GCSE WJEC Chemistry: Covalent Bonding | WJEC 化学：共价键考点精讲

📚 GCSE WJEC Chemistry: Covalent Bonding | WJEC 化学：共价键考点精讲

Covalent bonding is one of the central topics in GCSE WJEC Chemistry, explaining how non-metal atoms join together by sharing pairs of electrons. A clear understanding of covalent bonds helps you predict the properties of simple molecules, giant covalent structures, and many everyday substances like water, carbon dioxide, and diamond. This article breaks down every key concept you need for the exam, from dot-and-cross diagrams to structure-property relationships.

共价键是 GCSE WJEC 化学中的核心主题之一，它解释了非金属原子如何通过共用电子对结合。清晰理解共价键有助于你预测简单分子、巨型共价结构以及许多常见物质（如水、二氧化碳和金刚石）的性质。本文逐一剖析考试所需的每个关键概念，涵盖点叉图到结构与性质的关系。

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1. What is Covalent Bonding? | 什么是共价键？

A covalent bond is the strong electrostatic attraction between the shared pair of electrons and the positive nuclei of two non-metal atoms. Atoms share electrons so that each atom can achieve a stable full outer shell, typically like the electron configuration of a noble gas. This sharing allows both atoms to lower their overall energy and become more stable.

共价键是共用电子对与两个非金属原子的带正电的原子核之间的强静电吸引力。原子共用电子，使每个原子都能达到稳定的满外层结构，通常类似于稀有气体的电子排布。这种共用使两个原子整体能量降低，变得更加稳定。

For example, in a hydrogen molecule (H₂), two hydrogen atoms each contribute one electron to form a shared pair, giving both atoms the helium electron arrangement with two outer electrons. The bond is often represented as a single line, H–H.

例如，在氢分子（H₂）中，两个氢原子各自提供一个电子形成共用电子对，使两个原子都拥有氦的电子排布（最外层两个电子）。该键通常用一条单线表示，即 H–H。

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2. How Covalent Bonds Form: Electron Sharing | 共价键的形成：电子共用

Non-metal atoms form covalent bonds because they have high ionisation energies, making electron transfer energetically unfavourable. Instead of losing or gaining electrons completely, they share electrons to fill their outer shells. The shared electrons are attracted to both nuclei, holding the atoms tightly together.

非金属原子形成共价键是因为它们具有较高的电离能，使得电子完全转移在能量上不利。它们不是完全失去或获得电子，而是通过共用电子来填充外层。共用电子受到两个原子核的吸引，将原子紧密地结合在一起。

Each covalent bond provides one extra shared electron to each atom’s outer shell count. For instance, a chlorine atom has seven outer electrons; by sharing one pair with another chlorine atom, both effectively gain one more electron and attain a stable octet. Thus Cl₂ is formed with a single covalent bond.

每条共价键为每个原子的外层电子数额外提供一个共用电子。例如，一个氯原子有7个外层电子；与另一个氯原子共用一对电子后，两者都有效地多获得一个电子，达到稳定八隅体。因此 Cl₂ 通过单共价键形成。

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3. Dot and Cross Diagrams | 点叉图

Dot-and-cross diagrams are the standard way to illustrate covalent bonding at GCSE. They show the outer electrons of each atom using different symbols (dots for one atom, crosses for the other) so that the origin of each electron is clear. The shared pair is drawn in the overlap region between the two atoms.

点叉图是 GCSE 中表示共价键的标准方式。它们使用不同的符号（一个原子用点，另一个用叉）来表示每个原子的外层电子，使电子的来源清晰可见。共用电子对画在两个原子之间的重叠区域。

Key molecules to practise include H₂, Cl₂, O₂, N₂, H₂O, NH₃, CH₄, CO₂, and C₂H₄. For oxygen, a double bond is formed: two pairs of electrons are shared, so the O=O diagram shows four electrons in the overlap. For nitrogen, a triple bond (N≡N) involves three shared pairs.

需要练习的关键分子包括 H₂、Cl₂、O₂、N₂、H₂O、NH₃、CH₄、CO₂ 和 C₂H₄。对于氧气，形成双键：共用两对电子，因此 O=O 图中重叠区域显示四个电子。对于氮气，三键（N≡N）包含三对共用电子。

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4. Single, Double and Triple Bonds | 单键、双键与三键

The number of shared electron pairs defines the bond order. A single covalent bond contains one shared pair (2 electrons), a double bond contains two shared pairs (4 electrons), and a triple bond contains three shared pairs (6 electrons). Higher bond orders are shorter and stronger.

共用电子对的数量决定了键级。单共价键包含一对共用电子（2个电子），双键包含两对（4个电子），三键包含三对（6个电子）。键级越高，键长越短，键能越大。

In the exam, you need to recognise that double and triple bonds are common in elements like O₂ (O=O) and N₂ (N≡N), and in compounds like CO₂ (O=C=O) and ethene (H₂C=CH₂). Always show the correct number of electron pairs in your diagrams.

在考试中，你需要识别出双键和三键在 O₂（O=O）、N₂（N≡N）等单质以及 CO₂（O=C=O）和乙烯（H₂C=CH₂）等化合物中很常见。在绘图中务必显示正确数量的电子对。

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5. Simple Molecular Substances: Structure | 简单分子物质：结构

Simple molecular substances consist of small molecules held together by strong covalent bonds within the molecules, but only weak intermolecular forces between them. These substances include water, carbon dioxide, hydrogen, oxygen, ammonia and the halogens at room temperature.

简单分子物质由小分子组成，分子内部有强共价键，但分子之间只有微弱的分子间作用力。这些物质包括水、二氧化碳、氢气、氧气、氨气以及室温下的卤素单质。

Because the intermolecular forces are weak, little energy is required to overcome them. This explains why simple molecular substances typically have low melting and boiling points, and many are gases or liquids at room temperature.

由于分子间作用力弱，克服它们所需能量很少。这就解释了为什么简单分子物质通常具有较低的熔点和沸点，许多在室温下是气体或液体。

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6. Properties of Simple Molecular Substances | 简单分子物质的性质

Simple molecular substances do not conduct electricity because there are no free ions or delocalised electrons. The molecules are neutral overall, and electrons are locked in covalent bonds or within the atoms. This is a frequent exam question when comparing with ionic or metallic substances.

简单分子物质不导电，因为没有自由离子或离域电子。分子整体呈电中性，电子被锁定在共价键或原子内部。与离子或金属物质进行对比时，这是常见的考试问题。

They also tend to be insoluble in water unless they can form hydrogen bonds with water molecules, as with small alcohols or ammonia. However, many simple molecular substances are soluble in organic solvents. The weak intermolecular forces result in low viscosity and high volatility for liquids.

它们通常也不溶于水，除非能像小分子醇或氨那样与水分子形成氢键。但许多简单分子物质可溶于有机溶剂。微弱的分子间作用力导致液体具有低粘度和高挥发性。

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7. Giant Covalent Structures | 巨型共价结构

Giant covalent structures, also called macromolecules, contain billions of atoms joined by strong covalent bonds in a continuous lattice. Unlike simple molecules, there are no separate small units; the whole structure is one giant network. Diamond, graphite, silicon dioxide and silicon are typical examples.

巨型共价结构（也称高分子）包含数十亿个原子，通过强共价键连接成连续的点阵结构。与简单分子不同，不存在独立的小单元；整个结构是一个巨型网络。金刚石、石墨、二氧化硅和硅是典型的例子。

Because all atoms are linked by strong covalent bonds, these substances have very high melting and boiling points. A huge amount of energy is needed to break the many bonds throughout the structure. Most are solids at room temperature.

由于所有原子通过强共价键连接，这些物质具有极高的熔点和沸点。需要巨大的能量才能破坏结构中的大量共价键。大多数在室温下为固体。

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8. Diamond vs. Graphite: Allotropes of Carbon | 金刚石与石墨：碳的同素异形体

Diamond and graphite are both allotropes of carbon but have very different properties because of their distinct bonding arrangements. In diamond, each carbon atom forms four strong single covalent bonds in a tetrahedral arrangement, creating a rigid three-dimensional network. This makes diamond extremely hard, with a very high melting point, and it does not conduct electricity because all four outer electrons per carbon are used in bonding.

金刚石和石墨都是碳的同素异形体，但由于键合排列不同，性质差异很大。在金刚石中，每个碳原子以四面体方式形成四个强单共价键，构成刚性的三维网络。这使得金刚石极度坚硬、熔点极高，并且由于每个碳原子的四个外层电子都用于成键，所以不导电。

In graphite, each carbon atom bonds to only three others in flat hexagonal layers. The fourth outer electron of each carbon becomes delocalised and can move freely between the layers. This allows graphite to conduct electricity (a key exam point) and makes it useful for electrodes. The layers can slide over each other because of weak forces between them, making graphite soft and slippery – ideal for lubricants and pencils.

在石墨中，每个碳原子只与其他三个碳原子结合，形成扁平的六角形层状结构。每个碳原子的第四个外层电子变成离域电子，可在层间自由移动。这使得石墨能够导电（一个关键考点），因此可用作电极。层间因微弱作用力可相对滑动，使石墨柔软润滑——适合做润滑剂和铅笔芯。

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9. Silicon Dioxide (Silica) and Silicon | 二氧化硅（硅石）与硅

Silicon dioxide, SiO₂, has a giant covalent structure similar to diamond. Each silicon atom is bonded to four oxygen atoms, and each oxygen is bonded to two silicon atoms, forming a continuous tetrahedral network. SiO₂ is very hard, has a high melting point, and does not conduct electricity. It is the main component of sand and quartz.

二氧化硅（SiO₂）具有类似于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，形成连续的四面体网络。SiO₂ 非常坚硬，熔点高，且不导电。它是沙子和石英的主要成分。

Silicon itself has the same giant covalent structure as diamond: each Si atom forms four Si–Si bonds. Like diamond, it is hard, has a very high melting point, and is a semiconductor when doped – but in its pure covalent network, it does not conduct electricity. Students often need to compare silicon with diamond in structure and properties.

硅本身具有与金刚石相同的巨型共价结构：每个 Si 原子形成四个 Si–Si 键。与金刚石类似，它坚硬、熔点极高，掺杂后可成为半导体——但在纯共价网络状态下不导电。学生常需将硅与金刚石在结构和性质上进行比较。

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10. Comparing Bonding and Structure Types | 键合与结构类型的比较

The exam frequently asks you to link the type of bonding and structure to properties such as melting point, conductivity, and hardness. Use a simple table to compare ionic, simple molecular, giant covalent and metallic substances. Here is an at-a-glance summary tailored for WJEC GCSE Chemistry:

考试经常要求你将键合和结构类型与熔点、导电性及硬度等性质联系起来。可以用一个简单的表格来比较离子、简单分子、巨型共价和金属物质。以下是针对 WJEC GCSE 化学的快速总结：

Structure type | 结构类型	Particles | 粒子	Melting/boiling points | 熔沸点	Conductivity | 导电性
Ionic | 离子	Ions in lattice | 晶格中的离子	High | 高	When molten/dissolved | 熔融或溶解时
Simple molecular | 简单分子	Small molecules | 小分子	Low | 低	None | 无
Giant covalent | 巨型共价	Atoms in network | 网络中的原子	Very high | 极高	No, except graphite | 否，石墨除外
Metallic | 金属	Positive ions in sea of delocalised electrons | 正离子在离域电子海中	High | 高	Yes (solid and liquid) | 是（固态和液态）

Remember, giant covalent structures are the exception when it comes to non-conductivity: graphite conducts, while diamond and SiO₂ do not. Always explain by referring to delocalised electrons around the layers in graphite.

记住，巨型共价结构在不导电方面存在例外：石墨导电，而金刚石和 SiO₂ 不导电。务必通过提及石墨层间离域电子来解释。

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11. Representing Covalent Bonding in Exams | 考试中如何表示共价键

WJEC expects you to draw dot-and-cross diagrams for specified molecules and to show the outer electron shells only. Use labels or a key to distinguish electrons from different atoms. The shared pair must be placed between the two atomic symbols. For double and triple bonds, clearly draw the extra pairs. Also, be able to represent molecules using displayed formulas (structural formulas showing all bonds as lines) and molecular formulas.

WJEC 考试要求你绘制指定分子的点叉图，并只显示外层电子。使用标签或图例区分不同原子的电子。共用电子对必须放在两个原子符号之间。对于双键和三键，要清楚画出额外的电子对。此外，要能够用结构式（显示所有键为线的结构式）和分子式表示分子。

Practice common examples: H₂O (two bonding pairs, two lone pairs on oxygen), NH₃ (three bonding pairs, one lone pair), CH₄ (four bonding pairs, no lone pairs). Recognise that lone pairs can affect the shape, but WJEC at GCSE mainly focuses on the bonding itself and properties, not advanced molecular shapes.

练习常见例子：H₂O（两对成键电子，氧上有两对孤电子对）、NH₃（三对成键电子，一对孤电子对）、CH₄（四对成键电子，无孤电子对）。要认识到孤电子对会影响分子形状，但 WJEC GCSE 主要关注键合本身和性质，不深入高级分子形状。

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12. Quick Revision Summary | 快速复习要点

Covalent bonds form between non-metal atoms by sharing electrons to achieve full outer shells. Simple molecular substances have strong bonds within molecules but weak intermolecular forces, resulting in low melting points and no electrical conductivity. Giant covalent structures like diamond and SiO₂ have extremely high melting points and are generally non-conductors, while graphite conducts due to delocalised electrons. Always link structure to properties in your answers.

共价键通过非金属原子间共用电子以达到满外层而形成。简单分子物质内部有强键，但分子间作用力弱，导致低熔点和不导电。金刚石和 SiO₂ 等巨型共价结构具有极高熔点，通常不导电，而石墨因离域电子可导电。答题时务必将结构与性质联系起来。

Published by TutorHao | GCSE Chemistry Revision Series | aleveler.com

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