📚 Covalent Bonding: IGCSE Edexcel Chemistry Key Points | IGCSE Edexcel 化学：共价键 考点精讲
Covalent bonding is one of the fundamental topics in IGCSE Edexcel Chemistry. Understanding how non-metal atoms share electrons to achieve stable electron arrangements is key to explaining the properties of many familiar substances, from water and carbon dioxide to diamond and graphite. This revision guide breaks down every essential point you need for the exam, from drawing dot-and-cross diagrams to comparing giant covalent structures.
共价键是 IGCSE Edexcel 化学中的基础主题之一。理解非金属原子如何通过共享电子达到稳定电子排布，是解释许多常见物质性质的关键——从水、二氧化碳到金刚石和石墨。本复习指南将为你拆解考试所需的每一个要点，从绘制点叉图到比较巨型共价结构。
1. What is a Covalent Bond? | 什么是共价键？
A covalent bond is a strong electrostatic attraction between a shared pair of electrons and the positively charged nuclei of two non-metal atoms. Both atoms donate electrons to the shared pair, allowing each to achieve a full outer shell (noble gas configuration).
共价键是共享电子对与两个非金属原子带正电的原子核之间的强静电吸引力。两个原子共同提供电子形成共享电子对，使每个原子都能达到满的外层电子结构（惰性气体构型）。
Covalent bonding occurs exclusively between non-metal elements. For example, two chlorine atoms each need one electron to complete their outer shell, so they share one pair of electrons to form a Cl₂ molecule.
共价键只发生在非金属元素之间。例如，两个氯原子各需要一个电子来填满最外层，因此它们共用一对电子形成 Cl₂ 分子。
It is important to remember that in a covalent bond, electrons are not transferred (as in ionic bonding) but are shared. The shared electrons count towards the octet for both atoms.
要记住，在共价键中，电子并没有被转移（如离子键那样），而是被共用。共享的电子同时计入两个原子的八隅体。
2. Formation of Covalent Bonds: Electron Sharing | 共价键的形成：电子共享
When two non-metal atoms approach each other, their electron clouds overlap. The nuclei attract the shared electrons more strongly than they repel each other, leading to a stable molecule. Each shared pair of electrons is one covalent bond.
当两个非金属原子相互靠近时，它们的电子云发生重叠。原子核对共享电子的吸引力强于彼此间的排斥力，从而形成稳定分子。每一对共享电子就代表一个共价键。
For instance, in hydrogen (H₂), each hydrogen atom has one electron. By sharing their single electrons, both atoms effectively gain a helium-like duplet (2 electrons). This is represented as H–H.
例如，在氢气（H₂）中，每个氢原子有一个电子。通过共用它们唯一的电子，两个原子都获得了类似氦的电子对（2 个电子）。用结构式表示为 H–H。
The number of covalent bonds an atom typically forms depends on how many electrons it needs to reach a full outer shell. Carbon needs 4, so it forms 4 bonds; oxygen needs 2; hydrogen needs 1.
一个原子通常形成多少个共价键，取决于它需要多少电子来填满外层。碳需要 4 个，所以形成 4 个键；氧需要 2 个；氢需要 1 个。
3. Single, Double and Triple Bonds | 单键、双键和三键
an share more than one pair of electrons. A single bond involves one shared pair (e.g., Cl– shared pairs (e.g., O=O in oxygen gas). A triple bond involves three shared pairs (e.g., N≡N in nitrogen gas).
原子可以共用不止一对电子。单键涉及一对共享电子（如 Cl–Cl）。双键涉及两对共享电子（如氧气中的 O=O）。三键涉及三对共享电子（如氮气中的 N≡N）。
Double and triple bonds are shorter and stronger than single bonds. Nitrogen gas is very unreactive because of the strong triple bond holding the two nitrogen atoms together.
双键和三键比单键更短且更强。氮气非常不活泼，就是因为两个氮原子之间极强的三键。
In dot-and-cross diagrams, you must show all the shared electrons in the overlapping region between atoms. For O₂, you draw four electrons shared between the two oxygen atoms.
在点叉图中，你必须在原子间的重叠区域画出所有共享电子。对于 O₂，需要在两个氧原子之间画出四个共享电子。
4. Drawing Dot-and-Cross Diagrams | 绘制点叉图
For IGCSE, you must be able to draw dot-and-cross diagrams for simple molecules: H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄, CO₂ and C₂H₄. Use dots for electrons from one atom and crosses for the other, or different symbols to show the origin of electrons.
在 IGCSE 考试中，你必须能够为简单分子绘制点叉图：H₂、Cl₂、O₂、N₂、HCl、H₂O、NH₃、CH₄、CO₂ 和 C₂H₄。用点表示一个原子的电子，用叉表示另一个原子的电子，或用不同符号表明电子的来源。
Remember to show only the outer shell (valence) electrons. Draw the atoms with their electrons and circle the shared pairs between them. Complete all octets/duplets where appropriate.
记住只画出最外层（价）电子。画出原子及其电子，并将原子间的共享电子对圈起来。适当情况下完成所有八隅体/电子对。
agram must show 4 electrons between C and each O, with the remaining lone pairs on oxygen.
对于 CO₂，碳与每个氧原子共用两对电子（O=C=O）。点叉图中必须在 C 与每个 O 之间显示 4 个电子，氧原子上剩余孤对电子。
5. Simple Molecular Covalent Compounds | 简单分子共价化合物
The following tables summarise the key molecules you need to know, their formulas and structural features.
下表总结了你需要掌握的关键分子、它们的化学式和结构特征。
| Substance | Formula | Bonding | Dot-and-cross essential |
|---|---|---|---|
| Hydrogen | H₂ | Single bond | One shared pair, duplet |
| Chlorine | Cl₂ | Single bond | Octet on each Cl, 3 lone pairs each |
| Oxygen | O₂ | Double bond | Two shared pairs, 2 lone pairs each |
| Nitrogen | N₂ | Triple bond | Three shared pairs, 1 lone pair each |
| Hydrogen chloride | HCl | Single bond | H gets duplet, Cl gets octet |
| Water | H₂O | Two single bonds | O shares 2e⁻, 2 lone pairs on O |
| Ammonia | NH₃ | Three single bonds | N shares 3e⁻, 1 lone pair on N |
| Methane | CH₄ | Four single bonds | C shares 4e⁻, no lone pairs |
| Carbon dioxide | CO₂ | Two double bonds | C shares 4e⁻ with each O; linear |
| Ethene | C₂H₄ | C=C double bond, 4 C–H | Each C shares 4e⁻ |
Always show outer electrons only. In ethene, each carbon is bonded to two hydrogens and the other carbon via a double bond.
务必只展示最外层电子。在乙烯中，每个碳与两个氢键合，并与另一个碳通过双键键合。
6. Properties of Simple Molecular Substances | 简单分子物质的性质
Substances with simple molecular structures (like H₂O, CO₂, CH₄) have low melting and boiling points. This is because the covalent bonds within molecules are very strong, but the intermolecular forces between molecules are weak and require little energy to overcome.
具有简单分子结构的物质（如 H₂O、CO₂、CH₄）熔点和沸点较低。这是因为分子内的共价键非常强，但分子间的分子间作用力较弱，只需很少能量就能克服。
They do not conduct electricity in any state because there are no free electrons or ions. The molecules are neutral and electrons are locked in covalent bonds or lone pairs.
它们在任何状态下都不导电，因为没有可以自由移动的电子或离子。分子是电中性的，电子被锁在共价键或孤对电子中。
+ Cl⁻). The solution then conducts electricity, but the pure covalent liquid/gas does not.
一些简单分子，如氯化氢气体，溶解在水中能产生离子（HCl → H⁺ + Cl⁻）。溶液随后能导电，但纯的共价液体/气体不能导电。
7. Giant Covalent Structures: Diamond | 巨型共价结构：金刚石
Diamond is a giant covalent structure where each carbon atom forms four strong covalent bonds to other carbon atoms in a tetrahedral arrangement. This three-dimensional network extends throughout the whole crystal.
金刚石是一种巨型共价结构，每个碳原子与其他碳原子形成四个强的共价键，呈四面体排列。这个三维网络贯穿整个晶体。
Because all atoms are held together by strong covalent bonds, diamond is extremely hard and has a very high melting point (above 3500 °C). It does not conduct electricity because there are no delocalised electrons.
由于所有原子都通过强共价键连接，金刚石极其坚硬，熔点非常高（高于 3500 °C）。它不能导电，因为没有离域电子。
In IGCSE exams, you may be asked to explain these properties in terms of bonding and structure: 'strong covalent bonds throughout the lattice require a great deal of energy to break, and no free charge carriers exist.'
在 IGCSE 考试中，你可能需要用键合和结构来解释这些性质：“整个晶格中的强共价键需要大量能量才能破坏，且不存在自由电荷载体。”
8. Giant Covalent Structure: Graphite | 巨型共价结构：石墨
Graphite is also a giant covalent form of carbon, but each carbon atom bonds to only three others, forming hexagonal layers. The fourth electron from each carbon becomes delocalised and can move between the layers.
石墨也是碳的一种巨型共价形式，但每个碳原子只与其他三个碳原子键合，形成六边形层状结构。每个碳的第四个电子成为离域电子，可在层间移动。
Graphite conducts electricity parallel to the layers due to these delocalised electrons. It is also soft and slippery because the layers can slide over each other, as the forces between layers are weak.
由于这些离域电子，石墨在沿着层的方向上可以导电。它柔软而润滑，因为层与层之间可以相互滑动，层间作用力很弱。
Graphite has a high melting point (similar to diamond) because the covalent bonds within each layer are very strong. This makes it useful as a lubricant and in electrodes.
石墨的熔点很高（与金刚石相似），因为每一层内的共价键非常强。这使它适合用作润滑剂和电极材料。
9. Silicon Dioxide (Silica) | 二氧化硅（硅石）
Silicon dioxide (SiO₂) is another giant covalent structure found in sand and quartz. Each silicon atom forms four covalent bonds to oxygen atoms, and each oxygen is bonded to two silicon atoms, creating a continuous tetrahedral network like diamond.
二氧化硅（SiO₂）是另一种巨型共价结构，存在于沙子和石英中。每个硅原子与氧原子形成四个共价键，每个氧原子与两个硅原子键合，形成一个与金刚石类似的连续四面体网络。
As a result, SiO₂ is very hard, has a very high melting point, and does not conduct electricity. You must be able to compare its structure with diamond, noting that an oxygen atom sits between each silicon pair.
因此，SiO₂ 非常坚硬，熔点极高，且不导电。你必须能够将其结构与金刚石进行比较，注意到每个硅原子对之间有一个氧原子。
10. Comparing Diamond and Graphite | 金刚石与石墨的比较
| Property | Diamond | Graphite |
|---|---|---|
| Bonding per C | 4 covalent bonds | 3 covalent bonds + 1 delocalised e⁻ |
| Hardness | Extremely hard | Soft and brittle |
| Melting point | Very high (>3500 °C) | Very high (sublimes at ~3650 °C) |
| Electrical conductivity | None | Conductor (along layers) |
| Structure | 3D tetrahedral network | Layered hexagonal sheets |
Both are allotropes of carbon and have giant covalent structures, which accounts for their high melting points. The difference in electrical conductivity arises from the availability of delocalised electrons in graphite.
两者都是碳的同素异形体，都具有巨型共价结构，这解释了它们的高熔点。电导率的差异源于石墨中存在可移动的离域电子。
11. Comparing Covalent and Ionic Compounds | 共价化合物与离子化合物的对比
It is a common IGCSE exam question to compare properties and explain them in terms of bonding and structure. Covalent compounds (simple molecular) have low m.p./b.p. and do not conduct electricity; ionic compounds have high m.p./b.p. and conduct when molten or dissolved.
比较性质并从键合和结构角度进行解释是 IGCSE 常见考题。共价化合物（简单分子）熔沸点低，不导电；离子化合物熔沸点高，熔融或溶解时导电。
Giant covalent substances, unlike simple molecular ones, have extremely high melting points and are generally hard, but only graphite conducts electricity. Ionic compounds are brittle, whereas giant covalent structures like diamond are extremely hard.
与简单分子物质不同，巨型共价物质有极高的熔点和一般较高的硬度，但只有石墨导电。离子化合物很脆，而金刚石等巨型共价结构极其坚硬。
Always link the property to the particle and the bonding: 'Simple molecules have weak intermolecular forces, so little energy is needed to separate them.' For ionic compounds, 'strong electrostatic forces between oppositely charged ions throughout the lattice require high energy to break.'
始终将性质与粒子和键合联系起来：“简单分子之间的分子间作用力弱，因此只需很少能量就能将它们分开。” 对于离子化合物，“整个晶格中相反电荷离子之间的强静电力需要高能量才能破坏。”
12. Common Exam Pitfalls and Revision Tips | 常见考试易错点与复习建议
One common mistake is showing ionic bonding for non-metal compounds. Always check the elements involved: non-metal + non-metal means covalent.
一个常见错误是对非金属化合物画出离子键。务必检查参与的元素：非金属 + 非金属意味着共价键。
In dot-and-cross diagrams, students often forget to draw the correct number of electrons for the central atom, or they omit lone pairs. Count the total valence electrons first and distribute them carefully.
在点叉图中，学生经常忘记为中心原子画出正确数量的电子，或遗漏孤对电子。先计算总价电子数，然后仔细分配它们。
When describing graphite, do not say 'free electrons' on their own – refer to them as 'delocalised electrons'. And when explaining diamond's hardness, mention the '3D network of strong covalent bonds' rather than 'strong bonds' alone.
描述石墨时，不要只说“自由电子”——应称其为“离域电子”。解释金刚石硬度时，要提到“强共价键的三维网络”，而不仅仅是“强键”。
Make sure you can label the differences between intermolecular forces and intramolecular covalent bonds. The covalent bond is strong, but it is the intermolecular force that is overcome during melting/boiling for simple molecules.
确保你能区分分子间作用力和分子内的共价键。共价键很强，但对于简单分子来说，熔化/沸腾时克服的是分子间作用力。
Finally, practise drawing the structures of diamond and graphite and linking them to uses: diamond for cutting tools, graphite for electrodes and lubricants.
最后，练习绘制金刚石和石墨的结构，并将它们与用途联系起来：金刚石用于切割工具，石墨用于电极和润滑剂。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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