCl₃ 单元的氯原子接受一对孤对电子。
5. Metallic Bonding | 金属键
Metallic bonding is the electrostatic attraction between a lattice of positive metal ions and a sea of delocalised electrons. Metal atoms lose their outer electrons, which become free to move throughout the structure. This model explains the typical properties of metals: high electrical and thermal conductivity, malleability, and ductility.
金属键是正金属离子晶格与离域电子海之间的静电吸引。金属原子失去其外层电子，这些电子可以在整个结构中自由移动。这一模型解释了金属的典型性质：高导电性、高导热性、延展性和可塑性。
The strength of metallic bonding increases with the number of delocalised electrons per atom and with the charge of the cation. For example, Mg (Mg²⁺ with two delocalised electrons) has stronger metallic bonding than Na (Na⁺ with one). This is reflected in their melting points. Transition metals often have higher melting points due to the involvement of d-electrons in bonding.
金属键的强度随每个原子所拥有的离域电子数以及阳离子电荷的增加而增强。例如，Mg（Mg²⁺，有两个离域电子）的金属键比Na（Na⁺，有一个离域电子）更强，这反映在它们的熔点上。由于d电子参与了成键，过渡金属通常具有更高的熔点。
6. 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. Across a period, electronegativity increases; down a group, it decreases. Fluorine is the most electronegative element. The Pauling scale is commonly used, with fluorine assigned a value of 4.0.
电负性是指原子在共价键中吸引成键电子对的能力。在同一周期中，电负性从左到右递增；在同一族中，自上而下递减。氟是电负性最强的元素。通常使用鲍林标度，氟的电负性值定为4.0。
When two atoms with different electronegativities form a bond, the electrons are not shared equally. This produces a polar covalent bond. The more electronegative atom gains a partial negative charge (δ⁻), and the other atom gets δ⁺. If the electronegativity difference is large, the bond is ionic rather than covalent. There is no sharp boundary, but a difference greater than about 1.7 often indicates ionic character.
当两个电负性不同的原子成键时，电子不是均匀共享的，从而产生极性共价键。电负性较大的原子带部分负电荷（δ⁻），另一个原子则带δ⁺。如果电负性差值很大，该键就是离子键而非共价键。虽然二者没有严格的界限，但当差值大于约1.7时，通常表明该键具有离子性。
Some molecules with polar bonds are non-polar overall because the bond dipoles cancel out due to symmetry. Carbon dioxide has two C=O polar bonds, but the linear shape makes the molecule non-polar. Water, however, is bent, so the O-H bond dipoles do not cancel, giving water a net dipole moment.
一些含有极性键的分子整体是非极性的，因为对称性使得键的偶极矩相互抵消。二氧化碳有两个C=O极性键，但直线形结构使分子为非极性分子。然而，水分子是角形的，O-H键的偶极矩不能抵消，因此水分子具有净偶极矩。
7. Shapes of Molecules (VSEPR Theory) | 分子形状（价层电子对互斥理论）
Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom arrange themselves to minimise repulsion. Both bonding pairs and lone pairs are considered. Lone pairs repel more strongly than bonding pairs, reducing bond angles.
价层电子对互斥理论（VSEPR）认为，中心原子周围的电子对会自行排列，以将排斥力降至最低。成键电子对和孤对电子对都被考虑在内。孤对电子对的排斥力强于成键电子对，因此会使键角变小。
The shape is determined by the number of electron pairs around the central atom. With two electron pairs, the shape is linear, 180°. With three pairs, trigonal planar, 120°. With four pairs, tetrahedral, 109.5°. The presence of lone pairs changes the molecular shape: one lone pair gives a pyramidal shape (e.g., NH₃, 107°), and two lone pairs give a bent shape (e.g., H₂O, 104.5°).
分子的形状取决于中心原子周围的电子对数。两对电子对：直线形，键角180°。三对：平面三角形，120°。四对：正四面体形，109.5°。孤对电子的存在会改变分子形状：一对孤对电子导致三角锥形（如NH₃，键角107°），两对孤对电子导致角形（如H₂O，键角104.5°）。
For five electron pairs, the arrangement is trigonal bipyramidal (90° and 120°); for six pairs, octahedral (90°). Students should be able to predict and name the shapes of molecules and ions such as BF₃, CH₄, SF₆, and PCl₅, and include the effect of lone pairs.
对于五对电子对，排列方式为三角双锥形（键角90°和120°）；六对时则为八面体形（键角90°）。学生应能够预测并命名诸如BF₃、CH₄、SF₆ 和PCl₅ 等分子和离子的形状，并考虑孤对电子的影响。
Summary of molecular shapes:
| Electron Pairs | Lone Pairs | Shape / 形状 | Bond Angle / 键角 | Example / 例子 |
| 2 | 0 | Linear / 直线形 | 180° | BeCl₂, CO₂ |
| 3 | 0 | Trigonal Planar / 平面三角形 | 120° | BF₃ |
| 4 | 0 | Tetrahedral / 正四面体形 | 109.5° | CH₄, NH₄⁺ |
| 4 | 1 | Pyramidal / 三角锥形 | 107° | NH₃ |
| 4 | 2 | Bent / 角形 | 104.5° | H₂O |
| 5 | 0 | Trigonal Bipyramidal / 三角双锥形 | 90°, 120° | PCl₅ |
| 6 | 0 | Octahedral / 八面体形 | 90° | SF₆ |
8. Intermolecular Forces: van der Waals’ Forces | 分子间作用力：范德华力
Intermolecular forces are much weaker than covalent, ionic, or metallic bonds. They are responsible for the physical state of simple covalent substances. There are two main types of van der Waals’ forces: London (dispersion) forces and permanent dipole-dipole forces.
分子间作用力远弱于共价键、离子键或金属键。它们决定了简单共价物质的物理状态。范德华力主要有两种类型：伦敦力（色散力）和永久偶极-永久偶极力。
London forces arise from temporary fluctuations in electron distribution, creating an instantaneous dipole. This induces a dipole in a neighbouring molecule, resulting in a weak attraction. All atoms and molecules experience London forces. Their strength increases with the number of electrons and the surface area of the molecule. This explains why boiling points of noble gases and halogens increase down the group.
伦敦力源于电子分布瞬间波动所产生的瞬时偶极。该瞬时偶极会在相邻分子中诱导出偶极，从而产生微弱的吸引力。所有原子和分子都存在伦敦力。力的强度随电子数量的增加和分子表面积的增大而增强。这就解释了为什么稀有气体和卤素的沸点沿族往下依次升高。
Permanent dipole-dipole forces occur between molecules that possess a permanent dipole moment, such as HCl or CH₃Cl. These forces are generally stronger than London forces in molecules of similar size. They add to the total intermolecular attraction, giving higher boiling points than non-polar counterparts.
永久偶极-永久偶极力存在于具有永久偶极矩的分子之间，例如HCl或CH₃Cl。对于大小相近的分子，这种力通常比伦敦力更强。它们增加了总的分子间吸引力，因此沸点高于相应的非极性分子。
9. Hydrogen Bonding | 氢键
Hydrogen bonding is a special, stronger type of permanent dipole-dipole attraction. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom—fluorine, oxygen, or nitrogen—and is attracted to a lone pair on another such atom. The bond is usually represented as X-H···Y, where X and Y are F, O, or N.
氢键是一种特殊的、更强的永久偶极-永久偶极吸引力。它发生在氢原子与电负性极强的原子（氟、氧或氮）形成共价键，并被另一个同类原子上的孤对电子吸引时。氢键通常表示为X-H···Y，其中X和Y是F、O或N。
Hydrogen bonding has a profound effect on physical properties. Water has an unexpectedly high boiling point because each molecule can form up to two hydrogen bonds (using its two H atoms and two lone pairs on O). Ice is less dense than liquid water due to the open lattice structure created by hydrogen bonds.
氢键对物理性质有显著影响。水的沸点出乎意料地高，因为每个水分子最多可以形成两个氢键（利用它的两个氢原子和氧上的两对孤对电子）。冰的密度小于液态水，这是由于氢键形成的敞开晶格结构所致。
Hydrogen bonding is also crucial in biological molecules, such as holding together the two strands of DNA and stabilising protein secondary structures like α-helices and β-sheets. In alcohols, hydrogen bonding leads to higher boiling points compared to alkanes of similar relative molecular mass.
氢键在生物分子中也至关重要，例如将DNA的两条链维系在一起，并稳定α-螺旋和β-折叠片等蛋白质二级结构。在醇类中，氢键使得它们的沸点高于相对分子质量相近的烷烃。
10. Bonding and Physical Properties | 化学键与物理性质
Understanding bonding allows us to explain trends in melting and boiling points, electrical conductivity, and solubility. Ionic compounds have high melting points, are brittle, and conduct electricity only when molten or dissolved. Metals are good conductors, malleable, and have varying melting points. Giant covalent substances are very high-melting and non-conductive (graphite being an exception due to delocalised electrons between layers).
理解化学键可以让我们解释熔点、沸点、导电性和溶解度的变化趋势。离子化合物熔点高，脆，且仅在熔融或溶解时导电。金属是良导体，具有延展性，并且熔点范围很宽。巨型共价物质熔点极高，且不导电（石墨例外，因为其层间存在离域电子）。
Simple covalent substances have low melting points and do not conduct electricity because they consist of neutral molecules. Within a homologous series, the boiling point increases as the chain length increases due to stronger London forces. For molecules capable of hydrogen bonding, boiling points are significantly higher than would otherwise be expected.
简单共价物质熔点低，不导电，因为它们由中性分子组成。在同系物中，由于碳链增长使伦敦力增强，沸点随之升高。对于能够形成氢键的分子，其沸点显著高于本来应有的数值。
Solubility follows the rule ‘like dissolves like’. Ionic and highly polar substances tend to dissolve in polar solvents like water, while non-polar substances dissolve in non-polar solvents such as hexane. Knowledge of bonding is essential for interpreting these macroscopic observations and is frequently examined.
溶解性遵循“相似相溶”规则。离子和强极性物质倾向于溶于水等极性溶剂，而非极性物质则溶于正己烷等非极性溶剂。化学键的知识对于解释这些宏观观察至关重要，也是考试中常出现的内容。
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