📚 Mastering Covalent Bonding for IGCSE CCEA Chemistry | IGCSE CCEA 化学：共价键 考点精讲
Covalent bonding is one of the fundamental topics in the CCEA IGCSE Chemistry specification. Understanding how non-metal atoms share electrons to achieve stability explains the structures and properties of countless substances, from the water you drink to the diamond in jewellery. This article breaks down every key concept you need to master, including dot-and-cross diagrams, simple molecular substances, and giant covalent structures.
共价键是 CCEA IGCSE 化学大纲中的核心主题之一。理解非金属原子如何通过共享电子达到稳定结构，能够解释从饮用水到珠宝钻石等无数物质的结构与性质。本文将逐一拆解你需要掌握的每个关键概念，包括点叉图、简单分子物质和巨型共价结构。
1. What Is a Covalent Bond? | 什么是共价键？
A covalent bond is a strong electrostatic attraction between the positively charged nuclei of two non-metal atoms and a shared pair of electrons that lies between them. This shared pair of electrons is often referred to as a bonding pair, and it allows both atoms to achieve a full outer electron shell, similar to the electron configuration of a noble gas.
共价键是两个非金属原子的带正电的原子核与它们之间的一对共享电子之间的强静电吸引力。这对共享电子通常被称为成键电子对，它使两个原子都能达到全满的最外层电子层，类似于惰性气体的电子构型。
Atoms form covalent bonds because it lowers their overall energy — a full outer shell is more stable. This is often summarised by the octet rule: atoms tend to share electrons until they have eight electrons in their outermost shell (except hydrogen, which aims for two).
原子形成共价键是因为这样可以降低整体能量——全满的最外层更稳定。这通常用八隅体规则来概括：原子倾向于共享电子，直到它们的最外层拥有八个电子（氢除外，它只需两个）。
2. How Covalent Bonds Form: Sharing Electrons | 共价键如何形成：共享电子
In a covalent bond, each atom contributes one or more electrons to the shared pair. For example, in a hydrogen molecule (H₂), each hydrogen atom has one electron. By sharing their electrons, both atoms can count the shared pair as part of their own outer shell, effectively achieving the helium electronic configuration.
在共价键中，每个原子为共享电子对贡献一个或多个电子。例如，在氢分子 (H₂) 中，每个氢原子都有一个电子。通过共享电子，两个原子都可以将共享电子对算作自己最外层的一部分，从而有效达到氦的电子构型。
The shared electron pair is attracted to both nuclei, pulling the atoms together and forming a bond. The distance between the nuclei where the attractive and repulsive forces balance is called the bond length.
共享电子对被两个原子核吸引，将原子拉在一起并形成化学键。原子核之间吸引力与排斥力达到平衡的距离称为键长。
The covalent bond itself is very strong, requiring a lot of energy to break it. However, the forces between separate molecules (intermolecular forces) are much weaker, which governs the melting and boiling points of simple molecular substances.
共价键本身非常强，断裂它需要很多能量。然而，独立分子之间的作用力（分子间力）则弱得多，这决定了简单分子物质的熔点和沸点。
3. Drawing Dot-and-Cross Diagrams | 绘制点叉图
Dot-and-cross diagrams are used to show the origin of electrons in a covalent bond. Electrons from one atom are drawn as dots, and electrons from the other atom are drawn as crosses. Only the outer shell electrons are shown. For example, in a hydrogen molecule, the two shared electrons are placed between the two H symbols, one dot and one cross.
点叉图用于显示共价键中电子的来源。一个原子的电子用点表示，另一个原子的电子用叉表示。只画出最外层的电子。例如，在氢分子中，两个共享电子放在两个 H 符号之间，一个是点，一个是叉。
CCEA examiners may ask you to draw dot-and-cross diagrams for molecules such as H₂O, NH₃, CH₄, Cl₂, O₂, N₂, CO₂, C₂H₆ and C₂H₄. Practise by ensuring that after sharing, each atom (except H) is surrounded by eight electrons. Draw overlapping circles to represent the shared pair within the bond.
CCEA 考官可能会要求你画出 H₂O、NH₃、CH₄、Cl₂、O₂、N₂、CO₂、C₂H₆ 和 C₂H₄ 等分子的点叉图。练习时要确保共享后每个原子（氢除外）周围都有八个电子。可用重叠的圆圈来表示键中的共享电子对。
It is crucial to label which electron belongs to which atom using a key (dot = atom A, cross = atom B). In structures with multiple bonds, each additional shared pair is drawn as a second dot and cross pair between the symbols.
关键是要使用图例标明哪个电子属于哪个原子（点 = 原子 A，叉 = 原子 B）。在具有多重键的结构中，每多一对共享电子对，就在符号之间额外画一对点和叉。
4. Single, Double, and Triple Covalent Bonds | 单键、双键和三键
A single covalent bond is formed when two atoms share one pair of electrons, represented as A—B. Examples include H—H, Cl—Cl and C—C bonds in alkanes. A double bond involves two shared pairs (A=B), found in O=O and C=C in ethene. A triple bond has three shared pairs (A≡B), as in the nitrogen molecule N≡N.
单共价键由两个原子共享一对电子形成，表示为 A—B。例子包括烷烃中的 H—H、Cl—Cl 和 C—C 键。双键涉及两对共享电子 (A=B)，见于 O=O 和乙烯中的 C=C。三键有三对共享电子 (A≡B)，如氮分子 N≡N。
Bond strength and bond length depend on how many electron pairs are shared. Triple bonds are the shortest and strongest, while single bonds are the longest and weakest among the multiple bonds. This trend is important when discussing bond energies and reactivity.
键能和键长取决于共享电子对的数量。三键最短、最强，而单键在多重键中最长、最弱。这一趋势在讨论键能和反应活性时非常重要。
Carbon dioxide (CO₂) has two double bonds: O=C=O. Ethene (C₂H₄) contains a C=C double bond, while ethane (C₂H₆) contains only C—C and C—H single bonds. Being able to recognise and represent these bonds is essential for CCEA IGCSE.
二氧化碳 (CO₂) 有两个双键：O=C=O。乙烯 (C₂H₄) 含有一个 C=C 双键，而乙烷 (C₂H₆) 只含有 C—C 和 C—H 单键。能够识别并表示这些键对 CCEA IGCSE 至关重要。
5. Examples of Simple Molecular Substances | 简单分子物质示例
Simple molecular substances consist of small molecules held together by strong covalent bonds within the molecule but only weak intermolecular forces between molecules. Common examples include water (H₂O), methane (CH₄), ammonia (NH₃), oxygen (O₂), chlorine (Cl₂), carbon dioxide (CO₂), iodine (I₂) and the hydrocarbons like ethane and ethene.
简单分子物质由小分子组成，分子内部由强共价键结合，但分子之间只有弱的分子间作用力。常见的例子包括水 (H₂O)、甲烷 (CH₄)、氨 (NH₃)、氧气 (O₂)、氯气 (Cl₂)、二氧化碳 (CO₂)、碘 (I₂) 以及乙烷和乙烯等碳氢化合物。
These substances are usually gases or liquids at room temperature, though some may be volatile solids. For instance, iodine is a solid at room temperature because its larger relative molecular mass leads to stronger London dispersion forces between I₂ molecules, but it still sublimes easily.
这些物质在室温下通常为气体或液体，尽管有些可能是易挥发的固体。例如，碘在室温下是固体，因为其较大的相对分子质量导致 I₂ 分子间具有更强的色散力，但它仍然容易升华。
6. Properties of Simple Molecular Substances | 简单分子物质的性质
Simple molecular substances have low melting and boiling points. This is because when you heat them, the energy supplied is only enough to overcome the weak intermolecular forces between molecules, not the strong covalent bonds within each molecule. Therefore, the molecules separate from one another relatively easily.
简单分子物质具有较低的熔点和沸点。这是因为加热时，所供给的能量仅足以克服分子之间弱的分子间作用力，而不足以破坏每个分子内强共价键。因此，分子彼此分离相对容易。
They do not conduct electricity in any state. Even when molten or dissolved in water, simple molecular substances remain as neutral molecules with no free ions or delocalised electrons to carry charge. This is a key difference from ionic compounds.
它们在任何状态下都不导电。即使熔融或溶于水，简单分子物质仍以中性分子存在，没有可自由移动的离子或离域电子来携带电荷。这是与离子化合物的一个关键区别。
Many simple molecular substances are insoluble in water but will dissolve in non-polar solvents. For example, iodine dissolves better in hexane than in water. Remember: the intermolecular forces are what govern solubility, not the covalent bonds inside the molecule.
许多简单分子物质不溶于水，但能溶解在非极性溶剂中。例如，碘在己烷中的溶解性比在水中好。请记住：控制溶解度的是分子间作用力，而不是分子内部的共价键。
7. Giant Covalent Structures: Diamond | 巨型共价结构：金刚石
Diamond is a giant covalent structure in which each carbon atom forms four strong covalent bonds with four neighbouring carbon atoms. This creates a rigid, tetrahedral three-dimensional network extending throughout the entire crystal. There are no separate molecules — the whole crystal can be thought of as one giant molecule.
金刚石是一种巨型共价结构，其中每个碳原子与四个相邻的碳原子形成四个强共价键。这形成贯穿整个晶体的刚性、四面体三维网络。不存在单独的小分子——整个晶体可以被视为一个巨型分子。
Because all four outer-shell electrons of each carbon atom are used in bonding, there are no free electrons. Diamond therefore does not conduct electricity. It is an excellent electrical insulator. It is also the hardest known natural material and has a very high melting point (around 3550 °C) because a large amount of energy is needed to break the many covalent bonds.
由于每个碳原子的所有四个外层电子都用于成键，没有自由电子，因此金刚石不导电。它是一种优良的电绝缘体。它也是已知最硬的天然材料，具有极高的熔点（约 3550 °C），因为需要大量能量来打破大量的共价键。
8. Giant Covalent Structures: Graphite | 巨型共价结构：石墨
Graphite is another allotrope of carbon with a giant covalent structure, but its bonding arrangement is very different from diamond. Each carbon atom forms three covalent bonds with three other carbons in the same layer, creating flat hexagonal sheets. These sheets are held together by weak intermolecular forces, allowing them to slide over each other easily.
石墨是碳的另一种同素异形体，具有巨型共价结构，但其键合排列与金刚石大不相同。每个碳原子与同一层中的另外三个碳原子形成三个共价键，形成平面的六边形片层。这些片层之间由弱的分子间作用力结合，使它们能够轻易相互滑动。
The fourth outer-shell electron of each carbon atom is delocalised and forms a “sea” of free electrons that can move within the layers. This allows graphite to conduct electricity parallel to the layers, making it useful for electrodes and electrical contacts. The strong covalent bonds within the layers give graphite a very high melting point, similar to diamond.
每个碳原子的第四个外层电子是离域的，形成可在层内移动的“电子海”。这使得石墨能够沿层的方向导电，因而可用于电极和电触点。层内强共价键使石墨具有与金刚石相似的高熔点。
Graphite’s layered structure and weak interlayer forces make it soft and slippery, which is why it is used as a lubricant and in pencil “lead”. It is important to note that graphite is the only common non-metal that conducts electricity under normal conditions.
石墨的层状结构和弱的层间力使其柔软且滑腻，这就是它被用作润滑剂和铅笔“铅”的原因。需要注意的是，石墨是唯一在通常条件下导电的常见非金属。
9. Giant Covalent Structures: Silicon Dioxide | 巨型共价结构：二氧化硅
Silicon dioxide (SiO₂), commonly known as silica or quartz, has a giant covalent structure similar to diamond. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. This forms a continuous, highly rigid three-dimensional network.
二氧化硅 (SiO₂)，通常称为硅石或石英，具有与金刚石类似的巨型共价结构。每个硅原子与四个氧原子形成共价键，每个氧原子与两个硅原子成键。这形成一个连续、高度刚性的三维网络。
Like diamond, silicon dioxide has a very high melting point (around 1700 °C), is very hard, and does not conduct electricity. It is found naturally as sand and quartz, and is a major component of glass and ceramics. When drawing its structure, students should be able to represent the repeating Si—O framework.
与金刚石类似，二氧化硅的熔点非常高（约 1700 °C），质地非常坚硬，且不导电。它以砂子和石英的形式存在于自然界，是玻璃和陶瓷的主要成分。学生应能画出其重复的 Si—O 骨架结构。
10. Comparing Diamond and Graphite | 金刚石与石墨对比
Although both are pure carbon allotropes with giant covalent structures, diamond and graphite have strikingly different properties due to their different bonding geometries. The table below summarises the key contrasts:
虽然两者都是具有巨型共价结构的纯碳同素异形体，但由于键合几何形态不同，金刚石和石墨的性质差异显著。下表总结了关键对比：
| Property | 性质 | Diamond | 金刚石 | Graphite | 石墨 |
|---|---|---|
| Bonding geometry | 键合几何 | 4 bonds per C, tetrahedral | 每个碳4个键，四面体 | 3 bonds per C in layers, trigonal planar | 每层每个碳3个键，三角平面 |
| Hardness | 硬度 | Extremely hard | 极硬 | Soft and slippery | 柔软滑腻 |
| Electrical conductivity | 导电性 | Non-conductor | 不导电 | Conductor (along layers) | 沿层面导电 |
| Melting point | 熔点 | Very high (~3550 °C) | 极高 | Very high (sublimes ~3650 °C) | 极高（约3650 °C升华） |
| Uses | 用途 | Cutting tools, jewellery | 切割工具、珠宝 | Electrodes, lubricants, pencils | 电极、润滑剂、铅笔 |
Both have high melting points because they require the breakage of strong covalent bonds throughout the giant structure. The conductivity difference arises from the availability of delocalised electrons: graphite has one free electron per carbon, whereas diamond uses all electrons in bonding.
两者都具有高熔点，因为它们都需要破坏整个巨型结构中的强共价键。导电性的差异源于是否有可用的离域电子：石墨每个碳原子有一个自由电子，而金刚石将所有电子用于成键。
11. Summary of Covalent Structures | 共价结构总结
When you encounter a question on covalent substances, first identify whether it is a simple molecular or giant covalent substance. Simple molecules have low melting points and do not conduct electricity; giant covalent structures have very high melting points, and their conductivity depends on electron mobility — graphite conducts, diamond and SiO₂ do not.
当你遇到有关共价物质的问题时，首先要判断它是简单分子物质还是巨型
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