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

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

Covalent bonding is a core topic in IGCSE WJEC Chemistry, describing how non-metal atoms share electrons to form molecules and giant structures. Understanding the nature of the shared electron pair, how to represent bonding with dot-and-cross diagrams, and the resulting physical properties is essential for exam success.

共价键是IGCSE WJEC化学中的核心主题，描述了非金属原子如何通过共享电子形成分子和巨型结构。理解共享电子对的本质、如何用点叉图表示成键，以及由此产生的物理性质，是考试成功的关键。

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

Covalent bonding occurs when two or more non-metal atoms share one or more pairs of electrons. By sharing, each atom can achieve a full outer shell, which gives it the stable electronic configuration of a noble gas. This type of bonding holds the atoms together strongly within a molecule or a giant lattice.

共价键发生在两个或多个非金属原子之间，它们共享一对或多对电子。通过共享，每个原子都能填满最外层，从而获得惰性气体的稳定电子构型。这种键合将原子牢固地连接在分子或巨型晶格中。

2. How Covalent Bonds Form | 共价键的形成方式

Atoms form covalent bonds because they have unfilled outer shells and need extra electrons to become stable. Instead of losing or gaining electrons (as in ionic bonding), non-metal atoms meet their needs by overlapping their outer shells and sharing electrons. The shared pair of electrons is attracted to the positive nuclei of both atoms, creating a strong electrostatic force of attraction that constitutes the covalent bond.

原子之所以形成共价键，是因为它们的最外层未满，需要额外电子才能变得稳定。非金属原子并非像离子键那样失去或获得电子，而是通过重叠最外层并共享电子来满足需求。共享电子对受到双方原子核的吸引，这种强大的静电吸引力就构成了共价键。

3. Single Covalent Bonds with Examples | 单共价键及示例

A single covalent bond involves one shared pair of electrons. Common examples include hydrogen (H₂), chlorine (Cl₂), hydrogen chloride (HCl), water (H₂O), ammonia (NH₃) and methane (CH₄). In H₂, each hydrogen atom contributes one electron, so the shared pair gives both atoms the helium configuration. In Cl₂, each chlorine atom has seven outer electrons; they share one pair so each atom effectively has eight outer electrons.

单共价键包含一对共享电子。常见例子有氢气 (H₂)、氯气 (Cl₂)、氯化氢 (HCl)、水 (H₂O)、氨 (NH₃) 和甲烷 (CH₄)。在 H₂ 中，每个氢原子提供一个电子，共享电子对使两个氢原子都获得氦的电子构型。在 Cl₂ 中，每个氯原子有七个最外层电子；它们共享一对电子，从而使每个原子都拥有八个最外层电子。

4. Double and Triple Bonds | 双键与三键

Sometimes atoms need to share more than one pair of electrons to achieve a full outer shell. A double bond consists of two shared pairs (four electrons), as in O₂ and CO₂. In an oxygen molecule, each oxygen atom shares two pairs, forming O=O. A triple bond involves three shared pairs (six electrons), such as in N₂ (N≡N), which makes nitrogen molecules very strong and unreactive.

有时原子需要共享一对以上的电子才能填满最外层。双键包含两个共享电子对（四个电子），例如 O₂ 和 CO₂。在氧分子中，每个氧原子共享两对电子，形成 O=O。三键包含三个共享电子对（六个电子），例如 N₂ (N≡N)，这使得氮分子非常牢固且不活泼。

5. Drawing Dot-and-Cross Diagrams | 绘制点叉图

Dot-and-cross diagrams are used to represent the outer electrons of atoms and the covalent bonds formed. Different symbols, usually dots ‘•’ and crosses ‘×’, are used for the electrons from different atoms. For a hydrogen molecule, the diagram shows H• and ×H overlapping so that one •× shared pair appears between the two H symbols. For water, the oxygen atom (with six outer electrons, shown as dots) shares one electron with each of two hydrogen atoms (electrons shown as crosses), forming two •× shared pairs; oxygen retains two lone pairs of dots.

点叉图用来表示原子的最外层电子和形成的共价键。通常用不同的符号，比如点 ‘•’ 和叉 ‘×’，来代表不同原子的电子。对于氢分子，图形显示 H• 和 ×H 重叠，在两个 H 符号之间出现一个 •× 共享电子对。对于水分子，氧原子（有六个最外层电子，用点表示）与两个氢原子（电子用叉表示）各共享一个电子，形成两个 •× 共享电子对；氧原子上还留有两对孤对电子点。

When drawing dot-and-cross diagrams for CO₂, carbon (four outer electrons) shares two pairs with each oxygen atom (six outer electrons), giving O=C=O. The carbon atom uses all its electrons in the double bonds, while each oxygen also ends up with two lone pairs.

在绘制 CO₂ 的点叉图时，碳原子（四个最外层电子）与每个氧原子（六个最外层电子）各共享两对电子，得到 O=C=O。碳原子全部电子都用于双键，而每个氧原子最后还保留两对孤对电子。

6. Simple Molecular Substances | 简单分子物质

Most covalent compounds exist as simple molecules. Examples include H₂O, CO₂, NH₃, CH₄ and the halogens (Cl₂, Br₂, I₂). These substances consist of small, discrete groups of atoms held together by strong covalent bonds. However, the forces of attraction between the molecules (intermolecular forces) are weak, which determines their bulk physical properties.

大多数共价化合物以简单分子形式存在。例子包括 H₂O、CO₂、NH₃、CH₄ 和卤素 (Cl₂、Br₂、I₂)。这些物质由强共价键连接在一起的、独立的小原子集团构成。然而，分子之间的吸引力（分子间作用力）很弱，这决定了它们的宏观物理性质。

7. Properties of Simple Molecular Covalent Compounds | 简单分子共价化合物的性质

Simple molecular substances have low melting points and boiling points. This is because only the weak intermolecular forces need to be overcome rather than the strong covalent bonds. Consequently, many are gases or liquids at room temperature. They do not conduct electricity at any state, as there are no free ions or delocalised electrons to carry charge. Even when dissolved in water, simple molecules like sugar do not produce ions, so the solution does not conduct electricity.

简单分子物质具有较低的熔点和沸点。这是因为只需克服微弱的分子间作用力，而无需破坏牢固的共价键。因此，许多物质在室温下是气体或液体。它们在任何状态下都不导电，因为没有自由移动的离子或离域电子来运载电荷。即使溶于水，像糖这样的简单分子也不会产生离子，所以其溶液不导电。

8. Giant Covalent Structures: Diamond | 巨型共价结构：金刚石

In a giant covalent structure, huge numbers of atoms are joined together by covalent bonds in a continuous three‑dimensional network. Diamond is an allotrope of carbon where every carbon atom forms four strong single covalent bonds to four other carbon atoms in a tetrahedral arrangement. This makes diamond extremely hard, with a very high melting point (over 3500 °C). All the outer electrons are fixed in bonds, so diamond cannot conduct electricity.

在巨型共价结构中，大量的原子通过共价键连接成一个连续的三维网络。金刚石是碳的一种同素异形体，其中每个碳原子与另外四个碳原子形成四个牢固的单共价键，呈四面体排列。这使得金刚石极度坚硬，熔点极高（超过3500 °C）。所有外层电子都定域在化学键中，因此金刚石不导电。

9. Graphite and Its Unique Properties | 石墨及其独特性质

Graphite is another allotrope of carbon but with a very different structure. Each carbon atom forms only three covalent bonds, creating flat layers of hexagons. The fourth outer electron from each carbon becomes delocalised, meaning it is free to move between the layers. This explains why graphite can conduct electricity. The layers are held together by weak forces, so they can slide past each other, making graphite soft and slippery—ideal for use as a lubricant and in pencil ‘lead’. Graphite still has a high melting point because the covalent bonds within the layers are strong.

石墨是碳的另一种同素异形体，但结构截然不同。每个碳原子只形成三个共价键，构成扁平的六边形层状结构。每个碳原子的第四个外层电子被离域，意味着它可以在层间自由移动。这就解释了为什么石墨能导电。层与层之间依靠微弱的作用力结合，因此它们可以相互滑动，使石墨柔软滑腻——非常适合用作润滑剂和铅笔芯。石墨的熔点仍然很高，因为层内的共价键很强。

10. Silicon Dioxide (Silica) | 二氧化硅

Silicon dioxide (SiO₂), often called silica, has a giant covalent structure similar to diamond. However, instead of only C–C bonds, there are alternating Si and O atoms. Each silicon atom forms four covalent bonds with oxygen atoms, and each oxygen atom bonds to two silicon atoms. The resulting network is very hard and has an extremely high melting point. Like diamond, silica does not conduct electricity because all of its electrons are tightly held in bonds. Sand and quartz are common forms of SiO₂.

二氧化硅 (SiO₂) 常被称为硅石，具有类似金刚石的巨型共价结构。但是，它并非只有 C–C 键，而是由硅原子和氧原子交替排列。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成键。由此形成的网络非常坚硬，熔点极高。与金刚石一样，硅石不导电，因为其所有电子都牢牢地固定在化学键中。沙子和石英是 SiO₂ 的常见形态。

11. Comparing Structures and Typical Exam Questions | 结构对比与常见考题

In WJEC IGCSE exams, you are often asked to compare diamond, graphite and silica. Diamond is a tetrahedral network with each C bonded to 4 others—hard, non‑conductor. Graphite is layered, each C bonded to 3—soft, conducts electricity. Silica has an Si–O framework, similar hardness to diamond, and is non‑conducting. Another common question is to explain why simple molecules like carbon dioxide are gases at room temperature while diamond is a solid: CO₂ has weak intermolecular forces that are easily broken, whereas diamond has a giant network of strong covalent bonds throughout.

在 WJEC IGCSE 考试中，经常要求你比较金刚石、石墨和硅石。金刚石是四面体网络，每个碳原子与另外四个相连——坚硬、不导电。石墨是层状结构，每个碳原子与三个相连——柔软、导电。硅石具有 Si–O 骨架，硬度与金刚石相似，不导电。另一个常见问题是解释为什么像二氧化碳这样的简单分子在室温下是气体而金刚石是固体：CO₂ 的分子间作用力微弱，容易打破，而金刚石整体由牢固的共价键形成的巨型网络构成。

You may also be asked to draw dot-and-cross diagrams for molecules like N₂, O₂ or CO₂, and to state the number of lone pairs and bonding pairs in a given molecule. Always practise: showing the electrons from different atoms with different symbols, and using the correct number of electrons to obey the octet rule (or the duet rule for hydrogen).

你还可能被要求绘制 N₂、O₂ 或 CO₂ 等分子的点叉图，并说出给定分子中孤对电子和共用电子对的数目。要经常练习：用不同符号表示不同原子的电子，并用正确的电子数来满足八隅体规则（或氢的二电子规则）。

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