📚 Covalent Bonding Explained for GCSE Chemistry | GCSE 化学：共价键 考点精讲
In GCSE Chemistry, covalent bonding describes how non‑metal atoms achieve full outer shells by sharing pairs of electrons. This article covers the essential concepts, from drawing dot‑and‑cross diagrams to understanding the properties of simple molecular substances and giant covalent structures. Each point is explained clearly in both English and Chinese to support bilingual revision.
在 GCSE 化学中，共价键描述非金属原子如何通过共用电子对来达到满壳层结构。本文涵盖核心概念，从绘制点叉图到理解简单分子物质和巨型共价结构的性质，每个要点均提供清晰的中英双语讲解，助力高效复习。
1. What is a Covalent Bond? | 什么是共价键？
A covalent bond forms when a pair of electrons is shared between two non‑metal atoms. Each atom contributes at least one electron to the shared pair, allowing both to achieve a stable noble‑gas electron configuration (usually a full outer shell). The attraction between the shared electrons and the nuclei holds the atoms together strongly.
共价键是两个非金属原子之间共用一对电子而形成的化学键。每个原子至少提供一个电子参与共用，使双方都能达到稳定的稀有气体电子排布（通常是满的最外层）。共用电子与原子核之间的吸引力将原子紧密地结合在一起。
2. Why Do Atoms Share Electrons? | 原子为何共用电子？
Non‑metal atoms have high electronegativity, meaning they tend to attract electrons rather than lose them. By sharing electrons, each nucleus can pull on the shared pair, effectively completing the outer energy level without a full transfer of electrons. This process lowers the overall energy of the system, making the bonded atoms more stable than separate atoms.
非金属原子具有高电负性，倾向于吸引电子而非失去电子。通过共用电子，每个原子核都可以吸引共用电子对，从而在不完全转移电子的情况下填满最外层。这个过程降低了体系的总能量，使成键原子比单个原子更稳定。
3. Dot‑and‑Cross Diagrams for Simple Covalent Molecules | 简单共价分子的点叉图
Dot‑and‑cross diagrams are a visual way to show how electrons are shared. Dots represent electrons from one atom, crosses represent electrons from the other. Only the outer (valence) electrons are drawn. The shared pair is shown in the overlap region between the two atoms’ shells. For example, in hydrogen (H₂), each H atom contributes one electron to form a single covalent bond, giving both H atoms the electron configuration of helium.
点叉图是一种形象展示电子共用方式的图示。点代表一个原子的电子，叉代表另一个原子的电子。只绘制最外层（价）电子。共用电子对显示在两个原子壳层的重叠区域。例如，在氢气 (H₂) 中，每个 H 原子提供一个电子形成单共价键，使两个 H 原子都达到氦的电子排布。
Example | 示例：
Hydrogen molecule (H₂) – Each H has 1 outer electron. After sharing, both have 2 electrons in their outer shell.
氢分子 (H₂) – 每个 H 有 1 个最外层电子，共用后双方最外层均达到 2 个电子。
4. Single, Double and Triple Covalent Bonds | 单键、双键与三键
Covalent bonds can involve one, two or three shared pairs of electrons. A single bond (─) consists of one pair, e.g. in H₂ or Cl₂. A double bond (=) contains two shared pairs, e.g. O₂ has an O=O double bond, and CO₂ has two C=O double bonds. A triple bond (≡) shares three pairs, as in N₂ (N≡N). The number of bonds between atoms depends on how many electrons each atom needs to gain to reach a full shell.
共价键可以共用一对、两对或三对电子。单键 (─) 包含一对共用电子，如 H₂ 或 Cl₂。双键 (=) 包含两对共用电子，例如 O₂ 含 O=O 双键，CO₂ 含两个 C=O 双键。三键 (≡) 共用三对电子，如 N₂ (N≡N)。原子间成键的数量取决于每个原子需要获得多少电子才能达到满壳层。
5. Structure of Common GCSE Molecules | 常见 GCSE 分子的结构
You need to be able to draw and interpret dot‑and‑cross diagrams for molecules like H₂O, NH₃, CH₄, CO₂, O₂, N₂, Cl₂, HCl, and HF. Water (H₂O) has two O─H single bonds and two lone pairs on oxygen. Methane (CH₄) has four C─H single bonds with no lone pairs on carbon. Carbon dioxide (CO₂) has two double bonds and is linear.
你需要能够绘制并解读以下分子的点叉图：H₂O、NH₃、CH₄、CO₂、O₂、N₂、Cl₂、HCl、HF。水 (H₂O) 有两个 O─H 单键和氧上的两对孤对电子。甲烷 (CH₄) 有四个 C─H 单键，碳上没有孤对电子。二氧化碳 (CO₂) 有两个双键，分子呈直线形。
6. The Octet Rule and Its Limitations | 八隅体规则及其局限性
Most atoms in covalent molecules aim to have eight electrons in their outer shell (the octet rule). Carbon, nitrogen, oxygen and the halogens obey this rule in many compounds. However, hydrogen only needs two electrons (duet rule). Some elements like boron and phosphorus can form compounds that do not follow the octet rule, but at GCSE you mainly focus on molecules that do satisfy it.
多数共价分子中的原子力求最外层达到八个电子（八隅体规则）。碳、氮、氧和卤素在许多化合物中遵循此规则。但氢只需两个电子（二隅体规则）。硼和磷等元素可形成不符合八隅体规则的化合物，但在 GCSE 阶段，你主要关注满足该规则的分子。
7. Molecular Shape and Bond Angles | 分子形状与键角
The shape of a covalent molecule is determined by the number of bonding pairs and lone pairs around the central atom. Lone pairs repel more strongly than bonding pairs, reducing bond angles. GCSE students should recall simple shapes: linear (CO₂, 180°), bent (H₂O, about 104.5°), trigonal pyramidal (NH₃, about 107°) and tetrahedral (CH₄, 109.5°).
共价分子的形状由中心原子周围的成键电子对和孤对电子数目决定。孤对电子的排斥力大于成键电子对，会减小键角。GCSE 学生需记住简单形状：直线形 (CO₂，180°)、V 形 (H₂O，约 104.5°)、三角锥形 (NH₃，约 107°) 和正四面体形 (CH₄，109.5°)。
8. Properties of Simple Molecular Substances | 简单分子物质的性质
Substances made of small covalent molecules have low melting and boiling points because the intermolecular forces between molecules are weak, even though the covalent bonds inside the molecules are very strong. They do not conduct electricity as there are no free ions or delocalised electrons. Many are gases or liquids at room temperature.
由小分子构成的共价物质具有较低的熔点和沸点，因为分子间的分子间作用力很弱，尽管分子内的共价键很强。它们不导电，因为没有自由离子或离域电子。许多在室温下为气体或液体。
9. Giant Covalent Structures | 巨型共价结构
Some elements and compounds form giant covalent lattices where atoms are bonded by strong covalent bonds throughout the entire structure. Examples include diamond (carbon), graphite (carbon), and silicon dioxide (SiO₂). These substances have very high melting and boiling points because many covalent bonds must be broken. They are usually hard, and except graphite, they do not conduct electricity.
某些单质和化合物形成巨型共价晶格，整个结构中原子均通过强共价键连接。例子包括金刚石（碳）、石墨（碳）和二氧化硅 (SiO₂)。这些物质熔点和沸点极高，因为需要破坏大量共价键。它们通常很硬，且除石墨外不导电。
10. Graphite as a Special Case | 石墨的特殊性
Graphite is a giant covalent structure where each carbon atom bonds to three others in flat layers. The fourth electron becomes delocalised and can move between layers, allowing graphite to conduct electricity. The layers can slide over each other, making graphite soft and slippery, unlike diamond.
石墨是一种巨型共价结构，每个碳原子与另外三个碳原子成键，形成平面层状。第四个电子成为离域电子，可在层间自由移动，使石墨能够导电。层与层之间可以滑动，因此石墨柔软滑腻，不同于金刚石。
11. Comparing Covalent and Ionic Bonding | 共价键与离子键的比较
Ionic bonding involves electron transfer from metal to non‑metal, forming oppositely charged ions held together by electrostatic attraction. Covalent bonding involves sharing electrons between non‑metals. Ionic compounds have high melting points and conduct electricity when molten or dissolved; simple covalent compounds have low melting points and do not conduct electricity. Understanding these differences is key for GCSE exam questions.
离子键涉及电子从金属原子转移到非金属原子，形成带相反电荷的离子并通过静电引力结合。共价键涉及非金属原子间共用电子。离子化合物熔点高，熔融或溶解时可导电；简单共价化合物熔点低，不导电。理解这些区别是 GCSE 考试题的关键。
12. Polymers – An Extension of Covalent Bonding | 聚合物——共价键的延伸
Polymers are long chains of repeating units (monomers) joined by covalent bonds. The strong covalent bonds within the chains give polymers strength, while weak intermolecular forces between chains allow some flexibility. Thermoplastic polymers soften when heated because these intermolecular forces can be overcome, a topic often explored in higher‑tier GCSE Chemistry.
聚合物是由共价键连接的长链重复单元（单体）。链内强共价键赋予聚合物强度，而链间弱的分子间作用力赋予其一定柔韧性。热塑性聚合物加热时会软化，因为可以克服这些分子间作用力，此主题常在 GCSE 化学高难度卷中考查。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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