📚 Covalent Bonding: A Comprehensive Guide | 共价键 考点精讲
In this revision guide, we explore the core principles of covalent bonding as required by the IB and Edexcel Chemistry specifications. Understanding how atoms share electrons to achieve stability is fundamental to explaining molecular structures, physical properties, and chemical reactivity. Whether you are preparing for an end-of-topic test or final examinations, this article will provide clear explanations, comparative tables, and exam-focused tips to help you master covalent bonding.
本复习指南将深入探讨 IB 和 Edexcel 化学课程中关于共价键的核心原理。理解原子如何通过共享电子达到稳定构型，是解释分子结构、物理性质和化学反应活性的基础。无论你正在准备单元测验还是最终大考，本文都将通过清晰的概念讲解、对比表格和应试技巧，帮助你彻底掌握共价键这一考点。
1. What is a Covalent Bond? | 什么是共价键？
A covalent bond is a chemical bond formed by the electrostatic attraction between two positive nuclei and a shared pair of electrons. Typically, this occurs between non-metal atoms that have similar electronegativities. Each atom contributes at least one electron to the shared pair, allowing both to attain a noble gas electron configuration. The bond is directional and can be represented by a single line (e.g., H–H).
共价键是通过两个带正电的原子核与一对共用电子之间的静电吸引力形成的化学键。通常，这种键合发生在电负性相近的非金属原子之间。每个原子至少提供一个电子参与共用，从而使双方都能达到稀有气体电子构型。这种键具有方向性，可以用一条短线表示（如 H–H）。
The concept of covalent bonding was first systematically developed by Gilbert N. Lewis, who proposed that atoms share electrons to complete their octets. For hydrogen, the ‘duet’ rule applies, meaning it needs only two electrons to resemble helium. The shared pair is localised between the two nuclei, creating a region of high electron density that holds the atoms together.
共价键的概念最早由吉尔伯特·路易斯系统提出，他认为原子通过共享电子来完成八隅体规则。对氢而言，适用的是“二隅体”规则，即它只需要两个电子就可以达到氦的结构。共用电子对定域在两个原子核之间，形成一个高电子密度区域，把两个原子紧密地维系在一起。
2. Formation of Covalent Bonds: Energetics and Octet Rule | 共价键的形成：能量变化与八隅体规则
Covalent bond formation is an exothermic process. When two atoms approach each other, the potential energy of the system decreases as the shared electrons are attracted to both nuclei. At the optimum internuclear distance (the bond length), the energy is at a minimum. Any closer approach causes repulsion between the nuclei, sharply increasing energy. This energy minimum corresponds to the bond energy – the energy released when the bond forms or the energy required to break it.
共价键的形成是放热过程。当两个原子相互接近时，由于共用电子同时受到两个原子核的吸引，体系的势能逐渐降低。在最佳的核间距（即键长）处，能量达到最小值。若距离进一步缩短，原子核之间的排斥力会急剧增大，使能量升高。这个能量最小值对应的就是键能——形成该键时放出的能量，或是断裂该键所需的能量。
Most covalent bonds obey the octet rule: atoms share electrons until they are surrounded by eight valence electrons. There are important exceptions, such as boron trifluoride (BF₃) where boron has only six electrons, or phosphorus pentachloride (PCl₅) where phosphorus has ten electrons. These exceptions often arise when elements from Period 3 or beyond use available d-orbitals to accommodate extra electrons, a concept known as ‘expansion of the octet’.
大多数共价键遵守八隅体规则：原子共享电子直至被八个价电子包围。但是也存在重要例外，例如三氟化硼（BF₃）中的硼仅拥有六个电子，或者五氯化磷（PCl₅）中的磷拥有十个电子。这些例外通常出现在第三周期及之后的元素，它们可以利用可用的 d 轨道容纳额外电子，即所谓的“八隅体扩展”。
3. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity is the ability of an atom to attract the shared pair of electrons in a covalent bond. The Pauling scale is commonly used, with fluorine assigned the highest value of 4.0. When two atoms forming a covalent bond have different electronegativities, the electron pair is pulled more strongly towards the more electronegative atom. This creates a polar covalent bond, with partial charges δ⁺ and δ⁻ on the atoms. The larger the difference, the more polar the bond.
电负性指原子在共价键中吸引共用电子对的能力。通常使用鲍林标度，氟被定义为最高值 4.0。当成键的两个原子电负性不同时，电子对会被拉向电负性更强的原子一方，形成极性共价键，原子上出现部分电荷 δ⁺ 和 δ⁻。电负性差值越大，键的极性越强。
A bond is generally considered non-polar covalent when the electronegativity difference is less than 0.4. If the difference is between 0.4 and about 1.7, the bond is polar covalent. Above about 1.7, the bond may be considered ionic. However, these boundaries are guidelines; many compounds exhibit intermediate character. For Edexcel and IB, you must be able to predict bond type from electronegativity values and to place partial charges correctly on Lewis structures.
通常，电负性差值小于 0.4 时，键被视为非极性共价键；差值在 0.4 至约 1.7 之间时，为极性共价键；大于约 1.7 时，键可被视为离子键。但这些界限只是参考，很多化合物表现出介于两者之间的性质。对于 Edexcel 和 IB 考试，你需能够根据电负性数值预测键的类型，并在路易斯结构式中正确标注部分电荷。
4. Coordinate (Dative) Covalent Bonds | 配位（给予）共价键
A coordinate covalent bond, also known as a dative bond, is a covalent bond in which both electrons in the shared pair come from the same atom. Once formed, a dative bond is indistinguishable from any other covalent bond. It is typically depicted using an arrow pointing from the donor atom to the acceptor atom. Common examples include the ammonium ion (NH₄⁺), where the nitrogen atom in ammonia donates a lone pair to an H⁺ ion, and carbon monoxide (CO).
配位共价键，也称给予键，是一种特殊的共价键，其中共用的电子对全部由同一个原子提供。一旦形成，给予键与普通共价键在外观和性质上没有区别。它通常用从供体原子指向受体原子的箭头来表示。常见的例子包括铵根离子（NH₄⁺），其中氨气的氮原子提供一对孤对电子给 H⁺；还有一氧化碳（CO）。
To identify a dative bond in a Lewis structure, count the electrons contributed by each atom assuming all bonds are normal covalent. If an atom appears to have gained an extra electron pair without providing electrons to the bond, a dative bond is likely present. In the ammonium ion, after forming three N–H single bonds, nitrogen still has a lone pair. The H⁺ ion has no electrons to share, so the N–H bond formed is entirely from nitrogen’s lone pair.
要在路易斯结构中辨认配位键，可以先假设所有键都是普通共价键，然后计算每个原子提供的电子数。如果某个原子似乎无缘无故多了一对电子参与成键，而另一个原子没有贡献电子，那么配位键就很可能存在。在铵根离子中，氮形成三个正常的 N–H 单键后，还剩一对孤对电子；而 H⁺ 没有电子可以提供，因此新形成的 N–H 键完全来自氮的孤对电子。
5. Sigma (σ) and Pi (π) Bonds | σ 键与 π 键
Covalent bonds can be classified by the symmetry of the orbital overlap. A sigma (σ) bond results from the head-on overlap of atomic orbitals along the internuclear axis. The electron density is concentrated directly between the two nuclei. A pi (π) bond is formed by the sideways overlap of two p-orbitals perpendicular to the internuclear axis. Electron density in a π bond lies above and below the plane of the nuclei.
共价键可以根据轨道重叠的对称性进行分类。σ 键是由原子轨道沿着核间轴“头对头”重叠而成，电子密度集中在两个原子核之间。π 键则是由两个 p 轨道垂直于核间轴“肩并肩”侧向重叠而成，其电子密度分布在原子核平面的上方和下方。
In a single bond, there is always one σ bond. A double bond consists of one σ bond and one π bond. A triple bond contains one σ bond and two π bonds. Because π bonds have less effective overlap, they are weaker than σ bonds, which explains why double and triple bonds are more reactive than single bonds. In molecules like ethene (C₂H₄) and ethyne (C₂H₂), the presence of π bonds restricts rotation around the bond axis, giving rise to cis-trans stereoisomerism.
单键中总是包含一个 σ 键。双键由一个 σ 键和一个 π 键组成。三键则包含一个 σ 键和两个 π 键。由于 π 键的重叠效率较低，导致其强度弱于 σ 键，这也解释了为什么双键和三键通常比单键更活泼。在像乙烯（C₂H₄）和乙炔（C₂H₂）这样的分子中，π 键的存在限制了键轴的旋转，从而产生顺反立体异构现象。
6. Bond Length and Bond Energy | 键长与键能
Bond length is the equilibrium distance between the nuclei of two bonded atoms. As the number of shared electron pairs increases, the bond length decreases: a triple bond is shorter than a double bond, which is shorter than a single bond. This is because a greater number of electrons pulls the nuclei closer together. Bond energy (or bond enthalpy) is the energy required to break one mole of a particular covalent bond in the gaseous state, averaged over a range of compounds.
键长是两个成键原子核之间的平衡距离。共用电子对的数量越多，键长越短：三键比双键短，双键又比单键短。这是因为更多的电子把两个原子核拉得更紧。键能（或称键焓）是指断裂气态中一摩尔某种共价键所需的能量，通常取一系列化合物的平均值。
For diatomic molecules, the bond dissociation energy is precisely defined. For example, the H–H bond energy is +436 kJ mol⁻¹, while the Cl–Cl bond energy is +243 kJ mol⁻¹. The triple bond in N₂ has a very high bond energy (+945 kJ mol⁻¹), explaining why nitrogen gas is relatively inert. A general trend exists: shorter bonds tend to be stronger. This inverse relationship is important when discussing reaction enthalpy changes using bond enthalpies.
对于双原子分子，键离解能是精确确定的。例如，H–H 的键能为 +436 kJ mol⁻¹，而 Cl–Cl 的键能为 +243 kJ mol⁻¹。N₂ 中的三键键能非常高（+945 kJ mol⁻¹），这解释了为什么氮气相对惰性。存在一个普遍趋势：较短的键通常更强。在使用键焓计算反应焓变时，这种反比关系至关重要。
7. 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 3D shapes of molecules and ions. It states that electron pairs around a central atom (both bonding pairs and lone pairs) repel each other and arrange themselves as far apart as possible to minimise repulsion. The resulting geometry is determined by the total number of electron domains (pairs). Lone pairs exert a greater repulsive force than bonding pairs, compressing bond angles slightly.
价层电子对互斥理论（VSEPR）可以用来预测分子和离子的三维形状。它指出，中心原子周围的所有电子对（包括成键电子对和孤对电子）会相互排斥，并尽可能远离，从而使排斥力最小。最终的空间构型取决于电子域（电子对）的总数。孤对电子的排斥力大于成键电子对，会稍微压缩键角。
Table: Common VSEPR Shapes and Bond Angles
| Electron Domains | Bonding Pairs | Lone Pairs | Shape | Bond Angle | Example |
|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° | BeCl₂, CO₂ |
| 3 | 3 | 0 | Trigonal Planar | 120° | BF₃, SO₃ |
| 3 | 2 | 1 | Bent / V-shaped | ~117° | SO₂, O₃ |
| 4 | 4 | 0 | Tetrahedral | 109.5° | CH₄, NH₄⁺ |
| 4 | 3 | 1 | Trigonal Pyramidal | ~107° | NH₃, PH₃ |
| 4 | 2 | 2 | Bent / V-shaped | ~104.5° | H₂O, OF₂ |
| 5 | 5 | 0 | Trigonal Bipyramidal | 90°, 120° | PCl₅ |
| 6 | 6 | 0 | Octahedral | 90° | SF₆ |
When determining shape, first draw the Lewis structure and count the number of bonding and non-bonding pairs on the central atom. Treat any multiple bond as one bonding domain. Then, choose the arrangement that minimises electron-pair repulsion. Remember: lone pair–lone pair repulsion > lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion. This explains why the bond angle in water (104.5°) is smaller than the tetrahedral angle (109.5°).
判断分子形状时，首先要画出路易斯结构，数出中心原子周围的成键电子对和孤对电子数。多重键被视为一个成键域。然后选择使电子对排斥力最小的排布方式。请记住：孤对-孤对排斥 > 孤对-成键对排斥 > 成键对-成键对排斥。这解释了为何水分子的键角（104.5°）小于标准四面体角（109.5°）。
8. Polarity of Molecules: From Bond Dipoles to Molecular Dipoles | 分子的极性：从键偶极到分子偶极
A molecule is polar if it has a net dipole moment – meaning the vector sum of all its bond dipoles is non-zero. For a diatomic molecule with a polar bond like HF, the molecule is automatically polar. For molecules with three or more atoms, molecular geometry determines whether individual bond polarities cancel. Symmetrical molecules such as CO₂ (linear) and CCl₄ (tetrahedral) are non-polar because the dipoles cancel. In contrast, H₂O is polar because the bond dipoles do not cancel; the bent shape creates a net dipole.
如果一个分子具有净偶极矩——即所有键偶极的矢量和不为零，那么它就是极性分子。对于像 HF 这样含有极性键的双原子分子，分子必然是极性的。对于三原子及以上的分子，空间构型决定了个别键的极性是否会相互抵消。对称分子，如直线型的 CO₂ 和四面体型的 CCl₄，由于偶极互相抵消而呈非极性。相比之下，H₂O 是极性分子，因为键偶极没有抵消，弯曲形状产生了净偶极。
In the IBO and Edexcel exams, you may be asked to predict and explain molecular polarity. The approach is to (1) identify polar bonds using electronegativity differences, (2) consider the VSEPR shape, and (3) determine whether the dipole moments cancel. For example, BF₃ is trigonal planar and the three equivalent B–F bonds are polar, but their vector sum is zero, so the molecule is non-polar. In NH₃, the trigonal pyramidal shape prevents cancellation, giving a net dipole.
在 IB 和 Edexcel 考试中，你可能会被要求预测并解释分子的极性。解题思路是：（1）根据电负性差判断极性键；（2）考虑 VSEPR 形状；（3）判断偶极矩是否抵消。例如，BF₃ 是平面三角形，三个等价的 B–F 键是极性的，但它们的矢量和为零，因此分子是非极性的。而在 NH₃ 中，三角锥形形状阻止了偶极抵消，分子具有净偶极。
9. Resonance Structures and Delocalisation | 共振结构与电子离域
Some molecules or polyatomic ions cannot be adequately described by a single Lewis structure. When two or more valid Lewis structures differ only in the position of electrons (not atoms), they are called resonance structures. The true structure is a resonance hybrid – a blend of the contributing structures with delocalised electrons. A classic example is the carbonate ion, CO₃²⁻, which has three equivalent resonance forms, each with one C=O double bond and two C–O single bonds.
有些分子或多原子离子无法用单一的路易斯结构充分描述。当存在两个或以上有效路易斯结构，且它们仅在电子位置上有差异（原子位置不变）时，就叫做共振结构。真实的结构是共振杂化体——是各贡献结构的混合，电子处于离域状态。一个经典例子是碳酸根离子 CO₃²⁻，它有三个等价的共振式，每个含有一个 C=O 双键和两个 C–O 单键。
Experimental evidence shows that all three carbon–oxygen bonds in CO₃²⁻ are identical in length and strength, intermediate between single and double bonds. Similarly, the benzene molecule (C₆H₆) is best represented by two resonance structures with alternating single and double bonds. The actual structure has six equal C–C bonds with bond order 1.5. Delocalisation of π electrons across the ring gives benzene extra stability, known as aromatic stability.
实验证据表明，CO₃²⁻ 中三个碳氧键长度和强度完全相同，介于单键和双键之间。类似地，苯分子（C₆H₆）的最佳描述是两个交替单双键的共振结构。真实结构中六个 C–C 键等长，键级为 1.5。π 电子环状离域赋予苯额外的稳定性，即芳香稳定性。
Resonance also explains why some molecules do not behave as predicted by the simple Lewis diagram. For example, SO₂ has two major resonance forms; the actual molecule has two equivalent S–O bonds and is bent. The concept of formal charge helps determine the most stable resonance contributor. The major contributor usually has the fewest atoms with non-zero formal charge, and negative charges reside on the most electronegative atoms.
共振也可以解释为什么某些分子的行为与简单路易斯图的预测不同。例如，SO₂ 有两个主要的共振式；实际分子有两个等价的 S–O 键并呈 V 形。形式电荷的概念有助于判定最稳定的共振贡献体。通常，主要贡献体拥有最少非零形式电荷的原子，且负电荷位于电负性最大的原子上。
10. Comparison of Bonding Types: Covalent, Ionic, and Metallic | 化学键类型比较：共价键、离子键与金属键
It is essential to distinguish covalent bonding from ionic and metallic bonding. Ionic bonding arises from the electrostatic attraction between oppositely charged ions, often formed between metals and non-metals with large electronegativity differences. Electrons are transferred, not shared. Metallic bonding involves a lattice of positive ions surrounded by a ‘sea’ of delocalised electrons, which accounts for electrical conductivity and malleability.
区分共价键与离子键、金属键十分重要。离子键产生于带相反电荷的离子之间的静电力，通常发生在电负性差异大的金属与非金属之间，电子发生转移而非共享。金属键则是由正离子组成的晶格浸没在离域电子的“海洋”中，这解释了金属的导电性和延展性。
| Property | Covalent | Ionic | Metallic |
|---|---|---|---|
| Particles | Molecules or giant network | Ions in a lattice | Positive ions and delocalised electrons |
| Melting/Boiling Points | Low for simple molecular, high for giant covalent | High to very high | Generally high |
| Electrical Conductivity | Poor (except graphite) | Only when molten or in solution | Excellent (solid and liquid) |
| Solubility | Often insoluble in water; soluble in organic solvents | Many soluble in water | Insoluble |
Giant covalent structures, such as diamond and silicon dioxide, consist of a vast network of covalent bonds extending throughout the crystal. They have high melting points and extreme hardness, but do not conduct electricity (except in the case of graphite, where delocalised electrons between layers allow conductivity). These properties are often examined, so be ready to compare and contrast with simple molecular substances like iodine.
巨型共价结构，如金刚石和二氧化硅，由贯穿整个晶体的庞大共价键网络组成。它们具有极高的熔点和硬度，但不导电（石墨除外，其层间的离域电子允许导电）。这些性质经常被考核，因此要准备好将之与碘等简单分子物质进行比较和对比。
11. Intermolecular Forces vs. Covalent Bonds | 分子间作用力与共价键
Do not confuse the strong covalent bonds holding atoms together within a molecule with the much weaker intermolecular forces that attract separate molecules. Covalent bond energies range from 150 to over 1000 kJ mol⁻¹, whereas intermolecular forces like van der Waals forces typically range from 0.5 to 30 kJ mol⁻¹. This distinction explains why simple molecular substances like chlorine (Cl₂) are gases at room temperature – only weak intermolecular forces need to be overcome during melting or boiling, not the Cl–Cl covalent bond.
请勿将分子内强大的共价键与分子间微弱的吸引力混淆。共价键的键能范围约为 150 到超过 1000 kJ mol⁻¹，而范德华力等分子间作用力通常只有 0.5 到 30 kJ mol⁻¹。这种区别解释了为什么像氯气（Cl₂）这样的简单分子物质在室温下是气体——熔化或沸腾时只需克服微弱的分子间作用力，而无需破坏 Cl–Cl 共价键。
Hydrogen bonding, a special strong dipole–dipole interaction, is still an intermolecular force and is far weaker than a covalent bond. In water, the O–H covalent bond energy is about 463 kJ mol⁻¹, while the hydrogen bond between water molecules is about 20 kJ mol⁻¹ per bond. IB and Edexcel questions frequently ask students to explain physical properties like melting points, boiling points, and solubility by referring to the type of bonding and intermolecular forces present, not by breaking covalent bonds.
氢键作为一种特殊的强偶极-偶极相互作用，仍然属于分子间作用力，远比共价键弱。在水中，O–H 共价键的键能约为 463 kJ mol⁻¹，而水分子间的氢键大约只有 20 kJ mol⁻¹。IB 和 Edexcel 的考题经常要求学生在解释熔点、沸点和溶解度等物理性质时，根据所存在的化学键和分子间作用力类型进行分析，而不是错误地认为破坏了共价键。
12. Common Exam Questions and Tips for Success | 常见考题与应试技巧
Covalent bonding appears extensively throughout the IB and Edexcel Chemistry assessments. Typical question styles include: drawing Lewis structures for unfamiliar molecules, determining molecular shape and bond angles using VSEPR, predicting polarity, explaining physical properties in terms of structure and bonding, and comparing bond lengths/strengths. In data-based questions, you may be given electronegativity values and asked to deduce bond type or draw appropriate dipoles.
共价键在 IB 和 Edexcel 化学考试中无处不在。常见题型包括：为陌生分子绘制路易斯结构式；利用 VSEPR 判定分子形状和键角；预测分子的极性；根据结构和键型解释物理性质；以及比较键长和键强。在数据题中，你可能会被给予电负性值，要求推断键型或绘制合适的偶极标识。
Key tips:
- Always check the number of valence electrons before drawing Lewis structures. Count lone pairs carefully – a common error is missing lone pairs or violating the octet rule for Period 2 elements.
- Memorise the basic VSEPR geometries and the effect of lone pairs on bond angles. Practice with both neutral molecules and ions.
- When explaining molecular polarity, explicitly mention symmetry. Use the phrase ‘the bond dipoles cancel due to the symmetrical arrangement’ for non-polar molecules with polar bonds.
- For comparison questions, make direct reference to the type of structure and forces that must be overcome – e.g., ‘in silicon dioxide, covalent bonds must be broken, whereas in CO₂ only weak intermolecular forces must be overcome’.
- Learn to draw resonance hybrids using dashed lines and partial bonds, especially for benzene, carbonate, and nitrate ions.
应试技巧：
- 绘制路易斯结构之前，务必先核对价电子总数。仔细计算孤对电子——常见错误是遗漏孤对电子或违反第二周期元素的八隅体规则。
- 牢记基本的 VSEPR 构型以及孤对电子对键角的影响。多练习中性分子和离子的题型。
- 解释分子极性时，要明确提到对称性。对于含有极性键的非极性分子，使用“键偶极因对称排列而抵消”这一短语。
- 在比较题中，直接指出需要克服的结构类型和作用力——例如，“在二氧化硅中，需要断裂共价键，而在 CO₂ 中仅需克服微弱的分子间作用力”。
- 学会用虚线和部分键来绘制共振杂化体，特别是苯、碳酸根和硝酸根离子。
Finally, always read the question command terms carefully. Words like ‘explain’, ‘predict’, ‘deduce’, and ‘compare’ each require a specific response structure. Practice past paper questions and mark your answers using the relevant mark schemes to internalise the level of detail expected.
最后，务必仔细阅读题目中的指令词。“解释”、“预测”、“推断”和“比较”等词语各自需要特定的回答结构。多做历年真题，并参照相应评分标准批改自己的答案，内化答题所需的详细程度。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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