Chemical Bonding | IGCSE WJEC 化学：化学键考点精讲

📚 Chemical Bonding | IGCSE WJEC 化学：化学键考点精讲

Understanding how atoms join together is central to explaining the properties of all substances. In IGCSE WJEC Chemistry, chemical bonding is divided into three main types: ionic, covalent, and metallic. This article covers the key concepts, diagrams, properties, and exam tips you need to master the topic.

理解原子如何结合是解释所有物质性质的核心。在 IGCSE WJEC 化学中，化学键主要分为三种类型：离子键、共价键和金属键。本文涵盖关键概念、示意图、性质以及你需要掌握的考试技巧。

1. Why Atoms Bond | 原子为何形成化学键

Atoms bond to achieve a full outer shell of electrons, typically 8 electrons (the octet rule) or 2 for hydrogen and helium. This gives the atom the stable electronic configuration of a noble gas. Bonding lowers the energy of the system, making compounds more stable than separate atoms.

原子结合是为了达到满的外层电子排布，通常是8个电子（八隅律），氢和氦则为2个。这使原子获得类似稀有气体的稳定电子构型。成键会降低体系能量，使化合物比孤立的原子更稳定。

In ionic bonding, atoms lose or gain electrons to form ions with full outer shells. In covalent bonding, atoms share electrons to fill their outer shells. In metallic bonding, atoms delocalise electrons into a ‘sea’ that holds positive ions together.

在离子键中，原子通过失去或获得电子形成具有满外层的离子。在共价键中，原子共用电子对来填满外层。在金属键中，原子将电子离域到“电子海”中，把正离子维系在一起。

2. Ionic Bonding: Electron Transfer | 离子键：电子转移

Ionic bonding involves the electrostatic attraction between positively charged metal cations and negatively charged non‑metal anions. The metal atom transfers one or more electrons to the non‑metal atom. This forms a giant ionic lattice in the solid state.

离子键涉及带正电的金属阳离子和带负电的非金属阴离子之间的静电吸引。金属原子将一个或多个电子转移给非金属原子。在固态中形成巨型离子晶格。

Example: sodium chloride, NaCl. Sodium (2,8,1) loses its 1 outer electron to become Na⁺ (2,8). Chlorine (2,8,7) gains 1 electron to become Cl⁻ (2,8,8). The formula is NaCl because the charges balance (1⁺ and 1⁻). For magnesium oxide, Mg loses 2 electrons to form Mg²⁺ and O gains 2 electrons to form O²⁻, giving MgO.

示例：氯化钠 NaCl。钠（2,8,1）失去1个最外层电子变成 Na⁺ (2,8)。氯（2,8,7）得到1个电子变成 Cl⁻ (2,8,8)。化学式为 NaCl，因为电荷平衡（1⁺ 与 1⁻）。对于氧化镁，镁失去2个电子形成 Mg²⁺，氧得到2个电子形成 O²⁻，得出 MgO。

Dot-and-cross diagrams show the outer electrons only. Electrons from different atoms are shown using dots and crosses. Brackets and charges are used for ions. For NaCl: Na is drawn with seven crosses (or two crosses in outer shell as 2,8,1 simplified) but usually we show outer shell. Actually for IGCSE, we draw full outer shell for sodium ion as empty, but we often show the ion with no outer shell, just [Na]⁺. Precise diagram rules: draw ion with full outer shell using the transferred electron now belonging to the non‑metal, indicated by a different symbol.

点叉图只表示最外层电子。不同原子的电子用点和叉标示。离子使用方括号和电荷标示。对于 NaCl：钠的离子外层没有电子（已失去），与氯离子外层8个电子一起显示，其中一个电子来自钠，用叉标出。注意，钠离子写成 [Na]⁺，而氯离子写成带8个电子的图示。

The formula of an ionic compound is determined by the ratio that makes the total positive and negative charges equal. The ending of the non‑metal name changes to -ide.

离子化合物的化学式由正负总电荷相等的比例决定。非金属名称词尾改为“-ide”（中文称“化某”）。

3. Properties of Ionic Compounds | 离子化合物的性质

Ionic compounds have high melting and boiling points because the giant lattice contains strong electrostatic forces between ions in all directions. A lot of energy is needed to overcome these forces.

离子化合物具有高熔点和高沸点，因为巨型晶格中离子之间在各个方向上都存在强大的静电引力。克服这些引力需要大量能量。

They do not conduct electricity when solid because the ions are locked in place and cannot move. When molten (liquid) or dissolved in water, the ions become mobile and can carry charge, so they conduct electricity.

它们在固态时不导电，因为离子被固定在晶格中不能移动。当熔融（液态）或溶于水时，离子变得可以自由移动，能够携带电荷，因此可以导电。

Ionic compounds are often soluble in water, but this varies. They are usually brittle and shatter when a force is applied because layers of ions shift and like charges repel, splitting the crystal.

离子化合物通常可溶于水，但溶解度各异。它们通常很脆，受力时会碎裂，因为离子层发生位移，同号电荷相互排斥，使晶体裂开。

4. Covalent Bonding: Electron Sharing | 共价键：电子共用

Covalent bonding occurs between non‑metal atoms. Atoms share pairs of electrons so that each atom obtains a full outer shell. A single covalent bond is one shared pair of electrons (represented by a line in structural formulas). Double bonds (two pairs, O₂, CO₂) and triple bonds (three pairs, N₂) are also possible.

共价键发生在非金属原子之间。原子共用电子对，使每个原子都获得满的外层电子排布。单共价键是一对共用电子（结构式中用一条线表示）。也存在双键（两对，如 O₂、CO₂）和三键（三对，如 N₂）。

Dot-and-cross diagrams for simple molecules show the outer shells overlapping, with the shared pair in the overlap region. Each atom counts the shared electrons as part of its own outer shell to check the noble gas configuration.

简单分子的点叉图显示重叠的外层电子，共用电子对位于重叠区域。每个原子将共用电子计入自己的外层，以检验是否达到稀有气体构型。

Examples: H₂ (H–H, each H gets 2), Cl₂ (Cl–Cl, each Cl gets 8), H₂O (O shares with two H atoms, two lone pairs on O), CH₄ (four C–H bonds), CO₂ (O=C=O, two double bonds).

示例：H₂（H–H，每个 H 得到2个电子）、Cl₂（Cl–Cl，每个 Cl 得到8个）、H₂O（O 与两个 H 共用，O 上有两对孤对电子）、CH₄（四条 C–H 键）、CO₂（O=C=O，两个双键）。

5. Simple Molecular Substances | 简单分子物质

Substances made of small covalent molecules have relatively low melting and boiling points because the individual molecules are held together by weak intermolecular forces (van der Waals forces). Only a small amount of energy is needed to overcome these forces, not the strong covalent bonds inside the molecules.

由小共价分子组成的物质具有相对较低的熔点和沸点，因为单个分子之间靠弱的分子间力（范德华力）维系。打破这些力只需要很少的能量，无需破坏分子内部牢固的共价键。

They do not conduct electricity in any state because there are no mobile charged particles (no ions, no free electrons). They are often gases or liquids at room temperature, or soft solids.

它们在任何状态下都不导电，因为没有可移动的带电粒子（无离子，无自由电子）。它们在室温下通常是气体或液体，或者是软固体。

As the size of the molecule increases, the intermolecular forces become stronger, and melting/boiling points rise. This explains trends in the halogens or alkanes.

随着分子体积增大，分子间力变强，熔沸点升高。这解释了卤素或烷烃的变化趋势。

6. Giant Covalent Structures | 巨型共价结构

Some covalent substances form giant structures where atoms are joined by strong covalent bonds in a continuous network. They have very high melting points and are extremely hard because many covalent bonds must be broken to melt or scratch them.

有些共价物质形成巨型结构，原子通过牢固的共价键连接成连续的网状结构。它们熔点非常高，并且极其坚硬，因为熔化或刮擦需要破坏大量的共价键。

Diamond: each carbon atom forms four strong covalent bonds in a tetrahedral arrangement. It is the hardest natural substance, has a very high melting point, and does not conduct electricity (no free electrons). Used for cutting tools and jewellery.

金刚石：每个碳原子以正四面体排列形成四个牢固的共价键。它是最硬的天然物质，熔点极高，不导电（无自由电子）。用于切割工具和珠宝。

Graphite: each carbon atom bonds to three others in flat layers. The fourth outer electron per carbon becomes delocalised between the layers. These delocalised electrons allow graphite to conduct electricity and heat. The weak forces between layers let them slide, making graphite soft and slippery – used as a lubricant and in pencil leads.

石墨：每个碳原子与另外三个碳原子键合，形成平面的层状结构。每个碳原子的第四个外层电子在层间离域。这些离域电子使石墨能够导电和导热。层与层之间微弱的力使得它们可以滑动，因此石墨柔软滑腻——用作润滑剂和铅笔芯。

Silicon dioxide (SiO₂) has a giant covalent structure similar to diamond: each silicon bonds to four oxygen atoms, and each oxygen to two silicon atoms. It is very hard, has a high melting point, and does not conduct. Found in quartz and sand.

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键合，每个氧原子与两个硅原子键合。它非常坚硬，熔点高，不导电。见于石英和沙中。

7. Metallic Bonding | 金属键

Metallic bonding is the electrostatic attraction between a lattice of positive metal ions and the sea of delocalised electrons. Metal atoms lose their outer electrons, which become free to move throughout the giant lattice.

金属键是正金属离子晶格与离域电子海之间的静电吸引。金属原子失去外层电子，这些电子可以在整个巨型晶格中自由移动。

The model explains typical metallic properties: malleable (layers can slide without breaking bonds because delocalised electrons continue to hold the ions), ductile, good conductors of heat and electricity (mobile electrons carry current and kinetic energy), high melting points (strong attraction requires lots of energy to overcome), and lustrous.

该模型解释了典型的金属性质：可锻（层可滑动而不破坏键，因为离域电子继续维系着离子）、延展性好、导热导电性好（流动的电子携带电流和动能）、熔点高（强大的吸引力需要大量能量克服），并且具有光泽。

Alloys are mixtures of a metal with other elements. The different sized atoms disrupt the regular layers, preventing them from sliding easily. This makes alloys harder and stronger than pure metals. Example: steel (iron with carbon), brass (copper with zinc).

合金是金属与其他元素混合而成的物质。不同大小的原子打乱了规则的层状排列，阻止层间轻易滑动。这使合金比纯金属更硬更强。例如：钢（铁与碳）、黄铜（铜与锌）。

8. Bonding and State Symbols in Equations | 化学方程式中的状态符号与键型

State symbols (s), (l), (g), (aq) show the physical state of each substance. They link to bonding: ionic solids are (s) but may be used as aqueous solutions (aq); small covalent molecules are often (g) or (l); metals are (s) except mercury (l). Understanding bonding helps predict state symbols.

状态符号 (s)、(l)、(g)、(aq) 表示每种物质的物理状态。它们与键型相关：离子固体为 (s)，但可能用于水溶液 (aq)；小共价分子常为 (g) 或 (l)；金属除汞为 (l) 外通常为 (s)。理解键型有助于预测状态符号。

When writing ionic equations for precipitation or displacement, the bonding of the reactants and products determines whether ions are free or locked. Only aqueous ionic compounds split into ions.

在书写沉淀反应或置换反应的离子方程式时，反应物和产物的键型决定了离子是自由移动还是被固定在晶格中。只有水溶液中的离子化合物才能拆分成离子。

9. Comparing Bonding Types | 键型比较

Property	Ionic	Covalent (simple)	Covalent (giant)	Metallic
Particles	Ions	Small molecules	Atoms (network)	Positive ions and delocalised electrons
Melting/boiling point	High	Low	Very high	High (except mercury)
Electrical conductivity	Only when molten or aqueous	None	None (except graphite)	Yes (solid and liquid)
Typical examples	NaCl, MgO, CaCl₂	H₂O, CO₂, C₆H₁₂O₆	Diamond, graphite, SiO₂	Cu, Fe, Al, alloys

A useful comparison: ionic and metallic substances conduct when mobile charged particles are present; covalent substances (except graphite) never conduct. Giant structures (ionic, metallic, giant covalent) have high melting points, while simple molecular substances have low ones.

一个有用的对比：离子和金属物质在有可移动带电粒子时导电；共价物质（除石墨外）永不导电。巨型结构（离子、金属、巨型共价）具有高熔点，而简单分子物质熔点低。

10. Drawing Dot-and-Cross Diagrams Accurately | 准确绘制点叉图

Use dots for electrons from one atom and crosses for the other. Show only the outer shell electrons. For ions, draw square brackets with the charge outside. For covalent molecules, draw overlapping circles/ovals with the shared pair in the intersection. Ensure each atom counts 8 electrons (or 2 for hydrogen) after sharing.

用一个原子的电子画点，另一个原子的电子画叉。只显示最外层电子。对于离子，画方括号，并在外面写上电荷。对于共价分子，画重叠的圆圈/椭圆，共用电子对位于交集处。确保共用后每个原子数到8个电子（或氢为2个）。

Common mistake: forgetting non‑bonding (lone) pairs on central atoms like oxygen in water or nitrogen in ammonia. Also, showing wrong number of shared pairs for double/triple bonds.

常见错误：忘记中心原子上的未成键电子对（孤对电子），例如水中的氧或氨中的氮。此外，在双键/三键中显示错误数量的共用电子对。

For ionic giant structures, the diagram only represents the ratio of ions in the formula, not the whole lattice. The WJEC exam often asks for dot-and-cross of NaCl, MgO, MgCl₂, Na₂O, and CaO. Practice balancing charges and showing brackets.

对于离子巨型结构，示意图仅表示化学式中离子的比例，而非整个晶格。WJEC 考试常要求画出 NaCl、MgO、MgCl₂、Na₂O 和 CaO 的点叉图。练习电荷平衡并画出方括号。

11. Interpreting Properties from Bonding | 根据键型解释性质

Exam questions frequently ask you to explain why a substance has a particular melting point or conductivity. Use “because” and link to the structure and bonding. For example: “Solid ionic compounds do not conduct electricity because the ions are fixed in the lattice and cannot move.”

考试题目常要求你解释为什么某种物质具有特定的熔点或导电性。使用“因为”并联系结构与键型。例如：“固态离子化合物不导电，因为离子被固定在晶格中，无法移动。”

For graphite conductivity: “Graphite conducts electricity because each carbon atom has one delocalised electron that can move freely along the layers.” Always mention the type of bonding and the particles involved.

对于石墨的导电性：“石墨导电是因为每个碳原子有一个离域电子，可以沿着层状结构自由移动。”务必提到键型和所涉及的粒子。

To explain high melting points: state the type of giant structure and the strong forces/bonds that must be overcome. For ionic, “strong electrostatic forces between oppositely charged ions”. For metallic, “strong attraction between metal cations and delocalised electrons”. For giant covalent, “many strong covalent bonds”.

解释高熔点：说明巨型结构的类型和必须克服的强作用力/键。对于离子型：“带相反电荷离子之间的强大静电引力”。对于金属：“金属阳离子与离域电子之间的强大吸引力”。对于巨型共价：“众多牢固的共价键”。

12. Quick Revision Checklist and Exam Tips | 快速复习清单与应试技巧

Define ionic, covalent, and metallic bonding with key words (transfer, sharing, delocalised).

用关键词定义离子键、共价键和金属键（转移、共用、离域）。
Draw dot-and-cross diagrams for NaCl, MgO, H₂O, CO₂, N₂, CH₄, and NH₃.

能画出 NaCl、MgO、H₂O、CO₂、N₂、CH₄ 和 NH₃ 的点叉图。
Explain properties of diamond, graphite, and silicon dioxide in terms of bonding.

用键型解释金刚石、石墨和二氧化硅的性质。
Connect melting point, conductivity, and strength to structure type. Remember: simple molecular substances are gases or liquids at room temperature due to weak intermolecular forces.

将熔点、导电性和强度与结构类型联系起来。记住：简单分子物质在室温下为气体或液体，因为分子间力弱。
In equations, give correct state symbols based on bonding and room temperature state.

在方程式中，根据键型和室温状态给出正确的状态符号。
When a question asks “Explain why…”, always start with the type of structure, name the particles, and describe the forces/bonds and what happens when you heat/apply voltage.

当题目问“解释为什么……”时，始终从结构类型开始，说出粒子名称，描述作用力/键，以及加热或加电压时发生的变化。

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Chemical Bonding | IGCSE WJEC 化学：化学键 考点精讲