Covalent Bonding in IGCSE Chemistry: Key Concepts | IGCSE 化学：共价键 考点精讲

📚 Covalent Bonding in IGCSE Chemistry: Key Concepts | IGCSE 化学：共价键 考点精讲

Covalent bonding is one of the fundamental types of chemical bonding that IGCSE Chemistry students must master. It explains how non-metal atoms share electrons to achieve stable electronic configurations, forming molecules with distinct shapes and properties. This article covers every essential aspect of covalent bonding: from the basic definition and electron sharing, to drawing dot-and-cross diagrams, understanding simple molecular and giant covalent structures, and linking bonding to physical properties like melting point and electrical conductivity. By the end, you’ll have a thorough grasp of the topic, ready to tackle any exam question with confidence.

共价键是 IGCSE 化学中必须掌握的基本化学键类型之一。它解释了非金属原子如何通过共享电子来达到稳定的电子构型，形成具有独特形状和性质的分子。本文涵盖了共价键的每个关键方面：从基本定义和电子共享，到绘制点叉图，理解简单分子与巨型共价结构，并将键合与熔点、导电性等物理性质联系起来。阅读之后，你将全面掌握该主题，足以自信应对任何考试题目。

1. What Is a Covalent Bond? | 什么是共价键？

A covalent bond is a chemical bond formed when two atoms share one or more pairs of electrons. This sharing allows each atom to attain the electronic configuration of a noble gas, typically an octet (8 electrons) in the outer shell, or a duplet (2 electrons) for hydrogen. Covalent bonding occurs predominantly between non-metal atoms.

共价键是两个原子通过共享一对或多对电子而形成的化学键。这种共享使得每个原子都能获得稀有气体的电子构型，通常是外层八电子（八隅体）结构，氢原子则为二电子（双电子）结构。共价键主要存在于非金属原子之间。

The shared pair of electrons is attracted to the nuclei of both atoms, creating a strong electrostatic attraction that holds the atoms together. Unlike ionic bonding, there is no transfer of electrons; instead, the electrons belong to both atoms simultaneously, forming a molecule.

共享的电子对受到两个原子核的吸引，产生强大的静电吸引力，将原子维系在一起。与离子键不同，这里没有电子转移；电子同时属于两个原子，从而形成分子。

The bond is often represented by a single line between atoms, e.g., H—H for a hydrogen molecule. This line represents one shared pair of electrons. Double and triple bonds involve two or three shared pairs respectively, shown as = or ≡.

共价键通常用原子间的单线表示，例如氢分子的 H—H。这条线代表一对共享电子。双键和三键分别涉及两对或三对共享电子，用 = 或 ≡ 表示。

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2. Why Do Atoms Form Covalent Bonds? | 原子为什么形成共价键？

Atoms form covalent bonds to achieve a more stable electronic arrangement. For most non-metals, this means filling their outermost shell to reach the stable octet configuration of the nearest noble gas. Hydrogen, lithium, and beryllium are exceptions, aiming for a duplet (2 electrons) like helium.

原子形成共价键是为了达到更稳定的电子排布。对大多数非金属而言，这意味着填满最外层，达到与最近稀有气体相同的稳定八电子结构。氢、锂和铍是例外，它们的目标是与氦相似的双电子结构。

By sharing electrons, both atoms can count the shared pair(s) toward their own valency requirement, effectively reducing the overall energy of the system. This energy lowering is what drives bond formation.

通过共享电子，两个原子都可以将共享电子对计入自己的化合价需求，从而有效降低体系的整体能量。这种能量降低正是驱动键形成的动力。

For example, a chlorine atom has 7 outer electrons; it needs one more to complete its octet. Two chlorine atoms each share one electron, forming a Cl—Cl single bond, giving both atoms access to 8 electrons in their outer shells.

例如，氯原子最外层有 7 个电子；需要再多一个电子才能达到八隅体。两个氯原子各共享一个电子，形成 Cl—Cl 单键，使每个原子的最外层都能拥有 8 个电子。

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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 toward itself. When two identical atoms bond, as in H—H or Cl—Cl, the electrons are shared equally, forming a non-polar covalent bond.

电负性是原子在共价键中吸引共享电子对的能力。当两个相同原子成键时，如 H—H 或 Cl—Cl，电子被均匀共享，形成非极性共价键。

If the two atoms have different electronegativities, the electron pair is pulled more toward the more electronegative atom. This creates a polar covalent bond, with a partial negative charge (δ⁻) on the more electronegative atom and a partial positive charge (δ⁺) on the other. For instance, in HCl, chlorine is more electronegative, so the bond is polar.

如果两个原子的电负性不同，电子对会更偏向电负性较大的原子。这就产生了极性共价键，电负性较大的原子带部分负电荷（δ⁻），另一个原子带部分正电荷（δ⁺）。例如在 HCl 中，氯的电负性更大，因此键为极性。

A large difference in electronegativity leads to ionic bonding rather than covalent. In IGCSE, a rule of thumb is that a difference greater than about 1.7 indicates ionic character; smaller differences suggest polar or non-polar covalent bonds. However, typical exam questions focus on non-metal vs. non-metal and the concept of unequal sharing.

电负性差异过大会导致离子键而非共价键。在 IGCSE 中，一个经验法则是差值大于约 1.7 时往往呈现离子键特性；差值较小则表现为极性或非极性共价键。不过，常见考题仍集中在非金属之间的成键，以及共享不均的概念上。

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4. Dot-and-Cross Diagrams | 点叉图

Dot-and-cross diagrams are the standard way to represent covalent bonding in IGCSE. They show the outer electrons of each atom, using dots for electrons from one atom and crosses for those from the other. The shared pair(s) are placed in the overlapping region of the shells.

点叉图是 IGCSE 中表示共价键的标准方法。它们展示每个原子的最外层电子，用一个原子的电子画点、另一原子的电子画叉。共享电子对放置在电子层重叠区域。

When drawing these diagrams, you must show only the outer shell electrons. Write the chemical symbols, then arrange dots and crosses around them to represent the formation of single, double, or triple bonds. Ensure that each atom (except hydrogen) ends up with 8 electrons in its outer shell after sharing.

绘制这些图时，必须仅展示最外层电子。写出化学符号，然后在它们周围排列点和叉，以表示单键、双键或三键的形成。确保每个原子（氢除外）在共享后最外层都拥有 8 个电子。

Common examples you need to know: hydrogen (H₂), chlorine (Cl₂), oxygen (O₂, double bond), nitrogen (N₂, triple bond), water (H₂O), methane (CH₄), ammonia (NH₃), carbon dioxide (CO₂), ethene (C₂H₄), and hydrogen chloride (HCl). Practice drawing each one from memory.

你需要掌握的常见例子有：氢气 (H₂)、氯气 (Cl₂)、氧气 (O₂，双键)、氮气 (N₂，三键)、水 (H₂O)、甲烷 (CH₄)、氨气 (NH₃)、二氧化碳 (CO₂)、乙烯 (C₂H₄) 和氯化氢 (HCl)。要练习默画每一个图。

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5. Single, Double, and Triple Bonds | 单键、双键和三键

A single covalent bond involves one shared pair of electrons, represented by a single line (e.g., H—H). A double bond has two shared pairs (O=O), and a triple bond has three shared pairs (N≡N). The bond strength and bond length vary: triple bonds are the strongest and shortest, while single bonds are the longest and weakest among them.

单共价键涉及一对共享电子，用单线表示（如 H—H）。双键有两对共享电子 (O=O)，三键有三对共享电子 (N≡N)。键的强度和键长各不相同：三键最强且最短，单键则最长且最弱。

Double and triple bonds are found in many important molecules. Oxygen gas is O=O, carbon dioxide is O=C=O, and nitrogen gas is N≡N. Ethene (C₂H₄) contains a C=C double bond, which is a typical feature of alkenes. Understanding multiple bonds helps explain molecular shapes and reactivity.

双键和三键存在于许多重要分子中。氧气是 O=O，二氧化碳是 O=C=O，氮气是 N≡N。乙烯 (C₂H₄) 含有 C=C 双键，这是烯烃的典型特征。理解多重键有助于解释分子的形状和反应活性。

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6. Simple Molecular Structures | 简单分子结构

Many covalent compounds exist as simple molecules with a fixed number of atoms held together by strong covalent bonds. Examples include H₂O, CO₂, CH₄, and NH₃. Within each molecule, the covalent bonds are strong, but the forces between molecules (intermolecular forces) are weak.

许多共价化合物以简单分子的形式存在，由固定数量的原子通过强共价键连接在一起。例子有 H₂O、CO₂、CH₄ 和 NH₃。在分子内部，共价键很强，但分子之间的作用力（分子间力）很弱。

Because of weak intermolecular forces, simple molecular substances typically have low melting and boiling points. Only a small amount of energy is needed to overcome these forces when changing state, not to break the covalent bonds themselves. This is a common exam question: why does iodine (a simple molecular solid) have a low melting point despite strong covalent bonds within I₂ molecules?

由于分子间力较弱，简单分子物质通常具有较低的熔点和沸点。状态变化时只需要少量能量来克服这些分子间力，而不是破坏共价键。这是常见考题：为什么碘（一种简单分子固体）尽管 I₂ 分子内有强共价键，熔点却很低？

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7. Giant Covalent Structures | 巨型共价结构

Some covalent substances form giant covalent structures (also called macromolecules), where all atoms are bonded in a continuous network by strong covalent bonds. Diamond, graphite, silicon dioxide (SiO₂), and silicon carbide (SiC) are key examples.

一些共价物质形成巨型共价结构（也称大分子），其中所有原子通过强共价键连接成连续的网络。金刚石、石墨、二氧化硅 (SiO₂) 和碳化硅 (SiC) 是重要例子。

In diamond, each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a rigid three-dimensional lattice. This makes diamond extremely hard and gives it a very high melting point (over 3500 °C). It does not conduct electricity because all electrons are locked in bonds.

在金刚石中，每个碳原子以共价键与四个其他碳原子相连，呈四面体排列，形成刚性的三维晶格。这使得金刚石极度坚硬，熔点极高（超过 3500 °C）。它不导电，因为所有电子都被束缚在键中。

Graphite, another allotrope of carbon, has a layered structure. Each carbon is bonded to three others in flat hexagonal sheets. The fourth outer electron per carbon becomes delocalised between layers, allowing graphite to conduct electricity along the planes. The layers can slide over each other, making graphite soft and slippery, useful as a lubricant and in pencils.

石墨是碳的另一种同素异形体，具有层状结构。每个碳原子与另外三个碳原子键合，形成平面的六角形片层。每个碳的第四个外层电子在层间离域，使石墨能沿平面导电。层间可以滑动，使石墨质地柔软且有滑腻感，可作润滑剂和铅笔芯。

Silicon dioxide (silica) has a structure similar to diamond, with each silicon atom bonded to four oxygen atoms and each oxygen to two silicon atoms. This results in a very hard, high-melting solid that is the main component of sand and quartz.

二氧化硅（硅石）的结构与金刚石相似，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。这形成了一种非常坚硬、高熔点的固体，是沙子和石英的主要成分。

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8. Properties of Covalent Compounds | 共价化合物的性质

The physical properties of covalent compounds depend heavily on whether they are simple molecular or giant covalent. Simple molecular substances are usually gases or liquids at room temperature, or soft solids with low melting/boiling points. They are often insoluble in water (unless they can form hydrogen bonds, like sugars) and do not conduct electricity in any state because there are no free ions or delocalised electrons.

共价化合物的物理性质很大程度上取决于它们是简单分子还是巨型共价结构。简单分子物质在室温下通常是气体或液体，或者是具有低熔点/沸点的柔软固体。它们通常不溶于水（除非能形成氢键，如糖类），在任何状态下都不导电，因为没有自由离子或离域电子。

Giant covalent compounds have very high melting and boiling points due to the need to break many strong covalent bonds throughout the structure. They are generally hard, though graphite is an exception because of its layered structure. Electrical conductivity varies: diamond and SiO₂ are insulators; graphite conducts due to delocalised electrons.

巨型共价化合物具有极高的熔点和沸点，因为需要破坏整个结构中大量的强共价键。它们通常很硬，但石墨因其层状结构而例外。导电性各有不同：金刚石和 SiO₂ 是绝缘体；石墨由于离域电子而导电。

Volatility is another concept: simple covalent liquids like ethanol or hexane evaporate easily at room temperature, which is linked to their weak intermolecular forces. This property is often contrasted with giant covalent or ionic substances.

挥发性是另一个概念：简单的共价液体如乙醇或己烷在室温下容易蒸发，这与它们微弱的分子间力有关。这一性质常与巨型共价或离子物质相对比。

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9. Molecular Shapes and Bond Angles | 分子形状与键角

The shape of a covalent molecule is determined by the number of electron pairs (bonding and lone pairs) around the central atom, according to VSEPR theory (Valence Shell Electron Pair Repulsion). Electron pairs repel each other and arrange themselves as far apart as possible to minimise repulsion.

根据 VSEPR 理论（价层电子对互斥理论），共价分子的形状由中心原子周围的电子对（成键电子对和孤对电子）数量决定。电子对相互排斥，会尽可能远地排列以最小化排斥力。

Key shapes for IGCSE include: linear (e.g., CO₂, 180° bond angle), bent or V-shaped (e.g., H₂O, about 104.5° due to two lone pairs), trigonal pyramidal (e.g., NH₃, about 107° due to one lone pair), and tetrahedral (e.g., CH₄, 109.5°). While precise angles are not always required, you should be able to describe and recognise these geometries.

IGCSE 需掌握的关键形状包括：直线形（如 CO₂，键角 180°）、弯曲形或 V 形（如 H₂O，由于两对孤对电子，键角约 104.5°）、三角锥形（如 NH₃，由于一对孤对电子，键角约 107°）和四面体形（如 CH₄，109.5°）。虽然精确角度不总是要求，但应能描述并识别这些几何形状。

Lone pairs exert greater repulsion than bonding pairs, which reduces bond angles. Thus, water (2 lone pairs) has a smaller angle than ammonia (1 lone pair). Understanding this helps explain many physical and chemical properties.

孤对电子的排斥力比成键电子对大，因此键角减小。所以，水（2 对孤对电子）的键角小于氨气（1 对孤对电子）。理解这一点有助于解释许多物理和化学性质。

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10. Covalent Bonding in Organic Chemistry | 有机化学中的共价键

Organic chemistry is built on covalent bonding. Carbon has four outer electrons, so it forms four covalent bonds, leading to a vast variety of structures: saturated alkanes with single C—C bonds, unsaturated alkenes with C=C double bonds, and alkynes with triple bonds. Functional groups like alcohols (—OH), carboxylic acids (—COOH), and esters contain polar covalent bonds that influence reactivity.

有机化学建立在共价键的基础上。碳原子有四个外层电子，因此形成四个共价键，导致了种类繁多的结构：具有单 C—C 键的饱和烷烃、具有 C=C 双键的不饱和烯烃，以及具有三键的炔烃。官能团如醇（—OH）、羧酸（—COOH）和酯含有极性共价键，影响反应活性。

For IGCSE, you must be able to draw displayed formulae showing all atoms and bonds. Understanding that carbon forms four bonds, hydrogen one, oxygen two, and nitrogen three (in most cases) is essential for constructing correct structures and avoiding common mistakes.

在 IGCSE 中，你必须能绘制显示所有原子和键的展示式。理解碳形成四个键、氢形成一个、氧形成两个、氮形成三个（大多数情况下）对于构建正确结构并避免常见错误至关重要。

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11. Comparing Covalent and Ionic Bonding | 共价键与离子键的比较

Feature | 特征	Covalent Bonding | 共价键	Ionic Bonding | 离子键
Particle formed | 形成微粒	Molecule or giant lattice | 分子或巨型晶格	Ions in a giant lattice | 巨型晶格中的离子
Bonding mechanism | 键合机制	Electron sharing | 电子共享	Electron transfer | 电子转移
Typical elements | 典型元素	Non-metal + non-metal | 非金属 + 非金属	Metal + non-metal | 金属 + 非金属
Melting/boiling point | 熔沸点	Simple molecules: low; Giant: very high | 简单分子：低；巨型：很高	High | 高
Electrical conductivity | 导电性	Usually not, except graphite | 通常不导电，石墨除外	When molten or dissolved | 熔融或溶解时导电

This comparison is a frequent exam topic. Be prepared to explain differences in terms of structure and bonding. Use particle diagrams and dot-and-cross representations to support your answers.

这一比较是常考的题目。要准备好从结构和键合角度解释差异。使用微粒图和点叉图来佐证你的答案。

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12. Common Misconceptions and Exam Tips | 常见误区与考试技巧

One common mistake is confusing intermolecular forces with covalent bonds. When explaining low melting points of simple molecular substances, students often say “covalent bonds are broken,” which is incorrect. The correct explanation is that weak intermolecular forces are overcome; the covalent bonds within molecules remain intact.

一个常见误区是将分子间力与共价键混淆。解释简单分子物质低熔点时，学生常说“共价键被破坏”，这是错误的。正确的解释是微弱的分子间力被克服；分子内的共价键保持完整。

Another pitfall: not showing all outer electrons in dot-and-cross diagrams, or forgetting to use different symbols (dots and crosses) for the two atoms. Always count electrons carefully to ensure octet/duplet rules are satisfied.

另一个陷阱：在点叉图中未画出所有外层电子，或忘记对两个原子使用不同的符号（点和叉）。务必仔细数清电子数，确保满足八隅体/双电子规则。

In giant covalent structures, students sometimes draw a few bonds and think that’s enough. Remember that the structure extends in all directions; you should indicate this with a diagram showing a portion of the lattice and continuation bonds.

在巨型共价结构中，学生有时只画几个键就认为够了。要记住结构向所有方向延伸；应通过图示展示晶格的一部分，并画出延续键来表示。

Finally, when asked to explain why graphite conducts electricity but diamond does not, link the answer to the presence of one delocalised electron per carbon atom in graphite that can move and carry charge, while in diamond all four outer electrons are used in covalent bonds and are not free to move.

最后，当被要求解释为何石墨导电而金刚石不导电时，答案应联系到石墨中每个碳原子有一个离域电子可以移动并携带电荷，而在金刚石中，所有四个外层电子都参与共价键，不能自由移动。

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