📚 Covalent Bonding | GCSE AQA Chemistry Key Points | 共价键 考点精讲
Covalent bonding is one of the three main types of chemical bonding you need to master for AQA GCSE Chemistry. It occurs between non‑metal atoms that share pairs of electrons to achieve a full outer shell. In this revision guide, we break down everything from simple molecules like water and methane to giant covalent structures such as diamond and graphite, so you can tackle any exam question with confidence.
共价键是 AQA GCSE 化学中必须掌握的三大化学键之一。它发生在非金属原子之间，原子通过共用电子对来达到稳定的满外层电子结构。在这份考点精讲中，我们将从水、甲烷等简单分子一直讲到金刚石、石墨等巨型共价结构，帮助你从容应对所有考试题型。
1. What is a Covalent Bond? | 什么是共价键？
A covalent bond is the strong electrostatic attraction between a shared pair of negatively charged electrons and the positively charged nuclei of the bonded atoms. The bond forms when the outer shells of two non‑metal atoms overlap, allowing each atom to count the shared electrons as part of its own outer shell. This gives both atoms a more stable electronic arrangement, often a full outer shell like that of a noble gas.
共价键是共用电子对与成键原子带正电的原子核之间的强静电吸引力。当两个非金属原子的最外层轨道重叠时，它们共用一对或多对电子，每个原子都可以把这些共享电子视为自己最外层的一部分，从而获得更稳定的电子构型——通常是类似稀有气体的满壳层结构。
2. Why Do Atoms Share Electrons? | 原子为什么要共用电子？
Non‑metal atoms have fairly high electronegativities, meaning they all need to gain electrons to fill their outer shells. If two non‑metals reacted by transferring electrons, one would have to lose electrons, which is energetically unfavourable. Instead, they share electrons so that both can achieve a full outer shell without either atom completely losing control of its electrons. Each shared pair of electrons counts towards the outer‑shell total of both atoms.
非金属原子的电负性都较高，都倾向于获得电子来填满最外层。如果两个非金属通过转移电子的方式结合，其中一个就不得不失去电子，这在能量上是不利的。因此，它们选择共用电子，这样双方都能达到满外层，而不需要任一原子完全失去对电子的控制。每一对共用电子同时计入两个原子的最外层电子总数。
3. Dot and Cross Diagrams | 点叉图（电子式）
A dot and cross diagram is the standard way to represent the sharing of electrons in a covalent molecule. Electrons from one atom are shown as dots and from the other as crosses, making it clear where each electron originated. Only the outer‑shell electrons are drawn. You need to be able to draw these for molecules like H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄, CO₂, C₂H₆ and C₂H₄.
点叉图是表示共价分子中电子共享的标准方法。一个原子的电子用圆点表示，另一个原子的电子用叉号表示，这样可以清楚追踪电子的来源。绘图时只画出最外层电子。你需要能够画出 H₂、Cl₂、O₂、N₂、HCl、H₂O、NH₃、CH₄、CO₂、C₂H₆ 和 C₂H₄ 等分子的点叉图。
₂, two hydrogen atoms each contribute one electron. The shared pair is shown between the two H symbols: H• n a water molecule H₂O, oxygen shares two of its electrons with two hydrogen atoms, forming two single bonds and leaving two lone pairs on the oxygen.
例如，在氢分子 H₂ 中，两个氢原子各提供一个电子，共用电子对画在两个 H 符号之间：H• ×H，形成单共价键。在水分子 H₂O 中，氧原子用两个电子分别与两个氢原子共用，形成两根单键，氧原子上还剩下两对孤对电子。
4. Single, Double and Triple Bonds | 单键、双键与三键
ed pair (2 electrons), a double bond contains two shared pairs (4 electrons), and a triple bond contains three shared pairs (6 electrons). For example, O₂ has a double bond O=O, and N₂ has a triple bond N≡N.
原子可以共用一对、两对或三对电子，分别形成单键、双键和三键。单键含有一对共用电子（2个电子），双键含有两对共用电子（4个电子），三键含有三对共用电子（6个电子）。例如，O₂ 含有 O=O 双键，N₂ 含有 N≡N 三键。
The more electron pairs that are shared, the stronger and shorter the bond. This explains why nitrogen gas N₂ is so unreactive – the triple bond requires a lot of energy to break.
共用的电子对越多，共价键越强、越短。这解释了为什么氮气 N₂ 非常不活泼——三键需要很高的能量才能断裂。
| Bond type | Shared pairs | Example |
|---|---|---|
| Single | 1 (2 e⁻) | H₂, Cl₂, H₂O, CH₄ |
| Double | 2 (4 e⁻) | O₂, CO₂ |
| Triple | 3 (6 e⁻) | N₂ |
5. Simple Molecular Substances | 简单分子物质
Most covalent compounds exist as simple molecules, such as H₂O, CO₂, NH₃ and C₂H₅OH (ethanol). In these substances, the atoms within each molecule are held together by very strong covalent bonds. However, the intermolecular forces between one molecule and its neighbours are weak. These weak forces determine the bulk properties of the substance.
大多数共价化合物以简单分子形式存在，如 H₂O、CO₂、NH₃ 和 C₂H₅OH（乙醇）。在这些物质中，每个分子内部的原子通过很强的共价键结合在一起。但是，分子与相邻分子之间的分子间作用力很弱。正是这些弱作用力决定了物质的宏观性质。
It is a common exam mistake to confuse the strong covalent bonds inside the molecules with the weak intermolecular forces. Always be clear which you are describing when asked about melting points or physical state.
考试中常见的错误是混淆分子内部强的共价键与分子间弱的分子间作用力。在回答有关熔点或物态的问题时，一定要说清楚你描述的是哪种作用力。
6. Properties of Simple Covalent Compounds | 简单共价化合物的性质
Simple molecular substances have low melting and boiling points because only weak intermolecular forces need to be overcome when the substance changes state, not the strong covalent bonds. As a result, many are gases or liquids at room temperature. They do not conduct electricity in any state because the molecules are neutral and there are no free ions or mobile electrons.
简单分子物质具有较低的熔点和沸点，因为物态变化时只需要克服微弱的分子间作用力，而不需要破坏强的共价键。因此，它们在室温下多为气体或液体。在任何状态下它们都不导电，因为分子是电中性的，没有自由离子或可移动的电子。
For example, chlorine Cl₂ is a gas at room temperature, bromine Br₂ is a liquid, and iodine I₂ is a solid. All three consist of diatomic molecules with strong covalent bonds inside each molecule but weak forces between them.
例如，氯气 Cl₂ 在室温下为气体，溴 Br₂ 为液体，碘 I₂ 为固体。三者都由双原子分子组成，分子内部有很强的共价键，但分子间作用力很弱。
7. Giant Covalent Structures – An Introduction | 巨型共价结构简介
Some non‑metal elements and compounds form giant covalent lattices instead of separate molecules. In these structures, billions of atoms are joined by covalent bonds in a continuous network. There are no individual molecules; the whole structure is one giant molecule. The three main examples you need to know for AQA are diamond, graphite and silicon dioxide (silica).
有些非金属单质和化合物会形成巨型共价晶格，而不是独立的分子。在这些结构中，数十亿个原子通过共价键连接成一个连续的网络。没有单独的分子，整个结构就是一个巨大的分子。AQA 考试要求你掌握的三个主要例子是金刚石、石墨和二氧化硅（硅石）。
8. Diamond | 金刚石
In diamond, each carbon atom forms four strong covalent bonds to four other carbon atoms in a tetrahedral arrangement. This rigid, three‑dimensional network makes diamond the hardest natural substance. It has a very high melting point (over 3500 °C) because a huge amount of energy is needed to break the many strong covalent bonds throughout the lattice.
在金刚石中，每个碳原子与另外四个碳原子形成四根强的共价键，呈四面体排列。这种刚性的三维网络使金刚石成为最硬的天然物质。它具有极高的熔点（超过 3500 °C），因为需要极大的能量来破坏整个晶格中无数个强的共价键。
Diamond does not conduct electricity. All four outer‑shell electrons of each carbon atom are used in covalent bonds; there are no delocalised (free) electrons to carry a charge.
金刚石不导电。每个碳原子的四个最外层电子都用于形成共价键，没有离域（自由）电子来传输电荷。
9. Graphite | 石墨
Graphite is also made only of carbon atoms, but its structure is very different. Each carbon atom forms three covalent bonds to three others, creating flat layers of hexagonal rings. The fourth outer‑shell electron of each carbon becomes delocalised between the layers. These delocalised electrons can move freely, allowing graphite to conduct electricity – the only non‑metal giant covalent substance with this property.
石墨也仅由碳原子组成，但其结构与金刚石截然不同。每个碳原子与另外三个碳原子形成三根共价键，构成平面的六边形环层。每个碳原子的第四个最外层电子在层间离域。这些离域电子可以自由移动，使石墨能够导电——这是唯一具有此性质的非金属巨型共价物质。
The layers are held together by weak intermolecular forces, so they can slide over each other easily. This makes graphite soft and slippery, which is why it is used as a lubricant and in pencil 'lead'.
层与层之间靠微弱的分子间作用力结合，因此层间很容易滑动。这使得石墨柔软滑腻，因而被用作润滑剂和铅笔“铅芯”。
10. Silicon Dioxide (Silica) | 二氧化硅（硅石）
Silicon dioxide SiO₂ has a giant covalent structure similar to diamond. Each silicon atom bonds to four oxygen atoms, and each oxygen atom bonds to two silicon atoms, forming a continuous, hard lattice. Like diamond, silica has a very high melting point (around 1700 °C) and does not conduct electricity because there are no free electrons or ions.
二氧化硅 SiO₂ 具有类似金刚石的巨型共价结构。每个硅原子与四个氧原子成键，每个氧原子与两个硅原子成键，形成一个连续的坚硬晶格。与金刚石类似，硅石具有极高的熔点（约 1700 °C），并且因为没有自由电子或离子而不导电。
Sand and quartz are common forms of silicon dioxide. The formula SiO₂ is the empirical formula; it does not represent a simple molecule but the ratio of atoms in the giant lattice.
沙子和石英是二氧化硅的常见形式。SiO₂ 只是经验式，不代表简单分子，而是表示巨型晶格中硅原子与氧原子的比例。
11. Comparing Covalent Structures | 共价结构性质对比
It is essential to compare the properties of simple molecular substances and giant covalent structures systematically. The table below summarises the key differences that come up frequently in exams.
系统地比较简单分子物质与巨型共价结构的性质非常重要。下表总结了考试中经常出现的核心差异。
| Property | Simple molecular (e.g. H₂O, CO₂) | Giant covalent (e.g. diamond, SiO₂) |
|---|---|---|
| Melting / boiling point | Low – weak intermolecular forces | Very high – strong covalent bonds throughout |
| Electrical conductivity | Do not conduct | Diamond: no; Graphite: yes (delocalised electrons) |
| State at room temperature | Often gases or liquids | Always solids |
| Strength of bonds within structure | Strong covalent inside each molecule | Strong covalent throughout the lattice |
12. Modelling Covalent Molecules and Limitations | 分子模型及其局限
You may be asked about different ways to represent molecules: ball‑and‑stick models, space‑filling models, displayed formulae and 2D drawings. Ball‑and‑stick models show bonds and angles well but exaggerate the distance between atoms. Space‑filling models give a better sense of relative sizes but hide bond angles. No single model perfectly represents the three‑dimensional reality, so you need to recognise the advantages and limitations of each.
考试可能问及不同的分子表示方法：球棍模型、空间填充模型、结构式以及二维绘图。球棍模型能清楚显示化学键和键角，但夸大了原子间的距离。空间填充模型能更好地反映原子的相对大小，但隐藏了键角。没有哪一种模型能完美呈现三维真实结构，因此你需要了解每种模型的优缺点。
For giant covalent structures, we usually draw a small section of the lattice to show the bonding pattern. Remember that these drawings are just a tiny part of a structure that extends in three dimensions.
对于巨型共价结构，我们通常画出晶格的一小部分来表示成键方式。要记住这些图只是三维延伸结构的一个微小片段。
13. Common Exam Mistakes and Key Phrases | 常见失分点与关键表达
Use precise language: say 'strong covalent bonds' when referring to bonds inside molecules or throughout a giant lattice, and 'weak intermolecular forces' when explaining low melting points of simple molecular substances. Never say that covalent bonds themselves are broken when a simple molecular substance melts or boils – only the intermolecular forces are overcome.
使用精确的语言：描述分子内部或巨型晶格中的化学键时，说“强的共价键”；解释简单分子物质的低熔点时，说“微弱的分子间作用力”。千万不要说简单分子物质熔化或沸腾时破坏了共价键——只需克服分子间作用力。
For graphite, always state that the delocalised electrons can move, allowing it to conduct electricity. For diamond, stress that all electrons are used in bonds, so it cannot conduct. Learn to draw dot and cross diagrams for the molecules specified in the AQA specification – practice them until they are second nature.
对于石墨，一定要说明离域电子可以自由移动，因此它能导电。对于金刚石，要强调所有电子都用于成键，所以不导电。把 AQA 大纲中规定的分子的点叉图画熟练，直到成为本能。
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