Covalent Bonding in A-Level OCR Chemistry | A-Level OCR 化学：共价键考点精讲

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

Covalent bonding is a core concept in A-Level OCR Chemistry, forming the foundation for understanding molecular structure, physical properties, and the reactivity of non‑metal compounds. In covalent bonding, atoms achieve a more stable electron configuration by sharing pairs of electrons, and the interplay between shared electrons, bond polarity, and molecular shape determines virtually every chemical behaviour you will study. This article unpacks the key points you need to master, from dot‑and‑cross diagrams and dative bonds to bond energies, VSEPR theory, and giant covalent lattices.

共价键是 A‑Level OCR 化学中的核心概念，是理解分子结构、物理性质及非金属化合物反应性的基础。在共价键中，原子通过共享电子对而获得更稳定的电子构型，而共享电子、键极性与分子形状之间的相互作用，几乎决定着你将学到的每一种化学行为。本文梳理你需要掌握的关键考点，从点叉图、配位键到键能、价层电子对互斥理论和巨型共价晶格，逐一精讲。

1. The Nature of Covalent Bonding | 共价键的本质

A covalent bond is the electrostatic attraction between the nuclei of two atoms and the shared pair of electrons localised between them. Each atom contributes at least one electron to the shared pair, and the overlapping of atomic orbitals allows the electron density to concentrate in the internuclear region, pulling the positively charged nuclei together.

共价键是两个原子核与定域在它们之间的共享电子对之间的静电吸引力。每个原子至少提供一个电子给共享电子对，原子轨道的重叠使电子密度集中在核间区域，从而将带正电的原子核拉在一起。

Atoms form covalent bonds to attain a full outer shell of electrons, usually an octet (8 electrons) for Period 2 elements, although elements from Period 3 onwards can expand their octet by using low‑lying d orbitals. Hydrogen is an exception, needing only 2 electrons to achieve the stable helium configuration.

原子形成共价键是为了达到满的外层电子结构，通常是第二周期元素的八隅体（8 个电子），但第三周期及之后的元素可利用低能 d 轨道扩展八隅体。氢是一个例外，仅需 2 个电子便可达到氦的稳定结构。

Key to the covalent model is that the shared electrons are attracted to both nuclei simultaneously, which lowers the overall energy of the system compared with the isolated atoms, making the molecule more stable.

共价模型的关键在于共享电子同时被两个核吸引，与孤立原子相比，降低了体系的总能量，使分子更加稳定。

2. Lewis Structures and Dot‑and‑Cross Diagrams | 路易斯结构与点叉图

Lewis structures show how valence electrons are arranged around atoms in a molecule. Dots and crosses represent electrons from different atoms, making it easy to visualise which electrons originate from which atom. Single bonds contain one shared pair, double bonds contain two shared pairs, and triple bonds contain three shared pairs.

路易斯结构展示分子中价电子在原子周围的排布方式。点与叉代表来自不同原子的电子，便于看清电子的来源。单键含有一对共享电子，双键含有两对共享电子，三键含有三对共享电子。

Lone pairs are pairs of valence electrons that are not involved in bonding. They occupy space around the central atom and play a critical role in determining molecular geometry because they repel bonding pairs more strongly than bonding pairs repel each other.

孤电子对是不参与成键的价电子对。它们占据中心原子周围的空间，并且对分子几何构型起关键作用，因为孤电子对之间的排斥力强于键对之间的排斥力。

When drawing Lewis structures, you must count total valence electrons, distribute them to satisfy the octet rule (or expanded octet where applicable), minimise formal charges, and place multiple bonds if needed. Formal charge = valence electrons – (number of non‑bonding electrons + ½ number of bonding electrons). A structure with formal charges close to zero is usually the most stable.

绘制路易斯结构时需要计算总价电子数，分配电子以八隅体规则（或适当时用扩展八隅体），使形式电荷最小化，必要时引入多重键。形式电荷 = 价电子 –（非键电子数 + ½ 键合电子数）。形式电荷接近零的结构通常最稳定。

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

A dative covalent bond, also called a coordinate bond, forms when both electrons in the shared pair come from the same atom. Once formed, a dative bond is indistinguishable in strength and length from an ordinary covalent bond. The donor atom must have a lone pair, and the acceptor atom must be electron‑deficient, having an empty orbital available to accept the electron pair.

配位共价键又称配位键，是指共享电子对的两个电子均来自同一个原子。一旦形成，配位键的强度与键长与普通共价键不可区分。供体原子必须具有孤电子对，受体原子必须缺电子并具有空轨道来接受电子对。

Classic examples include the ammonium ion NH₄⁺, formed when ammonia donates its lone pair on nitrogen to a H⁺ ion, and the hydronium ion H₃O⁺ formed from water and H⁺. Another important example is aluminium chloride, Al₂Cl₆, where chlorine atoms donate lone pairs to aluminium atoms to complete the dimeric structure.

经典例子包括铵离子 NH₄⁺（氨中氮的孤电子对贡献给 H⁺）和水合氢离子 H₃O⁺（水与 H⁺ 形成）。另一个重要例子是氯化铝 Al₂Cl₆，其中氯原子提供孤电子对给铝原子以完成二聚结构。

4. Electronegativity and Bond Polarity | 电负性与键极性

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. Pauling’s scale is the most common, with fluorine assigned the highest value of 4.0. Across a period, electronegativity increases due to greater nuclear charge; down a group, it decreases due to increased atomic radius and shielding.

电负性是原子在共价键中吸引键合电子的能力。最常用的是鲍林标度，氟的最高值为 4.0。同一周期从左到右电负性增大（核电荷增加），同一族从上到下电负性减小（原子半径增大、屏蔽效应增强）。

A covalent bond between identical atoms is non‑polar because the electrons are shared equally. When atoms of different electronegativity bond together, the electron density is skewed towards the more electronegative atom, creating a polar bond with partial charges δ⁺ and δ⁻. A bond is considered ionic if the electronegativity difference is very large (typically greater than 1.7), but the boundary is not sharp.

相同原子间的共价键是非极性的，因为电子均等共享。当不同电负性的原子成键时，电子密度偏向电负性更大的原子，产生极性键，形成部分电荷 δ⁺ 和 δ⁻。当电负性差值很大（通常大于 1.7）时，键被视为离子键，但界限并不绝对。

Molecular polarity depends on both bond polarity and molecular geometry. A molecule can have polar bonds but be non‑polar overall if the shape is symmetric, like tetrahedral CCl₄ or linear CO₂, because the bond dipoles cancel out.

分子的极性取决于键的极性和分子几何形状。如果形状对称，如四面体 CCl₄ 或直线形 CO₂，即使含极性键，键的偶极矩相互抵消，分子整体仍为非极性。

5. σ Bonds and π Bonds | σ 键与 π 键

Covalent bonds can be classified by how atomic orbitals overlap. A σ (sigma) bond results from the head‑on overlap of orbitals along the internuclear axis. The electron density is concentrated directly between the two nuclei, allowing free rotation of atoms around the bond. All single bonds are σ bonds.

共价键可按原子轨道重叠方式分类。σ（西格玛）键来自沿核间轴的轨道头对头重叠，电子密度集中在两核之间，允许原子绕键自由旋转。所有单键均为 σ 键。

A π (pi) bond is formed by the sideways overlap of adjacent p orbitals above and below the plane of the atoms. Electron density lies above and below the internuclear axis, and the bond does not permit rotation because rotating would break the parallel alignment of p orbitals. Double bonds consist of one σ and one π bond, and triple bonds consist of one σ and two π bonds.

π（派）键由相邻 p 轨道在原子平面上下方侧向重叠形成。电子密度分布在核间轴的上方和下方，且该键不允许旋转，因为旋转会破坏 p 轨道的平行排列。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。

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

Bond length is the average distance between the nuclei of two bonded atoms. Multiple bonds are shorter than single bonds between the same pair of elements because additional electron pairs pull the nuclei more tightly together. Bond energy (enthalpy) is the energy required to break one mole of a given covalent bond in the gaseous state, averaged over a range of compounds for average bond energies.

键长是成键两原子核之间的平均距离。同一对元素之间的多重键比单键短，因为额外的电子对将原子核拉得更近。键能（焓）是气态下断裂 1 摩尔特定共价键所需的能量，平均键能则是在一系列化合物中的平均值。

Shorter bonds generally have higher bond energies. For example, C≡C (835 kJ mol⁻¹) > C=C (612 kJ mol⁻¹) > C–C (347 kJ mol⁻¹). Bond energy values are used in Hess cycles to calculate enthalpy changes of reactions and in discussing bond reactivity.

键越短，通常键能越高。例如 C≡C（835 kJ mol⁻¹）＞ C=C（612 kJ mol⁻¹）＞ C–C（347 kJ mol⁻¹）。键能数值可用于赫斯循环计算反应的焓变，以及讨论键的反应活性。

During a reaction, bonds in reactants are broken (endothermic) and new bonds form in products (exothermic). The enthalpy change of reaction can be estimated as Σ(bond energies broken) – Σ(bond energies made). This method works best for gaseous reactions.

反应中，反应物中的键断裂（吸热），产物中新键生成（放热）。反应的焓变可估算为 Σ（断裂的键能）− Σ（生成的键能）。此方法最适合气相反应。

7. VSEPR Theory and Molecular Shapes | 价层电子对互斥理论与分子形状

Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom arrange themselves as far apart as possible to minimise repulsion. Lone pair–lone pair repulsion > lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion. The presence of lone pairs therefore reduces bond angles from the ideal geometry.

价层电子对互斥（VSEPR）理论指出，中心原子周围的电子对会尽可能地远离，使排斥力最小化。孤电子对−孤电子对排斥＞孤电子对−键对排斥＞键对−键对排斥。因此孤电子对的存在会使键角偏离理想值。

2 bonding pairs, 0 lone pairs: linear, 180°. Example: BeCl₂, CO₂.
3 bonding pairs, 0 lone pairs: trigonal planar, 120°. Example: BF₃.
3 bonding pairs, 1 lone pair: bent (V‑shaped), <120°. Example: SO₂.
4 bonding pairs, 0 lone pairs: tetrahedral, 109.5°. Example: CH₄.
4 bonding pairs, 1 lone pair: trigonal pyramidal, ~107°. Example: NH₃.
4 bonding pairs, 2 lone pairs: bent (V‑shaped), ~104.5°. Example: H₂O.
5 bonding pairs: trigonal bipyramidal, 90° and 120°. Example: PCl₅.
6 bonding pairs: octahedral, 90°. Example: SF₆.

2 对键电子，0 对孤电子：直线形，180°，如 BeCl₂、CO₂。
3 对键电子，0 对孤电子：平面三角形，120°，如 BF₃。
3 对键电子，1 对孤电子：V 形，<120°，如 SO₂。
4 对键电子，0 对孤电子：四面体，109.5°，如 CH₄。
4 对键电子，1 对孤电子：三角锥形，约 107°，如 NH₃。
4 对键电子，2 对孤电子：V 形，约 104.5°，如 H₂O。
5 对键电子：三角双锥形，90° 和 120°，如 PCl₅。
6 对键电子：八面体，90°，如 SF₆。

To determine shape, count regions of electron density (bonds + lone pairs), decide the electron‑pair geometry, then describe the molecular shape considering positions of atoms only. Always quote the expected bond angle and justify any deviation.

确定形状时，先数电子区域（键 + 孤电子对）以确定电子对几何构型，然后仅考虑原子位置描述分子形状。始终给出预期键角并解释任何偏离。

8. Bond Polarity, Dipole Moments and Intermolecular Forces | 键极性、偶极矩与分子间作用力

A dipole moment arises when a bond or molecule has a separation of positive and negative charge. The overall dipole moment of a molecule is the vector sum of all bond dipoles. Polar molecules tend to have higher boiling points than non‑polar molecules of similar size because they experience permanent dipole–permanent dipole interactions, along with London dispersion forces.

当键或分子具有正负电荷分离时便产生偶极矩。分子的总偶极矩是所有键偶极矩的矢量和。极性分子的沸点往往高于尺寸相近的非极性分子，因为它们除了伦敦色散力之外，还存在永久偶极−永久偶极相互作用。

Hydrogen bonding is a special strong type of dipole–dipole interaction occurring when hydrogen is covalently bonded to very electronegative atoms with lone pairs—specifically nitrogen, oxygen or fluorine. It explains the unexpectedly high boiling points of H₂O, NH₃ and HF, and is crucial in the structure of DNA and proteins.

氢键是一种特殊的强偶极−偶极相互作用，当氢与具有孤电子对的强电负性原子（氮、氧或氟）共价结合时产生。它能解释 H₂O、NH₃ 和 HF 反常的高沸点，并对 DNA 和蛋白质结构至关重要。

9. Giant Covalent Structures | 巨型共价结构

Some elements and compounds form giant covalent lattices, also called network solids, in which atoms are held together by strong covalent bonds extending in all directions. The bonding is directional and the structures have very high melting points and are generally hard.

某些元素和化合物形成巨型共价晶格（又称网络固体），原子通过向所有方向延伸的强共价键连接。键合具有方向性，这类结构熔点极高且通常坚硬。

Diamond: each carbon atom forms four σ bonds to four other carbon atoms in a tetrahedral arrangement (sp³ hybridised). The rigid 3‑D network makes diamond the hardest natural substance, an electrical insulator (all electrons localised in bonds), and a good thermal conductor.

金刚石：每个碳原子以 sp³ 杂化与其他四个碳原子形成四个 σ 键，呈四面体排列。刚性的三维网络使金刚石成为天然最硬物质、电绝缘体（所有电子定域在键中）、热的良导体。

Graphite: carbon atoms are sp² hybridised, arranged in planar hexagonal layers with delocalised π electrons above and below the planes. The layers can slide over each other due to weak van der Waals forces, making graphite soft and slippery, and an electrical conductor parallel to the layers.

石墨：碳原子为 sp² 杂化，排列成平面六边形层，层上下方有离域 π 电子。层间通过微弱的范德华力相互作用，可相对滑动，使石墨柔软润滑，并可平行于层面导电。

Graphene: a single layer of graphite, only one atom thick. It is incredibly strong for its mass, transparent, and an excellent electrical and thermal conductor. Its properties arise from the 2‑D honeycomb array of carbon atoms held by strong σ bonds and delocalised π electrons.

石墨烯：单层石墨，仅一个原子厚。它质量轻但强度极高，透明，是优良的电和热的导体，其性质源于碳原子组成的二维蜂窝排列及强的 σ 键与离域 π 电子。

Silicon dioxide (SiO₂): a tetrahedral network where each silicon is bonded to four oxygens and each oxygen to two silicons. It has a high melting point, is hard, and an electrical insulator, similar in structure to diamond but with Si–O bonds.

二氧化硅（SiO₂）：四面体网络，每个硅与四个氧成键，每个氧与两个硅成键。熔点高、坚硬、电绝缘，结构与金刚石类似但为 Si–O 键。

10. Covalent Character in Ionic Compounds | 离子化合物中的共价特性

No bond is 100% ionic or covalent. When a small, highly charged cation approaches a large, easily polarisable anion, the cation polarises the anion’s electron cloud, drawing electron density back into the region between the nuclei. This introduces partial covalent character into what would otherwise be a purely ionic model.

没有任何键是 100% 离子键或共价键。当小而高电荷的阳离子靠近易极化的大阴离子时，阳离子极化阴离子的电子云，将电子密度拉回至核间区域。这就为纯离子模型引入了部分共价特性。

Fajans’ rules summarise the factors that increase covalent character: (1) small, highly charged cation, (2) large, highly charged anion, (3) cation with non‑noble gas electron configuration (e.g. transition metals). The greater the polarisation, the more the compound’s properties deviate from purely ionic expectations—e.g., lower melting point, increased solubility in organic solvents, and more directional bonding.

法扬斯规则总结了增强共价特性的因素：（1）小且高电荷阳离子，（2）大且高电荷阴离子，（3）非稀有气体电子构型的阳离子（如过渡金属）。极化程度越大，化合物性质越偏离纯离子预期，例如熔点降低、在有机溶剂中溶解度增加、成键更具方向性。

11. Common Exam Pitfalls and Tips | 常见考试失分点与技巧

Students often lose marks by forgetting to include lone pairs on Lewis structures or failing to place correct formal charges. When drawing shapes, always show the 3‑D representation using wedges and dashes, accurately state bond angles, and name the shape correctly. If there are lone pairs, mention them and explain their effect on bond angles.

考生常因忘记绘制路易斯结构中的孤电子对或未标出正确的形式电荷而失分。画形状时，务必使用楔形线和虚线展示三维结构，准确标注键角，并正确命名形状。如有孤电子对，须提及它并解释其对键角的影响。

In bond energy calculations, carefully count the number of each bond type in reactants and products. Sum bond energies for bonds broken (reactants) and subtract sum of bond energies for bonds made (products). Remember that average bond energies give an estimate and may differ from actual values for specific molecules.

在键能计算中，要仔细计数反应物和产物中每种键的数量。将断裂键的键能总和（反应物）减去生成键的键能总和（产物）。注意平均键能只是估算值，可能与特定分子的实际值有出入。

When comparing giant covalent substances, link structure to properties: bonding type, presence of delocalised electrons, strength of intermolecular forces between layers or chains. Explain clearly why diamond is an insulator while graphite conducts electricity.

比较巨型共价物质时，应将结构与性质联系起来：键的类型、是否存在离域电子、层或链间分子间力的强度等。要清楚解释为什么金刚石是绝缘体而石墨能导电。

Finally, do not confuse electronegativity with electron affinity. Electronegativity refers to an atom in a bond, while electron affinity refers to an isolated atom gaining an electron.

最后，不要混淆电负性与电子亲和能。电负性指成键中原子的性质，而电子亲和能指孤立原子获得电子的能力。

12. Summary of Key Points for Revision | 复习要点总结

Covalent bonds: shared electron pairs, electrostatic attraction between nuclei and shared electrons.
Dative bonds: both electrons from one atom, common in transition metal complexes and ions like NH₄⁺.
Electronegativity controls bond polarity; polar bonds + asymmetric shape = polar molecule.
σ bonds allow rotation; π bonds restrict it. Double bond = 1σ + 1π; triple bond = 1σ + 2π.
Bond length and bond energy: multiple bonds are shorter and stronger.
VSEPR determines molecular shape: lone pairs reduce bond angles.
Giant covalent structures: diamond (3‑D, insulator), graphite (layered, conductor), graphene (single layer, exceptionally strong and conductive), SiO₂ (tetrahedral network, insulator).
Ionic compounds have some covalent character due to polarisation (Fajans’ rules).
Examiners love shape and polarity questions, bond energy calculations, and comparison of giant structures.

共价键：共享电子对，原子核与共享电子之间的静电吸引。
配位键：两电子均来自一个原子，常见于过渡金属配合物及 NH₄⁺ 等离子。
电负性决定键的极性；极性键 + 不对称形状 = 极性分子。
σ 键可旋转；π 键限制旋转。双键 = 1σ + 1π；三键 = 1σ + 2π。
键长与键能：多重键更短、更强。
VSEPR 决定分子形状：孤电子对使键角减小。
巨型共价结构：金刚石（三维，绝缘体），石墨（层状，导电），石墨烯（单层，超强且导电），SiO₂（四面体网络，绝缘体）。
离子化合物因极化而具有部分共价特性（法扬斯规则）。
考官青睐形状与极性题、键能计算题及巨型结构比较题。

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Covalent Bonding in A-Level OCR Chemistry | A-Level OCR 化学：共价键考点精讲