AS Chemistry: Covalent Bonding Essentials | AS 化学：共价键考点精讲

📚 AS Chemistry: Covalent Bonding Essentials | AS 化学：共价键考点精讲

Covalent bonding is a cornerstone of AS Chemistry, explaining how atoms share electrons to form molecules and giant structures. Understanding the nature of the covalent bond, its polarity, molecular shape, and associated forces is essential for predicting physical properties and reactivity. This article breaks down all the key concepts you need to master for your exams, with each point presented in English and Chinese to reinforce learning.

共价键是 AS 化学的基石，它解释了原子如何通过共享电子形成分子和巨型结构。理解共价键的本质、极性、分子形状及其相关作用力，对于预测物理性质和反应性至关重要。本文分解了你考试必须掌握的所有核心概念，每个要点均以中英文对照呈现，以强化学习。

1. What is a Covalent Bond? | 什么是共价键？

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These shared electrons are attracted to the nuclei of both atoms, which holds them together. Typically, covalent bonds form between non-metal atoms with similar electronegativities, allowing both to achieve a more stable electron configuration, often that of a noble gas.

共价键是由原子间共享电子对形成的化学键。这些共享电子被两个原子核所吸引，从而使原子结合在一起。通常，共价键形成于电负性相似的非金属原子之间，使双方都能获得更稳定的电子构型，通常是惰性气体的结构。

The simplest example is the hydrogen molecule, H₂. Each hydrogen atom has one electron; by sharing their electrons, both atoms gain the configuration of helium. The shared pair is localised between the two nuclei, forming a stable sigma bond.

最简单的例子是氢分子 H₂。每个氢原子有一个电子；通过共享它们的电子，两个原子都获得了氦的电子构型。共享电子对定域在两个原子核之间，形成一个稳定的 σ 键。

2. Lewis Structures and Octet Rule | 路易斯结构与八隅规则

Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist. The octet rule states that atoms tend to share, lose, or gain electrons until they are surrounded by eight valence electrons (like a noble gas), except for hydrogen which aims for two.

路易斯结构是显示分子中原子间连接键以及可能存在的孤对电子的示意图。八隅规则指出，原子倾向于共享、失去或获得电子，直到最外层有八个价电子（如惰性气体），氢则倾向于两个电子。

To draw a Lewis structure, count the total valence electrons, arrange atoms, place a bonding pair between each pair of adjacent atoms, then distribute remaining electrons as lone pairs to satisfy the octet rule. Double and triple bonds may be needed if there are too few electrons to complete octets.

画路易斯结构时，首先计算总价电子数，排列原子，在每对相邻原子间放置一对电子作为键，然后将剩余电子分配为孤对电子以满足八隅体。如果电子太少不足以完成八隅体，可能需要双键或三键。

Common exceptions include molecules like BF₃, where boron has only six electrons, and SF₆, where sulfur expands its octet to 12 electrons. Understanding these exceptions is crucial for exam questions.

常见的例外包括 BF₃ 分子，其中硼只有六个电子，以及 SF₆，其中硫将八隅体扩展至 12 个电子。理解这些例外对考试至关重要。

3. Types of Covalent Bonds: Single, Double, Triple | 共价键类型：单键、双键、三键

Atoms can share one, two, or three pairs of electrons, forming single, double, or triple covalent bonds respectively. A single bond (e.g., C–C) contains one σ bond. A double bond (e.g., C=C) comprises one σ bond and one π bond, while a triple bond (e.g., N≡N) has one σ and two π bonds.

原子可以共享一对、两对或三对电子，分别形成单键、双键和三键。单键（如 C–C）包含一个 σ 键。双键（如 C=C）由一个 σ 键和一个 π 键构成，而三键（如 N≡N）由一个 σ 键和两个 π 键构成。

Bond multiplicity strongly influences bond length and strength. Multiple bonds are shorter and stronger than single bonds between the same atoms. For instance, the carbon–carbon bond length decreases from 154 pm in ethane (C–C) to 134 pm in ethene (C=C) and 120 pm in ethyne (C≡C).

键的级数强烈影响键长和键能。在相同原子之间，多重键比单键更短、更强。例如，碳–碳键长从乙烷 (C–C) 的 154 pm 减小到乙烯 (C=C) 的 134 pm 和乙炔 (C≡C) 的 120 pm。

4. Bond Length and Bond Energy | 键长与键能

Bond length is the average distance between the nuclei of two bonded atoms. Bond energy (bond enthalpy) is the energy required to break one mole of bonds in the gaseous state. There is an inverse relationship: shorter bonds are generally stronger, i.e., have higher bond energies.

键长是两个成键原子核之间的平均距离。键能（键焓）是在气态下断裂 1 摩尔键所需的能量。两者之间呈反比关系：较短的键通常更强，即具有更高的键能。

Factors influencing bond length include atomic size, bond order, and the electronegativity difference. For example, H–F bond is shorter than H–I because fluorine is smaller. The table below shows typical values for selected bonds.

影响键长的因素包括原子大小、键级和电负性差异。例如，H–F 键比 H–I 键短，因为氟更小。下表展示了部分键的典型数据。

Bond	Bond length / pm	Bond energy / kJ mol⁻¹
C–C	154	347
C=C	134	614
C≡C	120	839
H–F	92	568
H–Cl	127	431

In exam settings, you might be asked to explain why bond length affects reactivity. Shorter, stronger bonds require more energy to break, making the molecule less reactive in reactions where bond breaking is rate-determining.

在考试中，你可能会被要求解释键长为何影响反应性。较短、较强的键需要更多能量才能断裂，因此在断键步骤为速率控制的反应中，分子活性较低。

5. Electronegativity and Bond Polarity | 电负性与键的极性

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. The Pauling scale is commonly used. When two atoms with different electronegativities form a bond, the electron pair is pulled more towards the more electronegative atom, creating a polar covalent bond with a dipole moment.

电负性是一个原子在共价键中吸引电子对的能力。通常使用鲍林标度。当两个电负性不同的原子成键时，电子对会被拉向电负性更高的原子一侧，形成具有偶极矩的极性共价键。

The difference in electronegativity (ΔEN) determines bond polarity. A small difference (e.g., < 0.5) gives a non-polar covalent bond. A moderate difference (approximately 0.5–1.7) yields a polar covalent bond. A large difference (> 1.7) tends to produce ionic bonding. However, the boundary is not absolute; exam boards often treat compounds like HF as polar covalent despite a ΔEN >1.7.

电负性差值（ΔEN）决定了键的极性。小差值（如 < 0.5）产生非极性共价键。中等差值（约 0.5–1.7）产生极性共价键。大差值（> 1.7）倾向于形成离子键。不过，这个界限并非绝对；考试局常将 HF 等化合物视为极性共价键，尽管 ΔEN > 1.7。

Molecular polarity also depends on molecular geometry. A molecule with polar bonds can be non-polar if the geometry cancels out the dipoles, as in CCl₄ (tetrahedral). Conversely, CO₂ has polar bonds but is linear and non-polar, while H₂O is bent and polar.

分子极性还取决于分子几何构型。具有极性键的分子，如果几何形状使偶极抵消，则可能为非极性分子，如 CCl₄（四面体）。与之相反，CO₂ 有极性键但呈直线形且为非极性，而 H₂O 为 V 形且具有极性。

6. Molecular Shapes: VSEPR Theory | 分子形状：价层电子对互斥理论

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the shape of molecules. It states that electron pairs around a central atom repel each other and arrange themselves as far apart as possible, minimising repulsion. Both bonding pairs and lone pairs must be considered; lone pairs occupy more space and compress bond angles.

价层电子对互斥（VSEPR）理论用于预测分子形状。该理论认为，中心原子周围的电子对相互排斥并尽可能远离，以最小化排斥。必须同时考虑成键电子对和孤对电子；孤对电子占据更大空间并会压缩键角。

Common shapes you must know include: linear (2 bond pairs, 0 lone pairs, angle 180°), trigonal planar (3,0, 120°), tetrahedral (4,0, 109.5°), trigonal pyramidal (3,1, ~107°), bent/v-shaped (2,2, ~104.5°), and trigonal bipyramidal (5,0, 90° and 120°). Each shape’s name, bond angles, and number of lone pairs are typical AS exam requirements.

你须掌握的常见形状包括：直线形（2 个成键对，0 个孤对，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°）、V 形（2,2, ~104.5°）以及三角双锥形（5,0, 90° 和 120°）。每种形状的名称、键角和孤对电子数都是 AS 考试的典型考点。

For example, NH₃ has three bonding pairs and one lone pair, giving it a trigonal pyramidal shape with a bond angle of about 107°. The lone pair repels bonding pairs more strongly, reducing the angle from the ideal tetrahedral 109.5°.

例如，NH₃ 有三对成键电子和一对孤对电子，呈三角锥形，键角约为 107°。孤对电子对成键电子对的排斥力更大，使键角从理想四面体的 109.5° 减小。

7. Sigma (σ) and Pi (π) Bonds | σ 键与 π 键

Sigma and pi bonds describe the type of orbital overlap. A sigma bond results from head-on overlap of atomic orbitals along the internuclear axis, allowing free rotation. All single bonds are σ bonds.

σ 键和 π 键描述了轨道重叠的类型。σ 键由原子轨道沿两核连线“头碰头”重叠形成，可自由旋转。所有的单键都是 σ 键。

A pi bond is formed by sideways overlap of p-orbitals above and below the internuclear axis. It is weaker than a sigma bond and restricts rotation because breaking the π overlap would be required. Double bonds contain one σ and one π; triple bonds have one σ and two π bonds.

π 键由 p 轨道在核间轴上方和下方并肩重叠形成。它比 σ 键弱，并限制旋转，因为旋转会破坏 π 重叠。双键包含一个 σ 键和一个 π 键；三键包含一个 σ 键和两个 π 键。

The presence of π bonds leads to geometric isomerism in alkenes, a key organic chemistry concept. You should be able to identify σ and π bonds in molecules like ethene (H₂C=CH₂) and nitrogen (N₂).

π 键的存在导致烯烃出现顺反异构，这是一个重要的有机化学概念。你应该能够识别如乙烯 (H₂C=CH₂) 和氮气 (N₂) 分子中的 σ 键和 π 键。

8. Coordinate (Dative) Bonds | 配位键（共价配位键）

A coordinate bond (or dative covalent bond) is a covalent bond in which both electrons come from the same atom. Once formed, it is indistinguishable from an ordinary covalent bond. The atom donating the pair is called the donor; the one accepting is the acceptor.

配位键（又称共价配位键）是指两个电子全部来自同一个原子的共价键。一旦形成，它与普通共价键无法区分。提供电子对的原子称为供体；接受电子对的原子称为受体。

Classic examples include the ammonium ion (NH₄⁺), where a nitrogen atom in ammonia donates a lone pair to a hydrogen ion, H⁺, and the hydroxonium ion (H₃O⁺). In transition metal complexes, ligands frequently form coordinate bonds with a central metal ion.

经典例子包括铵根离子 (NH₄⁺)，氨中的氮原子提供一个孤对电子给氢离子 H⁺；以及水合氢离子 (H₃O⁺)。在过渡金属配合物中，配体常与中心金属离子形成配位键。

NH₃ + H⁺ → NH₄⁺

You must be able to draw Lewis diagrams representing coordinate bonds using an arrow from the donor to the acceptor.

你必须能够使用从供体指向受体的箭头绘制表示配位键的路易斯结构图。

9. Resonance Structures | 共振结构

When a single Lewis structure cannot fully describe a molecule’s bonding, resonance structures are used. The actual structure is a hybrid of all possible Lewis structures, with delocalised electrons distributed over several atoms. This increases stability.

当单一路易斯结构无法完全描述分子的键合情况时，会使用共振结构。实际结构是所有可能路易斯结构的杂化体，电子离域分布在多个原子之上。这增加了稳定性。

Key examples include the carbonate ion (CO₃²⁻), the nitrate ion (NO₃⁻), and benzene. For CO₃²⁻, three equivalent structures exist with a double bond in different positions; the actual ion has three equal C–O bonds with partial double bond character, all the same length.

重要示例包括碳酸根离子 (CO₃²⁻)、硝酸根离子 (NO₃⁻) 和苯。以 CO₃²⁻ 为例，它有三个等价的共振结构，双键位于不同位置；实际离子中三个 C–O 键完全等同，具有部分双键性质，键长均相等。

O₃ ↔ O–O=O ↔ O=O–O

Delocalisation energy is the extra stability gained by spreading the electrons. In benzene, resonance stabilisation makes it less reactive than ethene towards addition reactions, a typical exam question.

离域能是电子分散所带来的额外稳定性。在苯中，共振稳定化使其在加成反应中的活性低于乙烯，这是考试中的常考问题。

10. Giant Covalent Structures | 巨型共价结构

Not all covalent substances exist as simple molecules. Some form giant covalent (macromolecular) structures, where atoms are linked by a network of covalent bonds throughout the whole substance. These materials have very high melting and boiling points because many strong covalent bonds must be broken.

并非所有共价物质都以简单分子形式存在。有些形成巨型共价（大分子）结构，其中原子通过共价键网络连接整个物质。这些材料具有非常高的熔点和沸点，因为必须断裂大量强共价键。

Diamond is one example: each carbon atom is tetrahedrally bonded to four others, forming a rigid three-dimensional network. It is exceptionally hard, an electrical insulator (no mobile electrons), and a thermal conductor.

金刚石是一个例子：每个碳原子以四面体方式与另外四个碳原子键合，形成刚性的三维网络。它极其坚硬，是电绝缘体（无自由移动的电子），但导热性好。

Graphite has a layered structure. Within each layer, each carbon is covalently bonded to three others, forming hexagonal rings. The fourth electron is delocalised in a cloud between layers, allowing graphite to conduct electricity and act as a lubricant because layers slide over one another.

石墨具有层状结构。在每一层内，每个碳原子与另外三个碳原子共价键合，形成六元环。第四个电子在层间离域，使石墨可导电，并且因层间可滑动而充当润滑剂。

Silicon dioxide (SiO₂) also forms a giant covalent lattice similar to diamond, where each silicon is bonded to four oxygens and each oxygen to two silicons. Quartz is a common form.

二氧化硅 (SiO₂) 也形成类似金刚石的巨型共价晶格，其中每个硅原子与四个氧原子键合，每个氧原子与两个硅原子键合。石英是其常见形式。

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AS Chemistry: Covalent Bonding Essentials | AS 化学：共价键 考点精讲