📚 Covalent Bonding: IB & CCEA Chemistry Key Points | IB CCEA 化学：共价键考点精讲
Covalent bonding is the central pillar of molecular chemistry, governing how atoms share electrons to form stable molecules with distinct shapes and properties. Whether you are studying the IB Diploma Programme or the CCEA specification, a robust understanding of covalent bonding – from Lewis structures and VSEPR theory to hybridisation and polarity – is essential for predicting molecular behaviour, reactivity, and physical characteristics.
共价键是分子化学的核心支柱，决定了原子如何通过共享电子对形成具有独特形状和性质的稳定分子。无论你学习 IB 文凭课程还是 CCEA 考试大纲，深入理解共价键——从路易斯结构和 VSEPR 理论到杂化与极性——对于预测分子行为、反应活性与物理性质至关重要。
1. What is a Covalent Bond? | 什么是共价键？
A covalent bond forms when two atomic nuclei simultaneously attract a shared pair of electrons. This electrostatic attraction between the positively charged nuclei and the negatively charged shared electron pair holds the atoms together. In most cases, the shared electrons occupy a region of space called a molecular orbital, allowing each atom to attain a more stable electron configuration, often resembling that of a noble gas.
当两个原子核同时吸引一对共用电子时，便形成了共价键。带正电的原子核与带负电的共用电子对之间的静电引力将原子结合在一起。在多数情况下，共用电子占据一个称为分子轨道的空间区域，使得每个原子都能获得更稳定的电子排布，通常达到类似稀有气体的构型。
For both IB and CCEA, the core definition centres on electron sharing between non-metal atoms. The bond is directional and can be single, double, or triple, depending on the number of shared electron pairs.
对于 IB 和 CCEA 考试，核心定义都聚焦于非金属原子间的电子共享。键具有方向性，根据共用电子对的数目可分为单键、双键或三键。
2. Lewis Structures and the Octet Rule | 路易斯结构与八隅规则
Lewis diagrams represent valence electrons as dots around atomic symbols and illustrate how electrons are shared to form bonds. The octet rule states that atoms tend to gain, lose, or share electrons in order to achieve a full outer shell of eight electrons. Exceptions include hydrogen (which seeks two electrons), beryllium and boron (which can be stable with fewer), and elements from period three onwards that can expand their octet.
路易斯结构图用点来表示原子符号周围的最外层电子，并展示电子如何共享形成键。八隅规则指出，原子倾向于通过得到、失去或共享电子以达到 8 个电子的全满外层。例外情况包括氢（只需 2 个电子）、铍和硼（可稳定存在少于 8 个电子），以及第三周期及以后的元素可以扩展八隅体。
When drawing structures, you should place the least electronegative atom (except hydrogen) at the centre, distribute electrons to satisfy the octet for terminal atoms first, then assign any remaining electrons as lone pairs on the central atom. Multiple bonds are formed if the central atom still lacks an octet.
绘制结构时，应将电负性最小的原子（氢除外）置于中心，先满足末端原子的八隅体，再将剩余电子分配给中心原子作为孤对电子。如果中心原子仍未满足八隅体，则形成多重键。
3. Coordinate (Dative) Covalent Bonds | 配位共价键
A coordinate or dative covalent bond arises when both shared electrons originate from the same atom. Once formed, it is indistinguishable from any other covalent bond. Familiar examples include the ammonium ion NH₄⁺, the hydronium ion H₃O⁺, and the bond between NH₃ and BF₃.
配位共价键（亦称配价键）发生在共用电子对均由同一个原子提供的情况。一旦形成，它与其他共价键没有区别。常见的例子包括铵根离子 NH₄⁺、水合氢离子 H₃O⁺，以及 NH₃ 与 BF₃ 之间的键。
IB expects you to identify the donor and acceptor species; CCEA likewise asks for dot-and-cross diagrams showing the dative arrow pointing from donor to acceptor. Note that the donor must possess a lone pair, and the acceptor must be electron-deficient.
IB 要求能识别电子给予体和接受体；CCEA 同样要求用点叉图表示，从给予体指向接受体的配位键箭头。注意给予体必须拥有孤对电子，而接受体必须是缺电子物种。
4. Resonance: When One Structure Isn’t Enough | 共振：当一个结构不够用时
Some molecules or ions cannot be accurately described by a single Lewis structure. The actual structure is a resonance hybrid – a weighted blend of several plausible contributing forms. Classic examples include the carbonate ion CO₃²⁻, ozone O₃, and benzene C₆H₆. In the carbonate ion, all three C-O bonds are equivalent, with a bond order of 1.33.
有些分子或离子无法用单一的路易斯结构精确描述。实际结构是共振杂化体——若干合理贡献形式的加权混合。典型实例包括碳酸根离子 CO₃²⁻、臭氧 O₃ 和苯 C₆H₆。在碳酸根离子中，所有三条碳氧键等价，键级为 1.33。
Resonance stabilises the molecule by delocalising electrons over several atoms, lowering the overall energy. Both IB and CCEA syllabuses require you to draw resonance structures, use double-headed arrows between them, and understand the concept of delocalisation.
共振通过使电子在多个原子上离域来稳定分子，降低整体能量。IB 和 CCEA 考试大纲都要求绘制共振结构，在它们之间使用双箭头，并理解离域概念。
5. Bond Order, Length and Energy | 键级、键长与键能
Bond order refers to the number of shared electron pairs between two atoms. A single bond has bond order 1, a double bond 2, and a triple bond 3. In resonance hybrids, bond order can be fractional. As bond order increases, bond length decreases and bond energy (strength) increases.
键级指两个原子间共用电子对的数目。单键键级为 1，双键为 2，三键为 3。在共振杂化体中，键级可以是非整数。随着键级增加，键长变短，键能（强度）增大。
For a typical comparison: C−C single bond (154 pm, 348 kJ mol⁻¹), C=C double bond (134 pm, 614 kJ mol⁻¹), C≡C triple bond (120 pm, 839 kJ mol⁻¹). These values explain why many reactions involve breaking weaker π bonds while leaving the stronger σ framework intact.
典型对比：C−C 单键（154 pm, 348 kJ mol⁻¹），C=C 双键（134 pm, 614 kJ mol⁻¹），C≡C 三键（120 pm, 839 kJ mol⁻¹）。这些数值解释了为何许多反应中断裂较弱的 π 键，而保留较强的 σ 骨架。
6. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity (Pauling scale) measures an atom’s ability to attract bonding electrons. When atoms with different electronegativities form a bond, the electron density shifts towards the more electronegative atom, creating a dipole. If the difference is large (typically >1.7), the bond exhibits significant ionic character. Pure covalent bonds occur only between identical atoms.
电负性（鲍林标度）衡量原子吸引键合电子的能力。当电负性不同的原子形成化学键时，电子密度向电负性较大的原子偏移，产生偶极。如果差值很大（通常 > 1.7），键表现出明显的离子性。纯共价键仅存在于相同原子之间。
IB uses a continuum from non-polar to polar covalent to ionic; CCEA expects understanding of bond polarity and the use of partial charges (δ⁺ and δ⁻). The polarity of a bond influences the molecule’s overall dipole moment and intermolecular forces.
IB 将键型看作从非极性到极性共价再到离子键的连续体；CCEA 要求理解键的极性并使用部分电荷符号（δ⁺ 和 δ⁻）。键的极性会影响分子的整体偶极矩以及分子间作用力。
7. Dipole Moments and Molecular Polarity | 偶极矩与分子极性
A molecule possesses a permanent dipole moment if it contains polar bonds arranged asymmetrically. The vector sum of bond dipoles determines the overall molecular polarity. Carbon dioxide (CO₂) has two polar C=O bonds but is linear, so the dipoles cancel, resulting in a non-polar molecule. Water (H₂O) is bent, with bond dipoles reinforcing each other, making it highly polar.
如果分子含有不对称排列的极性键，则具有永久偶极矩。键偶极的矢量和决定了分子的整体极性。二氧化碳（CO₂）含有两条极性 C=O 键，但分子呈直线形，偶极抵消，因而是非极性分子。水分子（H₂O）为弯曲形，键偶极相互加强，因此极性很强。
Both specifications require you to deduce molecular polarity from shape and bond polarities. This directly links to solvation properties and intermolecular forces such as dipole-dipole interactions and hydrogen bonding.
两种大纲都要求根据分子形状和键的极性推断分子极性。这直接关系到溶剂化性质以及偶极-偶极相互作用、氢键等分子间作用力。
8. 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 adopt geometries that minimise repulsion. Electron domains (bonding pairs and lone pairs) arrange themselves as far apart as possible. The shape is named after the positions of atoms only.
价层电子对互斥理论（VSEPR）指出，中心原子周围的电子对采取使排斥力最小的几何排列。电子域（键对和孤对电子）尽量彼此远离。分子形状仅根据原子的位置命名。
Key geometries to memorise (with total electron pairs and bond angles): 2 pairs – linear (180°), 3 pairs – trigonal planar (120°), 4 pairs – tetrahedral (109.5°), 4 pairs with one lone pair – trigonal pyramidal (~107°), 4 pairs with two lone pairs – bent (~104.5°), 5 pairs – trigonal bipyramidal (90°, 120°), 6 pairs – octahedral (90°). IB typically covers up to 6 electron domains; CCEA focuses on up to 4 domains but includes shapes like pyramidal and bent.
需记忆的关键几何构型（连同总电子对数和键角）：2 对 – 直线形 (180°)，3 对 – 平面三角形 (120°)，4 对 – 四面体形 (109.5°)，4 对含 1 对孤对电子 – 三角锥形 (~107°)，4 对含 2 对孤对电子 – 弯曲形 (~104.5°)，5 对 – 三角双锥形 (90°, 120°)，6 对 – 八面体形 (90°)。IB 通常要求掌握多达 6 个电子域的构型；CCEA 主要集中在 4 个电子域内，但包括三角锥形和弯曲形等。
9. Hybridisation of Atomic Orbitals | 原子轨道的杂化
Hybridisation explains how atomic orbitals of the central atom mix to form new, equivalent hybrid orbitals that point towards the corners of the molecular geometry. The number of hybrid orbitals equals the number of electron domains around the central atom.
杂化理论解释了中心原子的原子轨道如何混合，形成一组指向分子几何顶点的新的等价杂化轨道。杂化轨道的数量等于中心原子周围的电子域数目。
sp hybridisation leads to linear geometry (2 domains), sp² to trigonal planar (3 domains), sp³ to tetrahedral (4 domains), sp³d to trigonal bipyramidal (5 domains), and sp³d² to octahedral (6 domains). IB requires detailed knowledge of hybridisation and its link to σ/π bonding; CCEA may introduce hybridisation conceptually but the emphasis in the advanced specifications is on shape.
sp 杂化对应直线形（2 个域），sp² 对应平面三角形（3 个域），sp³ 对应四面体形（4 个域），sp³d 对应三角双锥形（5 个域），sp³d² 对应八面体形（6 个域）。IB 要求详细掌握杂化及其与 σ/π 键的关系；CCEA 可能从概念上引入杂化，但更高级别的重点在分子形状。
10. Sigma and Pi Bonds | σ键与π键
Covalent bonds are classified by the way orbitals overlap. A sigma (σ) bond results from end-on overlap of orbitals, concentrating electron density directly between the two nuclei. All single bonds are σ bonds. A pi (π) bond is formed by sideways overlap of adjacent p orbitals, with electron density above and below the internuclear axis. Double bonds consist of one σ and one π bond; triple bonds contain one σ and two π bonds.
共价键按轨道重叠方式分类。σ 键由轨道端对端重叠形成，电子密度集中在两原子核之间。所有单键都是 σ 键。π 键由相邻 p 轨道肩并肩重叠形成，电子密度分布在核间轴的上方和下方。双键含有一个 σ 键和一个 π 键；三键含有一个 σ 键和两个 π 键。
IB explicitly links hybridisation to σ and π bonding – e.g., in ethene, the sp² hybrid orbitals form σ bonds and the unhybridised p orbital forms the π bond. CCEA expects you to understand the formation of π bonds in alkenes and their restricted rotation.
IB 明确将杂化与 σ 和 π 键联系起来——例如在乙烯中，sp² 杂化轨道形成 σ 键，未杂化的 p 轨道形成 π 键。CCEA 要求理解烯烃中 π 键的形成及其旋转受限。
11. Giant Covalent Structures | 巨型共价结构
Some substances consist of an extended network of covalent bonds continuing throughout the entire crystal. Diamond, graphite, graphene, and silicon dioxide are prime examples. In diamond, each carbon is sp³ hybridised and tetrahedrally bonded to four others, creating a rigid, insulating, extremely hard structure. In graphite, carbon atoms are sp² hybridised in planar sheets with delocalised electrons between layers, giving it softness, lubrication properties, and electrical conductivity.
某些物质由贯穿整个晶体的广阔共价键网络构成。金刚石、石墨、石墨烯和二氧化硅是典型例子。在金刚石中，每个碳原子采用 sp³ 杂化，以四面体方式与另外四个碳成键，形成坚硬、绝缘、极硬的结构。在石墨中，碳原子 sp² 杂化排列成平面层，层间有离域电子，赋予其柔软、润滑和导电性。
IB requires comparison of bonding, properties, and uses of giant covalent substances; CCEA similarly examines diamond and graphite in the context of structure-property relationships. Silicon dioxide (SiO₂) is another network solid where each Si is tetrahedrally surrounded by four oxygen atoms, leading to high melting point and hardness.
IB 要求比较巨型共价物质的键合、性质与用途；CCEA 同样在结构-性质关系的背景下考查金刚石和石墨。二氧化硅（SiO₂）是另一种网络固体，其中每个 Si 被四个氧原子四面体包围，导致高熔点和硬度。
12. Comparison with Ionic and Metallic Bonding | 与离子键和金属键的对比
Covalent bonding is just one type of strong intramolecular force. A systematic comparison helps solidify understanding. Ionic bonding involves electron transfer and electrostatic attraction between oppositely charged ions, forming lattice structures with high melting points, brittleness, and conductivity only when molten or dissolved. Metallic bonding consists of delocalised electrons attracted to a lattice of cations, giving metals their malleability, ductility, and electrical conductivity.
共价键只是强分子内力中的一种。系统对比有助于巩固理解。离子键涉及电子转移和相反电荷离子间的静电引力，形成具有高熔点、脆性、仅熔融或溶解时导电的晶格结构。金属键包含被阳离子晶格吸引的离域电子，赋予金属延展性、展性和导电性。
By contrast, simple covalent molecules have relatively weak intermolecular forces, hence lower melting and boiling points. Giant covalent materials have an intermediate position, with extremely high melting points but variable conductivity. Both IB and CCEA frequently set questions that require linking bonding type to physical properties.
相比之下，简单共价分子分子间作用力相对较弱，因而熔沸点较低。巨型共价材料处于中间位置，具有极高的熔点，但导电性不一。IB 和 CCEA 常出题要求将键合类型与物理性质联系起来。
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