Covalent Bonding: Essential Exam Points for IB WJEC Chemistry | 共价键考点精讲

📚 Covalent Bonding: Essential Exam Points for IB WJEC Chemistry | 共价键考点精讲

Covalent bonding is the glue that holds most molecular substances and giant covalent structures together. Mastering this topic is essential for both IB Standard and Higher Level Chemistry, as well as for WJEC specifications. This article consolidates the key concepts, from Lewis diagrams and VSEPR theory to sigma/pi bonding and hybridisation, all examined through typical exam-style reasoning. Use it as a revision checklist and see your understanding – and your grades – strengthen like a shared pair of electrons.

共价键是把大多数分子物质和巨型共价结构维系在一起的“胶水”。无论是IB标准水平还是高级水平化学，还是WJEC考试大纲，掌握共价键都至关重要。本文整合了从路易斯结构、VSEPR理论到σ/π键和杂化等核心概念，全部采用考试常见的逻辑进行剖析。把它当作一份复习清单，让你的理解和成绩都像共用电子对一样牢固地结合起来。

1. The Shared Pair: Definition of a Covalent Bond | 共用电子对：共价键的定义

A covalent bond is formed by the electrostatic attraction between the nuclei of two atoms and a shared pair of electrons that occupy a molecular orbital. Electrons are not transferred; they are co-owned. This occurs primarily between non-metal atoms with similar or identical electronegativities. In the simplest case, each atom contributes one electron to the bond, giving both atoms access to a stable electron configuration, typically an octet.

共价键是由两个原子的原子核与一对占据分子轨道的共用电子之间的静电引力形成的。电子没有被转移，而是被共享。这主要发生在电负性相近或相同的非金属原子之间。最简单的情形是每个原子贡献一个电子形成键，使两个原子都能获得稳定的电子排布，通常是八隅体。

Bonding pairs are often represented by a single line (H—H, Cl—Cl) or by a pair of dots in Lewis diagrams. Unshared valence electrons are called lone pairs. The number of covalent bonds an atom can form is usually determined by how many electrons it needs to complete its outer shell: carbon forms four bonds, nitrogen three, oxygen two, and the halogens one. In IB and WJEC exams, you must be able to draw clear Lewis structures and justify the bonding using the concept of orbital overlap.

成键电子对通常用一条短线（H—H、Cl—Cl）或路易斯图中的一对点来表示。未共享的价电子称为孤对电子。一个原子能形成的共价键数目通常取决于它需要多少电子来填满最外层：碳形成四键，氮三键，氧两键，卤素一键。在IB和WJEC考试中，你必须能画出清晰的路易斯结构并用轨道重叠的概念解释成键。

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

Drawing correct Lewis structures is a fundamental skill. Start by counting all valence electrons. Place the least electronegative atom (except hydrogen) in the centre. Connect outer atoms with single bonds, then distribute remaining electrons as lone pairs to satisfy octets. If there are not enough electrons, introduce multiple bonds. The octet rule is a guideline, not an absolute law; species such as BF₃, PCl₅ and SF₆ are stable exceptions that IB and WJEC candidates must recognise.

画出正确的路易斯结构是一项基本技能。先计算所有价电子数。将电负性最小的原子（氢除外）放在中心。用单键连接外围原子，然后将剩余电子作为孤对电子分配，使各原子满足八隅体。如果电子不够，就要引入多重键。八隅体规则是指导原则而非绝对法则；像BF₃、PCl₅和SF₆这类稳定的例外情况，IB和WJEC考生必须能识别。

Formal charge is used to determine the most plausible Lewis structure, especially for ions or molecules with resonance. The formal charge = (valence e⁻ of the free atom) – (number of bonds) – (number of unshared electrons). Structures in which formal charges are closest to zero are preferred. For example, the sulphate ion SO₄²⁻ shows S—O bonds with formal charges that can be minimised through resonance, an idea explored further in higher-level papers.

形式电荷用来判断最合理的路易斯结构，尤其对于离子或有共振的分子。形式电荷 = (自由原子的价电子数) – (键的数目) – (未共享电子数)。形式电荷最接近于零的结构占优。例如硫酸根离子SO₄²⁻显示的S—O键可通过共振使形式电荷降到最低，这一概念将在高级水平的试卷中进一步考查。

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3. Bond Polarity and Electronegativity | 键的极性与电负性

A covalent bond is polar if the two atoms involved have different electronegativities. The more electronegative atom pulls the bonding electrons towards itself, creating a dipole: one end is slightly negative (δ⁻) and the other slightly positive (δ⁺). Electronegativity values follow the Pauling scale. In the periodic table, electronegativity increases across a period and decreases down a group. Fluorine (4.0) is the most electronegative element.

如果成键的两个原子电负性不同，共价键就是极性的。电负性较大的原子会把成键电子拉向自己，产生一个偶极：一端略带负电（δ⁻），另一端略带正电（δ⁺）。电负性值遵循鲍林标度。在周期表中，电负性同周期从左到右递增，同族从上到下递减。氟（4.0）是电负性最强的元素。

Key thresholds: a difference of 0.0 → 0.4 is considered non-polar covalent, 0.5 → 1.7 is polar covalent, and above 1.7 the bond tends to be ionic. However, this is a continuum; no bond is 100% ionic. IB exam questions often ask you to predict bond type from electronegativity differences and to explain molecular polarity, while WJEC papers may link this to physical properties like solubility and boiling point.

关键阈值：差值0.0–0.4为非极性共价键，0.5–1.7为极性共价键，大于1.7则倾向于离子键。但这其实是一个渐变过程，没有100%的离子键。IB试题常要求根据电负性差预测键型并解释分子的极性，WJEC试卷则可能将极性与溶解度、沸点等物理性质联系起来。

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4. Molecular Polarity: Sum of Dipole Vectors | 分子极性：偶极矢量和

A molecule is polar only if it has polar bonds that do not cancel out due to symmetry. The net dipole moment is the vector sum of individual bond dipoles. Carbon dioxide, O=C=O, has two polar C=O bonds but is linear and symmetrical, so the dipoles cancel and the molecule is non-polar. Water, H₂O, is bent; the O—H bond dipoles add to give a net dipole, making water a polar molecule.

一个分子只有在含有极性键且这些极性键的偶极因对称性不抵消时，才具有极性。净偶极矩是各键偶极的矢量和。二氧化碳O=C=O有两个极性C=O键，但分子呈直线形且对称，偶极互相抵消，分子为非极性。水分子H₂O呈折线形，O—H键偶极相加产生净偶极，使水成为极性分子。

In IB assessments, you must deduce molecular polarity from shape and bond polarity, especially for planar triangular, tetrahedral, trigonal bipyramidal and octahedral geometries. WJEC will often expect you to label δ⁺ and δ⁻ on diagrams and predict solubility behaviour. A common pitfall is assuming that any molecule with polar bonds is automatically polar; always check three-dimensional shape.

在IB评估中，你必须从分子形状和键的极性推断分子极性，特别是平面三角形、四面体、三角双锥和八面体构型。WJEC通常要求你在图上标出δ⁺和δ⁻，并预测溶解行为。常见的误区是认为任何含极性键的分子都自动是极性分子；一定要检查三维形状。

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5. Dative (Coordinate) Covalent Bonding | 配位（配价）共价键

A dative covalent bond, also called a coordinate bond, is a covalent bond in which both electrons come from the same atom. The atom that donates the electron pair is the donor; the atom that accepts is the acceptor. Once formed, a dative bond is indistinguishable from an ordinary covalent bond. Examples include the ammonium ion NH₄⁺, where a nitrogen lone pair on NH₃ forms a bond with H⁺, and the hydronium ion H₃O⁺. Transition metal complexes also feature dative bonds from ligands to the central metal ion.

配位共价键又称配价键，是一种共价键，其中两个电子都来自同一个原子。提供电子对的原子是给体，接受电子对的是受体。一旦形成，配位键与普通共价键无法区分。例子包括铵根离子NH₄⁺，其中NH₃中氮原子的孤对电子与H⁺成键，以及水合氢离子H₃O⁺。过渡金属配合物中也有配体向中心金属离子提供孤对电子形成的配位键。

In Lewis diagrams, a dative bond is sometimes drawn as an arrow pointing from the donor to the acceptor, though a single line is also accepted once the bond is formed. Drawing the arrow can help earn marks in mechanisms where lone pair donation occurs, such as in the formation of BF₃—NH₃ adducts. Both IB and WJEC specifications require students to identify and draw dative bonds correctly.

在路易斯结构中，配位键有时被画成从给体指向受体的箭头，不过一旦成键也可以用单线表示。在有孤对电子参与的反应机理中，画出箭头有助于得分，例如BF₃—NH₃加合物的形成。IB和WJEC大纲都要求学生能正确识别并画出配位键。

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

Bond length is the equilibrium distance between the nuclei of two bonded atoms. It decreases with increasing bond order: triple bonds are shorter than double bonds, which are shorter than single bonds. Bond energy (bond enthalpy) is the energy required to break one mole of covalent bonds in the gaseous state. Triple bonds are the strongest, followed by double and single bonds. These data help explain the reactivity of molecules; for example, the C=C bond in ethene is not simply twice as strong as a C—C bond because the π bond is weaker than the σ bond.

键长是两个成键原子核之间的平衡距离。键长随键级的增大而缩短：三键比双键短，双键又比单键短。键能（键焓）是破坏气体状态下1摩尔共价键所需的能量。三键最强，其次是双键和单键。这些数据有助于解释分子的反应性；例如乙烯中的C=C键并非简单是C—C键的两倍强度，因为π键比σ键弱。

A useful exam skill is to compare bond energies and lengths to deduce the effect of resonance. In a delocalised system such as benzene, all C—C bonds are of equal length, intermediate between a single and a double bond. IB problem sets often give a table of experimental bond lengths and ask you to infer bond order. WJEC may provide bond energy data to calculate enthalpy changes or to discuss thermodynamic stability.

一个有用的考试技巧是比较键能和键长来推断共振效应。在像苯这样的离域体系中，所有碳碳键的长度都相等，介于单键和双键之间。IB习题常给出一张实验键长的表格，要求推断键级。WJEC可能会提供键能数据，用于计算焓变或讨论热力学稳定性。

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7. VSEPR Theory and Molecular Shapes | VSEPR理论与分子形状

Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom arrange themselves to be as far apart as possible to minimise repulsion. The shape of a molecule is determined by the number of bonding pairs and lone pairs. Lone pairs exert greater repulsion than bonding pairs, often compressing bond angles. Key shapes for IB HL and WJEC include: linear (2 bond pairs, 180°), trigonal planar (3 bp, 120°), tetrahedral (4 bp, 109.5°), trigonal pyramidal (3 bp + 1 lp, ~107°), bent (2 bp + 2 lp, ~104.5°), trigonal bipyramidal (5 bp, 90° and 120°) and octahedral (6 bp, 90°).

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彼此远离，以尽量减小排斥。分子的形状由成键电子对和孤对电子的数目决定。孤对电子的排斥力比成键电子对更大，常常会压缩键角。IB HL和WJEC要求掌握的关键形状包括：直线形（2对成键电子，180°）、平面三角形（3对成键，120°）、四面体形（4对成键，109.5°）、三角锥形（3对成键+1对孤对，~107°）、V形（2对成键+2对孤对，~104.5°）、三角双锥形（5对成键，90°和120°）和八面体形（6对成键，90°）。

Always state the number of electron domains and the presence of lone pairs to justify the shape. A table helps memorise:

Electron Domains	Bonding Pairs	Lone Pairs	Shape	Approx. Angle
2	2	0	Linear	180°
3	3	0	Trigonal planar	120°
4	4	0	Tetrahedral	109.5°
4	3	1	Trigonal pyramidal	~107°
4	2	2	Bent (V-shaped)	~104.5°
5	5	0	Trigonal bipyramidal	90°, 120°
6	6	0	Octahedral	90°

这份表格有助于记忆：

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8. Sigma and Pi Bonds (IB HL / WJEC Extension) | σ键与π键（IB HL / WJEC 拓展）

When two atomic orbitals overlap head-on, a sigma (σ) bond is formed. Electron density is concentrated along the internuclear axis. All single bonds are σ bonds. When parallel p orbitals overlap sideways, a pi (π) bond is formed, with electron density above and below the plane of the nuclei. A double bond consists of one σ and one π bond; a triple bond is one σ and two π bonds. The π bond restricts rotation, leading to geometrical (cis–trans) isomerism in alkenes.

当两个原子轨道以头对头的方式重叠，形成σ键。电子密度集中在核间轴上。所有单键都是σ键。当平行的p轨道以肩并肩的方式重叠，形成π键，电子密度分布在原子核平面的上下。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。π键限制了旋转，导致烯烃出现几何（顺反）异构。

In IB HL, you must be able to identify σ and π bonds in a molecule from its Lewis structure or orbital diagram. For example, ethene (C₂H₄) has 5 σ bonds (C—H and C—C σ) and 1 π bond. WJEC may ask you to link π bonding to the reactivity of alkenes with electrophiles, or to compare bond strengths. A common exam question: ‘Explain why the C=C bond is shorter and stronger than a C—C bond but not twice as strong.’ Answer: the σ bond is strong, but the π bond is weaker and more exposed.

在IB HL中，你必须能从路易斯结构或轨道示意图中识别σ键和π键。例如，乙烯(C₂H₄)有5个σ键（C—H键和C—Cσ键）和1个π键。WJEC可能要求你将π键与烯烃的亲电反应活性联系起来，或比较键的强度。一道常见的考题是：“解释为什么C=C键比C—C键短且强，但强度并非后者的两倍。”答案是：σ键很强，但π键较弱且暴露在外。

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9. Hybridisation of Atomic Orbitals (IB HL) | 原子轨道杂化（IB HL）

Hybridisation is the mixing of atomic orbitals to form new, degenerate hybrid orbitals that explain molecular geometry. In carbon, promotion and hybridisation of 2s and 2p orbitals yield sp, sp², or sp³ hybrid orbitals. sp³ gives four equivalent orbitals arranged tetrahedrally (e.g. CH₄). sp² results in three orbitals in a trigonal plane with an unhybridised p orbital for π bonding (e.g. C₂H₄). sp yields two linear orbitals and two unhybridised p orbitals (e.g. C₂H₂).

杂化是将原子轨道混合形成新的、简并的杂化轨道，用以解释分子几何形状。在碳原子中，2s和2p轨道的激发和杂化会产生sp、sp²或sp³杂化轨道。sp³杂化给出四个能量相等的轨道，呈四面体排布（如CH₄）。sp²杂化形成三个处于同一平面的轨道，留出一个未杂化的p轨道参与π键（如C₂H₄）。sp杂化生成两个直线形轨道和两个未杂化的p轨道（如C₂H₂）。

The type of hybridisation can be deduced from the number of electron domains around an atom: 2 domains → sp, 3 → sp², 4 → sp³, 5 → sp³d, 6 → sp³d². IB exam questions often ask: ‘State the hybridisation of the carbon atoms in the following molecules’ or ‘Explain the shape and bonding in benzene using hybridisation.’ WJEC may touch on hybridisation when discussing bond angles but does not require the same level of orbital detail as IB HL.

杂化类型可根据原子周围的电子域数来推断：2个电子域→sp，3→sp²，4→sp³，5→sp³d，6→sp³d²。IB考题常问：“指出下列分子中碳原子的杂化类型”或“利用杂化解释苯的形状和成键”。WJEC在讨论键角时可能会涉及杂化概念，但不像IB HL那样要求深入的轨道细节。

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10. Resonance and Delocalisation | 共振与离域

When a molecule or ion cannot be represented by a single Lewis structure, resonance occurs. The true structure is a hybrid of all contributing forms, with electrons delocalised over several atoms. Classic examples are the carbonate ion CO₃²⁻, ozone O₃, and benzene C₆H₆. In CO₃²⁻, all C—O bonds are of equal length and have a bond order of 1.33. Delocalisation lowers the overall energy of the system, increasing stability.

当某个分子或离子无法用单一路易斯结构表示时，就会出现共振。真实结构是所有参与形式的杂化体，电子在几个原子之间离域。经典例子包括碳酸根离子CO₃²⁻、臭氧O₃和苯C₆H₆。在CO₃²⁻中，所有C—O键长都相等，键级为1.33。离域会降低体系的总能量，从而增加稳定性。

In IB, you may be asked to draw resonance structures using curved arrows to show electron movement, and to explain why the bond lengths are intermediate. WJEC questions often highlight the additional stability of benzene due to delocalisation, compared to the hypothetical cyclohexatriene, and link this to enthalpy of hydrogenation data. Understanding resonance also helps explain the acidity of carboxylic acids and the colour of organic compounds with extended conjugation.

在IB考试中，你可能被要求用弯箭头画出共振结构，并解释为什么键长处于中间值。WJEC的题目常强调苯由于离域而获得额外的稳定性，与假想的环己三烯相比，并将其与氢化焓数据联系起来。理解共振也有助于解释羧酸的酸性，以及具有广泛共轭体系的有机化合物的颜色。

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11. Properties of Covalent Substances | 共价物质的物理性质

Covalent compounds can be simple molecular or giant covalent (network) solids. Simple molecular substances like iodine, water and methane have low melting and boiling points because the intermolecular forces (van der Waals’, dipole-dipole, hydrogen bonds) are weak compared to the strong covalent bonds within the molecules. They are often soft and do not conduct electricity in any state.

共价化合物可以是简单分子或巨型共价（网络）固体。像碘、水和甲烷这样的简单分子物质具有较低的熔点和沸点，因为分子间作用力（范德华力、偶极-偶极作用力、氢键）相对分子内部的强共价键很弱。它们通常质地柔软，在任何状态下都不导电。

Giant covalent structures, such as diamond (sp³, tetrahedral network), graphite (sp², layered with delocalised electrons), and silicon dioxide (SiO₂, tetrahedral network), have very high melting points due to the need to break numerous strong covalent bonds. Graphite conducts electricity parallel to its layers due to mobile π electrons, a classic WJEC and IB concept. The hardness of diamond versus the softness of graphite is a direct consequence of their bonding and structure.

巨型共价结构，如金刚石（sp³，四面体网络）、石墨（sp²，层状并有离域电子）和二氧化硅（SiO₂，四面体网络），由于需要破坏大量强共价键，熔点非常高。石墨由于可移动的π电子而具有层间导电性，这是WJEC和IB的经典概念。金刚石的硬度与石墨的柔软直接源于它们的成键和结构差异。

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12. Common Exam Pitfalls and Final Tips | 常见考试失分点与终极建议

Do not confuse intermolecular forces with covalent bonds. The covalent bond holds atoms together within a molecule; intermolecular forces act between molecules. When asked ‘explain the high boiling point of water’, mention hydrogen bonding between water molecules, not the O—H covalent bond. Similarly, in giant covalent substances, you must talk about breaking covalent bonds, not overcoming intermolecular forces.

不要混淆分子间作用力与共价键。共价键把分子内的原子联结在一起；分子间作用力存在于分子之间。当被要求“解释水的高沸点”时，要提到水分子之间的氢键，而不是O—H共价键。同理，对于巨型共价物质，你必须该讨论破坏共价键，而非克服分子间作用力。

Always link shape to bond angle logically: state total electron domains, number of lone pairs, and their effect. For IB, be precise with hybridisation and σ/π counts. For WJEC, emphasise practical applications. A good diagram is worth a thousand words, but labels must be accurate. Practise past paper questions on drawing Lewis structures with formal charges, deducing shapes, and explaining polarity. With consistent practice, covalent bonding becomes a reliable source of high marks.

始终有逻辑地将形状与键角联系起来：陈述总电子域数、孤对电子数及其影响。对IB来讲，杂化和σ/π键的数量要准确。对WJEC而言，要强调实际应用。一张好图胜过千言万语，但标注必须准确。练习过往试卷中关于画出带形式电荷的路易斯结构、推断形状和解释极性的题目。通过持续练习，共价键必将成为高分的可靠来源。

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