📚 Covalent Bonding in A-Level Edexcel Chemistry | A-Level Edexcel 化学：共价键考点精讲
Covalent bonding is a central topic in Edexcel A-Level Chemistry, forming the basis for understanding molecular structure, physical properties and chemical reactivity. From drawing Lewis diagrams to predicting molecular shapes and analysing bond polarity, mastering covalent bonding will give you a huge advantage in Paper 1 as well as in synoptic questions that link structure to function. This article breaks down every key specification point and gives you exam-ready explanations in a clear, paired English–Chinese format.
共价键是爱德思 A-Level 化学的核心主题，它是理解分子结构、物理性质和化学活性的基石。从绘制路易斯图到预测分子形状再到分析键的极性，掌握共价键将让你在 Paper 1 以及连接结构与功能的综合性题目中占据明显优势。本文拆解每一项考纲要点，并以中英对照的清晰讲解帮助你随时应考。
1. Introduction to Covalent Bonding | 共价键简介
A covalent bond is a chemical link formed by the electrostatic attraction between two nuclei and a shared pair of electrons. Unlike ionic bonding, where electrons are transferred, covalent bonding typically occurs between non‑metal atoms with similar electronegativities. The shared electrons occupy a region of space between the two nuclei, effectively binding them together.
共价键是由两个原子核与一对共享电子之间的静电吸引力所形成的化学键。与电子发生转移的离子键不同，共价键通常发生在电负性相近的非金属原子之间。共享电子占据两个原子核之间的空间区域，从而将它们有效地结合在一起。
2. Formation of Covalent Bonds | 共价键的形成
A covalent bond forms when two orbitals overlap and each atom contributes one electron to a shared pair. The reduction in potential energy as the atoms approach each other gives rise to a stable bond. For instance, two hydrogen atoms each with one 1s electron can overlap their orbitals, creating a sigma (σ) bond and forming H₂.
当两个轨道重叠、每个原子向共享电子对提供一个电子时，共价键便形成了。随着原子相互靠近，势能降低，产生稳定的键。例如，两个氢原子各带有一个 1s 电子，可以通过轨道重叠形成 σ 键，生成 H₂ 分子。
3. Lewis Structures and the Octet Rule | 路易斯结构与八隅体规则
Lewis structures show how valence electrons are arranged around atoms. The octet rule states that atoms tend to share electrons until they are surrounded by eight valence electrons, mimicking the electron configuration of a noble gas. You draw Lewis structures by counting total valence electrons, drawing a skeleton, and distributing electrons to satisfy the octet for each atom, using multiple bonds when necessary.
路易斯结构展示价电子如何在原子周围排布。八隅体规则指出，原子倾向于共享电子，直到外围有八个价电子，类似于稀有气体的电子构型。绘制路易斯结构时，先计算价电子总数，画出骨架，然后分配电子使每个原子满足八隅体，必要时使用多重键。
4. Coordinate (Dative) Bonds | 配位键（配价键）
A coordinate or dative covalent bond forms when both electrons in the shared pair come from the same atom. The donor atom must have a lone pair, and the acceptor atom must be electron‑deficient. Classic examples include the ammonium ion (NH₄⁺), where the nitrogen of ammonia donates a lone pair to a hydrogen ion, and the hydronium ion (H₃O⁺). Once formed, a dative bond is indistinguishable from an ordinary covalent bond.
配位共价键（又称配价键）形成时，共享电子对的两个电子都来自同一个原子。供体原子必须具备孤对电子，受体原子必须缺电子。典型例子包括铵离子（NH₄⁺），其中氨的氮原子将孤对电子提供给氢离子，以及水合氢离子（H₃O⁺）。配位键一旦形成，就与普通共价键没有区别。
5. Bond Polarity and Dipole Moments | 键的极性与偶极矩
When two atoms with different electronegativities form a covalent bond, the electron pair is not shared equally. The more electronegative atom pulls the electron density towards itself, creating a partial negative charge (δ⁻) and a partial positive charge (δ⁺) on the other atom. This separation of charge produces a bond dipole moment, measured in debye. The larger the electronegativity difference, the more polar the bond.
当两个电负性不同的原子形成共价键时，电子对并非均等共享。电负性较强的原子将电子密度拉向自身，产生部分负电荷（δ⁻），而在另一原子上产生部分正电荷（δ⁺）。这种电荷分离形成了键偶极矩，单位为德拜。电负性差值越大，键的极性越强。
6. Electronegativity and Polar Bonds | 电负性与极性键
Electronegativity is the power of an atom to attract the bonding pair of electrons in a covalent bond. The Pauling scale is commonly used. In Edexcel exams you are expected to link electronegativity trends (increasing across a period, decreasing down a group) to bond polarity and to deduce overall molecular polarity by considering both bond dipoles and molecular shape.
电负性是原子在共价键中吸引成键电子对的能力，常用鲍林标度表示。爱德思考试要求你能够将电负性变化规律（同周期从左到右增大，同族从上到下减小）与键的极性联系起来，并综合考虑键偶极和分子形状来推导分子的整体极性。
7. Resonance Structures | 共振结构
Some molecules cannot be adequately described by a single Lewis structure because the π electrons are delocalised over several atoms. Resonance structures are two or more Lewis structures that differ only in the arrangement of electrons while the atom positions stay fixed. The actual electronic structure is a resonance hybrid, an average of the contributing forms. Benzene, with its delocalised π system, and the nitrate ion (NO₃⁻) are key examples.
某些分子无法用单一的路易斯结构充分描述，因为 π 电子在多个原子之间离域。共振结构是两种或多种仅电子排布不同、而原子位置不变的路易斯结构。实际的电子结构是共振杂化体，即所有贡献形式的平均体。苯（带有离域 π 体系）和硝酸根离子（NO₃⁻）是重要例子。
8. Bond Energy and Bond Length | 键能与键长
ed atoms. Multiple bonds are shorter and stronger than single bonds between the same elements: e.g. C–C bond length ≈ 154 pm, C=C ≈ 134 pm, C≡ pm. In exothermic reactions, bond forming releases energy; bond breaking requires energy.
键能是断裂气态中一摩尔共价键所需的能量。键长是两个成键原子核之间的距离。同一对元素之间，多重键比单键更短、更强，例如 C–C 键长约为 154 pm，C=C 约为 134 pm，C≡C 约为 120 pm。在放热反应中，成键释放能量，断键吸收能量。
9. VSEPR Theory and Molecular Shapes | VSEPR 理论及分子形状
Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom repel one another and arrange themselves as far apart as possible. This determines the molecular geometry. The shape is defined by the number of bonding pairs and lone pairs. Common shapes tested by Edexcel include:
价层电子对互斥理论（VSEPR）指出，中心原子周围的电子对彼此排斥，并尽可能远离，从而决定了分子的几何构型。分子形状由成键电子对和孤对电子的数目决定。爱德思常考的形状包括：
| Bonding Pairs / 成键电子对 | Lone Pairs / 孤对电子 | Shape / 形状 | Bond Angle / 键角 | Example / 示例 |
|---|---|---|---|---|
| 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⁺ |
| 3 | 1 | Pyramidal / 三角锥形 | ~107° | NH₃ |
| 2 | 2 | V‑shaped (bent) / V 形（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 |
Lone pairs exert a greater repulsion than bonding pairs, reducing bond angles. For instance, the H–O–H angle in water is compressed to ~104.5° because of two lone pairs on oxygen.
孤对电子的排斥力大于成键电子对，从而使键角减小。例如，水分子中 H–O–H 键角被氧原子上的两对孤对电子压缩至约 104.5°。
10. Giant Covalent Structures | 巨共价结构
Some elements and compounds form giant covalent lattices where billions of atoms are held together by covalent bonds in a continuous network. Diamond, graphite, graphene and silicon dioxide (SiO₂) are classic examples. Diamond has a tetrahedral structure with each carbon bonded to four others, giving extreme hardness and no electrical conductivity. Graphite has layers of hexagonal rings; delocalised electrons between layers allow conductivity along the layers.
某些元素和化合物形成巨共价晶格，数十亿个原子通过共价键连接成一个连续的网络。金刚石、石墨、石墨烯和二氧化硅（SiO₂）是经典例子。金刚石是正四面体结构，每个碳原子与四个碳原子成键，硬度极高且不导电。石墨具有六边形环层状结构，层间离域电子使其沿层面方向能够导电。
11. Properties of Covalent Compounds | 共价化合物的性质
Simple molecular (covalent) substances have low melting and boiling points because only weak intermolecular forces need to be overcome, not the strong covalent bonds within molecules. They tend to be soft and often exist as gases or liquids at room temperature. Giant covalent substances, in contrast, have very high melting points and are usually hard because many covalent bonds must be broken. Most covalent compounds do not conduct electricity when solid or liquid (graphite being a notable exception).
简单分子共价物质熔沸点较低，因为只需克服微弱的分子间作用力，而不必破坏分子内牢固的共价键。它们通常较软，常温下多为气态或液态。相反，巨共价物质熔沸点极高，通常硬度很大，因为需要断裂大量共价键。大多数共价化合物在固态或液态时不导电（石墨是明显的例外）。
12. Summary of Exam Tips | 考点总结与应考技巧
For Edexcel exams, always show outer electrons clearly in dot‑and‑cross diagrams. Label lone pairs and correctly calculate total valence electrons. When predicting shapes, use VSEPR stepwise: count electron pairs, decide arrangement, state shape, and give the bond angle with the correct degree symbol. Use the correct terminology: 'linear', 'trigonal planar', 'tetrahedral', 'pyramidal', 'bent', 'trigonal bipyramidal', 'octahedral'. In polarity questions, discuss both bond polarity and molecular symmetry. Finally, remember that dative bonds are drawn with an arrow from donor to acceptor after the bond has formed.
在爱德思考卷中，务必在点叉图中清晰地显示最外层电子。标出孤对电子，正确计算价电子总数。在预测形状时，按步骤使用 VSEPR：计算电子对数 → 确定排布 → 说明形状 → 给出带正确度数符号的键角。要使用正确的术语：'linear'、'trigonal planar'、'tetrahedral'、'pyramidal'、'bent'、'trigonal bipyramidal'、'octahedral'。在涉及极性问题时，既要讨论键的极性，也要考虑分子对称性。最后，别忘记配位键已经形成后，须用从供体指向受体的箭头表示。
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