Covalent Bonding for CCEA A-Level Chemistry | A-Level CCEA 化学：共价键考点精讲

📚 Covalent Bonding for CCEA A-Level Chemistry | A-Level CCEA 化学：共价键考点精讲

Covalent bonding is the fundamental force holding molecules together through electron sharing. In CCEA A-Level Chemistry, a deep understanding of covalent bonds — from Lewis structures to hybridisation — is essential for success. This article covers every key specification point, with detailed English and Chinese explanations tailored for your revision.

共价键是通过电子共享将原子结合成分子的基本作用力。在 CCEA A-Level 化学中，透彻理解共价键——从路易斯结构到杂化理论——是取得高分的关键。本文覆盖所有核心考点，提供中英双语详尽解析，为你的备考保驾护航。

1. Definition and Nature of Covalent Bonding | 共价键的定义与本质

A covalent bond forms when two atoms share one or more pairs of electrons. The shared electrons are attracted to both nuclei, creating a stable balance between repulsive and attractive forces. Covalent bonding usually occurs between non‑metal atoms with similar electronegativities.

当两个原子共享一对或多对电子时，就形成了共价键。共享电子同时受到两个原子核的吸引，在排斥力与吸引力之间建立稳定平衡。共价键通常发生在电负性相近的非金属原子之间。

The bond can be represented by a single line (—) for one shared pair, a double line (=) for two shared pairs, or a triple line (≡) for three shared pairs. The electrostatic attraction between the shared electrons and the positive nuclei gives the bond its strength.

单键用一条短线（—）表示共享一对电子，双键（=）表示两对，三键（≡）表示三对。共享电子与带正电的原子核之间的静电吸引力赋予共价键其强度。

Key terms: bond pair – an electron pair involved in bonding; lone pair – an electron pair not involved in bonding, belonging entirely to one atom.

关键术语：键对（参与成键的电子对）；孤对电子（未参与成键、完全属于一个原子的电子对）。

2. Lewis Structures and the Octet Rule | 路易斯结构与八隅规则

Lewis structures are diagrams that show all valence electrons in a molecule, using dots for non‑bonding electrons and lines for bonding pairs. The octet rule states that atoms tend to share electrons until they are surrounded by eight valence electrons, achieving a noble‑gas configuration.

路易斯结构是展示分子中所有价电子的示意图，用点表示非成键电子，用线表示成键电子对。八隅规则指出，原子倾向于共享电子，直到其周围拥有八个价电子，达到稀有气体电子构型。

To draw a Lewis structure: count total valence electrons, identify the central atom (usually the least electronegative, except H), connect atoms with single bonds, complete octets of outer atoms, then place remaining electrons on the central atom. If the central atom lacks an octet, form multiple bonds.

绘制路易斯结构的步骤：计算总价电子数；确定中心原子（通常电负性最低，氢除外）；用单键连接原子；先使外围原子满足八隅体；然后将剩余电子放在中心原子上；若中心原子未满八隅体，则形成多重键。

Example: Water (H₂O). Oxygen has 6 valence electrons, each hydrogen has 1. Total = 8. Central O connects to two H atoms with single bonds, using 4 electrons. The remaining 4 electrons form two lone pairs on oxygen, giving O an octet and H a duet.

例子：水 (H₂O)。氧有 6 个价电子，每个氢有 1 个，共 8 个。中心氧与两个氢以单键连接，用去 4 个电子。剩余 4 个电子在氧上形成两对孤对电子，使氧达到八隅体，氢达到二隅体。

3. Exceptions to the Octet Rule | 八隅规则的例外

Several molecules do not obey the octet rule. Common exceptions include electron‑deficient species (e.g. BF₃, where B has only 6 electrons), odd‑electron species (radicals like NO, having an unpaired electron), and expanded octets (e.g. SF₆, PCl₅) where central atoms from period 3 or below can accommodate more than 8 electrons by using empty d‑orbitals.

有些分子不遵守八隅规则。常见例外包括缺电子物种（如 BF₃，硼仅有 6 个电子）、奇电子物种（自由基如 NO，含有未成对电子）以及扩展八隅体（如 SF₆, PCl₅），其中第三周期及以下的中心原子可利用空的 d 轨道容纳超过 8 个电子。

In BF₃, boron forms three bonds with fluorine but still has only 6 valence electrons. The molecule accepts a lone pair from another species to complete its octet, making it a Lewis acid. In SF₆, sulfur uses 3d orbitals to form six S–F bonds, resulting in 12 electrons around sulfur.

在 BF₃ 中，硼与氟形成三个键，却只有 6 个价电子。该分子可接受另一物种提供的孤对电子以完成八隅体，因此是路易斯酸。在 SF₆ 中，硫利用 3d 轨道形成六条 S–F 键，硫周围共有 12 个电子。

Radicals such as NO contain an odd number of valence electrons, leading to high reactivity. They are important in atmospheric chemistry and combustion processes.

自由基如 NO 含有奇数个价电子，因而具有高反应活性，在大气化学和燃烧过程中扮演重要角色。

4. Bond Length and Bond Energy | 键长与键能

Bond length is the average distance between the nuclei of two bonded atoms. Bond energy (bond dissociation enthalpy) is the energy required to break one mole of a specific covalent bond in the gaseous state. Multiple bonds are shorter and stronger than single bonds.

键长是两个成键原子核之间的平均距离。键能（键解离焓）是气态下断裂一摩尔特定共价键所需的能量。多重键比单键更短、更强。

Bond \| 键	Bond length (pm) \| 键长	Bond energy (kJ mol⁻¹) \| 键能
C–C	154	347
C=C	134	612
C≡C	120	837

Bond length decreases because the increased number of shared electrons pulls the nuclei closer together. Bond energy rises accordingly. The relationship is used to explain reactivity trends and physical properties of materials such as diamond (strong C–C single bonds) and graphite (delocalised π bonds).

键长因共享电子数增多而缩短，原子核被拉得更近；键能相应增大。这一关系被用来解释反应活性趋势以及材料物理性质，如金刚石（强 C–C 单键）和石墨（离域 π 键）。

5. Electronegativity and Bond Polarity | 电负性与键的极性

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. When two atoms with different electronegativities form a bond, the electron density is pulled towards the more electronegative atom, creating a polar covalent bond. The greater the difference, the more polar the bond.

电负性是原子在共价键中吸引成键电子的能力。两个电负性不同的原子成键时，电子云会被拉向电负性更强的原子，形成极性共价键。电负性差值越大，键的极性越强。

A bond is generally considered ionic if the electronegativity difference exceeds about 1.7–2.0, though polar covalent bonds exist over a continuous range. In a C–Cl bond, chlorine is more electronegative, gaining a partial negative charge (δ⁻) while carbon acquires a partial positive charge (δ⁺).

若电负性差值超过 1.7–2.0，键通常被视为离子键，但极性共价键存在于连续范围内。在 C–Cl 键中，氯电负性更大，带有部分负电荷 (δ⁻)，而碳带有部分正电荷 (δ⁺)。

The polarity of individual bonds and the molecular shape determine whether a molecule possesses a permanent dipole. Symmetrical molecules like CCl₄ have polar bonds but zero overall dipole.

单个键的极性与分子形状共同决定分子是否具有永久偶极。对称分子如 CCl₄ 键虽有极性，但整体偶极为零。

6. Coordinate (Dative) Covalent Bonds | 配位共价键

A coordinate or dative covalent bond forms when both electrons in the shared pair come from the same atom. Once formed, it is identical to a normal covalent bond. The atom donating the lone pair is called the donor; the atom accepting it is the acceptor.

配位共价键（配位键）中，共享电子对的两个电子均来自同一个原子。一旦形成，它与普通共价键无区别。提供孤对电子的原子称为供体，接受电子的原子称为受体。

Examples include the formation of the ammonium ion (NH₄⁺) when ammonia donates its lone pair to H⁺, and the reaction of BF₃ with F⁻ to give BF₄⁻. In transition metal complexes, ligands donate lone pairs to the central metal ion through coordinate bonds.

例子包括氨分子向 H⁺ 提供孤对电子形成铵根离子 (NH₄⁺)，以及 BF₃ 与 F⁻ 反应生成 BF₄⁻。在过渡金属配合物中，配体通过配位键向中心金属离子提供孤对电子。

The arrow (→) is used to represent a dative bond when showing the structure, pointing from the donor to the acceptor. However, once formed, all bonds in NH₄⁺ are equivalent.

绘制结构时用箭头 (→) 表示配位键，方向从供体指向受体。但一旦形成，NH₄⁺ 中的所有键都等效。

7. Sigma and Pi Bonds | σ 键与 π 键

Covalent bonds can be classified by the symmetry of the orbital overlap. A sigma (σ) bond results from head‑on overlap of orbitals along the internuclear axis. A pi (π) bond results from the sideways overlap of p‑orbitals above and below the axis.

共价键可根据轨道重叠的对称性分类。σ 键源于轨道沿核轴线的“头对头”重叠。π 键源于 p 轨道在轴线上方和下方的“肩并肩”重叠。

All single bonds are σ bonds. A double bond consists of one σ and one π bond; a triple bond consists of one σ and two π bonds. The σ bond is the first bond formed and is stronger than a π bond because of greater orbital overlap.

所有单键均为 σ 键。双键由一个 σ 键和一个 π 键组成；三键由一个 σ 键和两个 π 键组成。σ 键首先形成且因轨道重叠更大而比 π 键更强。

The presence of a π bond restricts rotation around the bond axis, leading to geometric (cis‑trans) isomerism in alkenes. This is a crucial structural concept in organic chemistry.

π 键的存在限制了绕键轴的旋转，导致烯烃出现顺反异构（几何异构）。这是有机化学中至关重要的结构概念。

8. Delocalised π Bonds | 离域 π 键

In some molecules, π electrons are not confined to a single pair of atoms but are spread over several atoms. This delocalisation provides extra stability. The classic examples are benzene (C₆H₆) and the carbonate ion (CO₃²⁻).

在某些分子中，π 电子并不局限在一对原子之间，而是分布在整个分子骨架上。这种离域作用提供额外的稳定性。经典例子有苯 (C₆H₆) 和碳酸根离子 (CO₃²⁻)。

In benzene, each carbon is sp² hybridised. The remaining p‑orbital on each carbon overlaps side‑on with its neighbours, forming a continuous π cloud above and below the ring. All C–C bonds are identical and intermediate in length between single and double bonds.

在苯中，每个碳均为 sp² 杂化。每个碳上剩余的 p 轨道与相邻碳侧向重叠，在环上下方形成连续的 π 电子云。所有 C–C 键等价，键长介于单键与双键之间。

In CO₃²⁻, resonance structures show the π electrons spread over three equivalent C–O bonds. The delocalised system lowers the overall energy, making the ion more stable than any individual resonance form.

在 CO₃²⁻ 中，共振结构显示 π 电子分布于三条等价的 C–O 键上。离域体系降低了整体能量，使离子比任一单独共振形式更稳定。

9. Valence‑Shell Electron‑Pair Repulsion (VSEPR) Theory | 价层电子对互斥理论

VSEPR theory predicts molecular shapes by assuming that electron pairs around a central atom repel each other and arrange themselves as far apart as possible. Both bonding pairs and lone pairs must be considered, but lone‑pair–lone‑pair repulsion is greater than lone‑pair–bonding‑pair repulsion, which is greater than bonding‑pair–bonding‑pair repulsion.

VSEPR 理论通过假设中心原子周围的电子对互相排斥并尽可能远离，来预测分子形状。需同时考虑键对和孤对电子，但孤对‑孤对排斥力 > 孤对‑键对排斥力 > 键对‑键对排斥力。

Key geometries:

2 electron pairs → linear, 180° (e.g. BeCl₂)
3 electron pairs → trigonal planar, 120° (e.g. BF₃)
4 electron pairs → tetrahedral, 109.5° (e.g. CH₄)
4 electron pairs with 1 lone pair → trigonal pyramidal, ~107° (e.g. NH₃)
4 electron pairs with 2 lone pairs → bent, ~104.5° (e.g. H₂O)
5 electron pairs → trigonal bipyramidal (e.g. PCl₅)
6 electron pairs → octahedral, 90° (e.g. SF₆)

关键构型：

2 个电子对 → 直线形，180°（如 BeCl₂）
3 个电子对 → 平面三角形，120°（如 BF₃）
4 个电子对 → 四面体形，109.5°（如 CH₄）
4 个电子对含 1 个孤对 → 三角锥形，约 107°（如 NH₃）
4 个电子对含 2 个孤对 → 角形，约 104.5°（如 H₂O）
5 个电子对 → 三角双锥形（如 PCl₅）
6 个电子对 → 八面体形，90°（如 SF₆）

The reduction in bond angles with increasing lone pairs is explained by the stronger repulsions of the more diffuse lone pairs. This concept is vital for predicting polarity and intermolecular forces.

随孤对电子增加键角减小的原因，是孤对电子云更弥散、排斥力更强。这一概念对预测极性和分子间作用力至关重要。

10. Hybridisation – sp, sp², sp³ | 杂化 – sp, sp², sp³

Hybridisation explains the observed shapes and bond angles by mixing atomic orbitals to form new, identical hybrid orbitals. In carbon‑based molecules, three types are essential: sp³ (four identical orbitals, tetrahedral, as in CH₄), sp² (three orbitals, trigonal planar, as in C₂H₄), and sp (two orbitals, linear, as in C₂H₂).

杂化理论通过混合原子轨道形成新的、等价的杂化轨道来解释观察到的分子形状和键角。在含碳分子中，三种杂化至为关键：sp³（四个等价轨道，四面体形，如 CH₄）、sp²（三个轨道，平面三角形，如 C₂H₄）、sp（两个轨道，直线形，如 C₂H₂）。

In sp³ hybridisation, one s and three p orbitals combine to give four sp³ orbitals, each with 25% s‑character. In sp², one s and two p orbitals give three sp² orbitals (33% s‑character) and one unhybridised p‑orbital for π bonding. In sp, one s and one p orbital give two sp orbitals (50% s‑character) and two unhybridised p‑orbitals.

sp³ 杂化中，一个 s 轨道与三个 p 轨道组合成四个 sp³ 轨道，每个含 25% s 成分。sp² 中，一个 s 与两个 p 形成三个 sp² 轨道（33% s 成分）和一个未杂化的 p 轨道用于 π 键。sp 中，一个 s 与一个 p 形成两个 sp 轨道（50% s 成分）和两个未杂化 p 轨道。

A higher s‑character means the hybrid orbital is closer to the nucleus, giving shorter, stronger bonds. This contributes to the acidity of terminal alkynes (sp C–H bond), compared to alkenes (sp²) and alkanes (sp³).

较高的 s 成分意味着杂化轨道更靠近原子核，键更短、更强。这解释了末端炔烃 (sp C–H) 相较于烯烃 (sp²) 和烷烃 (sp³) 的酸性更强的原因。

11. Bonding in Organic Molecules | 有机分子中的键合

The covalent bonding framework of organic molecules determines structure, reactivity, and physical properties. Functional groups contain specific bonds: alkanes possess only σ bonds allowing free rotation; alkenes have a rigid π bond causing cis‑trans isomerism; alkynes feature two π bonds giving a linear geometry.

有机分子的共价键骨架决定其结构、反应活性与物理性质。官能团含有特定键型：烷烃仅含 σ 键，允许自由旋转；烯烃具有刚性的 π 键，导致顺反异构；炔烃具有两个 π 键，呈直线形几何。

Alcohols, carbonyl compounds, carboxylic acids, and amines all feature polar bonds (C–O, C=O, C–N) that are sites for nucleophilic attack or hydrogen bonding. Understanding the electronic distribution in these bonds enables prediction of mechanisms.

醇、羰基化合物、羧酸和胺均含有极性键（C–O, C=O, C–N），这些是亲核进攻或氢键形成的位点。理解这些键的电子分布能够预测反应机理。

Resonance in amides (R–CO–NH₂) restricts rotation about the C–N bond, giving it partial double‑bond character. This affects protein folding and base‑pairing in DNA.

酰胺 (R–CO–NH₂) 中的共振效应限制了 C–N 键的旋转，使其具有部分双键性质。这在蛋白质折叠和 DNA 碱基配对中起到关键作用。

12. Summary and Common Exam Questions | 总结与常见考题

Covalent bonding underpins molecular chemistry. Master the drawing of Lewis structures, recognise octet‑rule exceptions, and use VSEPR to deduce shape. Be able to describe σ and π bonds, explain hybridisation, and predict polarity. Delocalised π systems and dative covalent bonds complete the essential toolset.

共价键是分子化学的基础。掌握路易斯结构的画法，识别八隅规则例外，运用 VSEPR 推导形状；能描述 σ 与 π 键，解释杂化并预测极性。离域 π 体系和配位共价键构成完整的知识工具包。

In CCEA exams, you may be asked to draw the shape and state the bond angle of a molecule, explain why a molecule is polar or non‑polar, compare bond lengths, or identify hybridisation states of carbons in a given structure. Practise with past‑paper questions on bonding regularly.

在 CCEA 考试中，你可能需要画出分子形状并注明键角、解释分子为何具有极性或非极性、比较键长，或鉴别给定结构中碳原子的杂化状态。请定期练习往年考题中的化学键部分。

A thorough command of covalent bonding will also illuminate topics such as organic mechanisms, structure determination, and energetics. Treat bonding as the language of chemistry and your revision will reap rewards.

扎实掌握共价键还将为有机机理、结构测定、能量学等专题提供清晰思路。把化学键视为化学的语言，你的复习必将硕果累累。

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Covalent Bonding for CCEA A-Level Chemistry | A-Level CCEA 化学：共价键 考点精讲