Chemical Bonding | 化学键

📚 Chemical Bonding | 化学键

Chemical bonding is the foundation of all chemical structures and reactions. This article reviews the key concepts of ionic, covalent and metallic bonding, intermolecular forces, and how bonding determines physical properties, aligned with the Cambridge A-Level Chemistry syllabus.

化学键是理解一切化学结构与反应的基石。本文系统梳理离子键、共价键与金属键，以及分子间作用力对物质性质的影响，完全对标剑桥A-Level化学考试大纲。

1. Why Atoms Bond | 原子为何成键

Atoms bond to achieve greater stability, usually by obtaining a full outer electron shell. This can be achieved by transferring electrons (ionic bonding), sharing electrons (covalent bonding) or through the delocalisation of electrons (metallic bonding).

原子通过成键获得更大的稳定性，通常是为了达到稳定的最外层电子构型。这种稳定可以通过电子转移（离子键）、共用电子对（共价键）或电子离域化（金属键）来实现。

Noble gases are stable because they have a complete outer shell; other atoms tend to match this configuration.
稀有气体稳定是因为其最外层已充满；其他原子倾向于获得类似构型。
The driving force for bonding is the lowering of potential energy of the system.
成键的根本驱动力是体系总势能降低。

2. Ionic Bonding | 离子键

Ionic bonding involves the complete transfer of electrons from a metal to a non-metal, forming oppositely charged ions. The electrostatic attraction between these ions constitutes the ionic bond.

离子键是电子从金属原子完全转移至非金属原子的过程，形成带相反电荷的离子，正负离子间的静电引力即为离子键。

Metals lose electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions).
金属失去电子形成正离子（阳离子）；非金属获得电子形成负离子（阴离子）。
The ionic bond is non-directional, so ionic compounds tend to form giant lattice structures.
离子键无方向性，因此离子化合物倾向于形成巨大的晶格结构。
Examples: NaCl, MgO, CaF₂.
例如：NaCl、MgO、CaF₂。

The strength of the ionic bond depends on ionic charge and ionic radius. According to Coulomb’s law, the lattice energy is proportional to the product of charges divided by the sum of radii.

离子键强度取决于离子电荷和离子半径。根据库仑定律，晶格能与离子电荷乘积成正比，与离子半径之和成反比。

F ∝ (Q⁺ × Q⁻) / (r⁺ + r⁻)²

Therefore MgO has a much higher melting point than NaCl because Mg²⁺ and O²⁻ carry higher charges with smaller radii.

因此MgO的熔点远高于NaCl，因为Mg²⁺和O²⁻电荷更高且半径更小。

3. Covalent Bonding | 共价键

Covalent bonding arises when two non-metals share one or more pairs of electrons. Each shared pair forms a single bond; two pairs form a double bond, and three pairs form a triple bond.

共价键由两个非金属原子共用一对或多对电子形成。一对共用电子构成单键，两对构成双键，三对构成三键。

A covalent bond is formed by the overlap of atomic orbitals, increasing electron density between the two nuclei.
共价键由原子轨道重叠形成，使两核之间电子密度增大。
This type of bond is directional, leading to specific molecular shapes.
共价键具有方向性，决定了分子的特定空间构型。
Examples: H₂, O₂, N₂, CH₄, CO₂.
例如：H₂、O₂、N₂、CH₄、CO₂。

When the two atoms are identical, the shared electrons are equally attracted and the bond is non-polar. When different elements are involved, the atom with higher electronegativity pulls the electron pair closer, creating a polar covalent bond.

若两个原子相同，共用电子的吸引力相同，则为非极性键。若涉及不同元素，电负性较高的原子会将共用电子对拉向自己，产生极性共价键。

In the Cambridge syllabus, you must be able to draw dot-and-cross diagrams for molecules such as Cl₂, H₂O, NH₃, CCl₄ and H₂O₂, and represent dative covalent bonds where both electrons come from one atom.

在剑桥考纲中，你需要会绘制Cl₂、H₂O、NH₃、CCl₄、H₂O₂等分子的电子点叉图，并能表示配位键（两个电子均来自同一原子）。

4. Dative Covalent Bond (Coordinate Bond) | 配位键

A dative covalent bond is a special type of covalent bond in which both shared electrons are supplied by one atom, but the resulting bond is identical to a normal covalent bond.

配位键是一种特殊的共价键，其中的共用电子对由同一个原子提供，但形成的键与普通共价键没有区别。

The donor atom must have a lone pair of electrons; the acceptor atom must have an empty orbital or be electron-deficient.
配位键的给体原子必须有孤对电子；受体原子必须有空轨道或为缺电子状态。
Common examples: NH₄⁺, H₃O⁺, and [Cu(NH₃)₄]²⁺.
常见实例：NH₄⁺、H₃O⁺和[Cu(NH₃)₄]²⁺。
In diagrams, an arrow is often used to show the direction of electron donation.
在插图中通常用箭头表示电子供应的方向。

In the Al₂Cl₆ molecule, each Al atom forms a dative bond with a Cl atom from the other AlCl₃ unit, allowing each aluminium to achieve a full octet.

在Al₂Cl₆分子中，每个Al原子与另一个AlCl₃单元中的Cl原子形成配位键，从而让每个铝原子都达到八隅体结构。

5. Metallic Bonding | 金属键

Metallic bonding is the electrostatic attraction between positive metal ions arranged in a lattice and a sea of delocalised electrons from the outer shells of the metal atoms.

金属键是正金属离子在晶格中排列，与其外部离域电子海之间的静电相互作用。

The delocalised electrons move freely throughout the structure, explaining high electrical and thermal conductivity.
离域电子可在整个结构中自由移动，这解释了金属良好的导电和导热性。
The attraction between layers of cations can be strong, making metals strong and malleable.
阳离子层间吸引力强，使金属具有高强度与延展性。
With increasing charge and smaller ion radius, the strength of metallic bonding increases, raising melting point.
随离子电荷增加、半径减小，金属键增强，熔点升高。

This model also explains why metals are shiny, ductile and why they alloy well with each other.

该模型还解释了金属具有光泽、延展性及易形成合金的原因。

6. Electronegativity and Bonding Type | 电负性与键型

Electronegativity is the ability of an atom in a molecule to attract bonding electrons towards itself. The difference in electronegativity (ΔEN) between two atoms determines the bond character.

电负性是分子中原子吸引成键电子的能力。两个原子之间电负性差值（ΔEN）决定成键特性。

ΔEN	Bond Type	键型
Less than 0.4	Non-polar covalent	非极性共价键
0.4 – 1.7	Polar covalent	极性共价键
Greater than 1.7	Mainly ionic	以离子键为主

These boundaries are approximate; many bonds are intermediate, showing both ionic and covalent character.

这些界限是经验性的，许多键处于中间状态，同时具有离子性与共价性。

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

Bond length is the distance between two bonded nuclei. Bond energy is the energy required to break one mole of a specific bond in a gaseous molecule.

键长是两个成键原子核之间的距离。键能是断裂气态分子中一摩尔特定化学键所需的能量。

Higher bond order (single, double, triple) leads to shorter bond length and higher bond energy.
键级越高（单键、双键、三键），键长越短，键能越大。
For example, the C–C bond length in alkanes is about 154 pm, C=C is 134 pm, and C≡C is 120 pm.
例如，烷烃中C–C键长约为154 pm，C=C为134 pm，C≡C为120 pm。
Bond energy affects reaction enthalpy; stronger bonds require more energy to break.
键能影响反应焓变；键能越大断裂所需能量越多。

In A-Level data tables, average bond energies are used to estimate enthalpy changes of reaction using the enthalpy cycle:

在A-Level数据表中，通常用平均键能来估算反应焓变，方法如下:

ΔH = Σ(bonds broken) − Σ(bonds formed)

Remember that bond breaking is endothermic, while bond formation is exothermic.

注意：断键吸热，成键放热。

8. Intermolecular Forces | 分子间作用力

Intermolecular forces (van der Waals forces) include London dispersion forces, permanent dipole-dipole forces, and hydrogen bonds. These are much weaker than chemical bonds, but they dominate the physical properties of molecular substances.

分子间作用力（范德华力）包括色散力、永久偶极-偶极作用力和氢键。这些力比化学键弱得多，但决定了分子物质的主要物理性质。

8.1 London Dispersion Forces | 色散力

London forces arise from instantaneous dipoles caused by random movement of electrons. They exist between all molecules, and their strength increases with the number of electrons and molecular surface area.

色散力源于电子随机运动引起的瞬时偶极，存在于一切分子之间，强度随分子内电子数目和表面积增大而增强。

Larger halogens like I₂ are solid at room temperature because their many electrons create stronger London forces.

像I₂这样较大的卤素在室温为固态，因为大量电子产生更强的色散力。

8.2 Permanent Dipole-Dipole Forces | 永久偶极-偶极作用力

Molecules with permanent dipoles experience attractions between the positive end of one molecule and the negative end of another. These forces are stronger than London forces of similar-sized non-polar molecules.

具有永久偶极的分子之间，一个分子的正端与另一个分子的负端相互吸引，这类作用力比相同大小非极性分子的色散力更强。

9. Hydrogen Bonding | 氢键

A hydrogen bond is a strong type of dipole-dipole interaction between a hydrogen atom covalently bonded to a highly electronegative atom (N, O or F) and a lone pair of electrons on an adjacent molecule.

氢键是一种较强的偶极-偶极相互作用：氢原子共价连接在高电负性原子（N、O、F）上后，再与邻近分子上的孤对电子相互作用。

Conditions: H directly bonded to N, O or F, and a lone pair on an electronegative atom in a nearby molecule.
条件：H直接连在N、O或F上，且附近分子有电负性原子上的孤对电子。
Hydrogen bonds cause water to have a high boiling point, high surface tension and unusual density behaviour (ice floats).
氢键使水具有高沸点、高表面张力以及反常的密度行为（冰浮在水上）。
They also determine the double-helix structure of DNA and secondary structure of proteins.
氢键也决定了DNA双螺旋结构和蛋白质的二级结构。

In ice, each water molecule forms four hydrogen bonds, producing an open tetrahedral lattice that is less dense than liquid water.

在冰中，每个水分子形成四个氢键，构成疏松的四面体晶格，密度低于液态水。

10. Structure and Physical Properties | 结构与物理性质

The type of bonding and structure determines whether a substance has a high or low melting point, conducts electricity, and dissolves in water.

键型和结构决定物质的熔点高低、导电性以及水溶性。

Structure	Bonding	Melting Point	Electrical Conductivity	示例
Giant ionic lattice	Ionic	High	When molten or aqueous	NaCl
Giant covalent	Covalent	Very high	Usually none (except graphite)	Diamond, SiO₂
Simple molecular	Intermolecular forces	Low	None	CO₂, H₂O
Metallic lattice	Metallic	Usually high	High in solid and liquid	Fe, Cu

Graphite is a special giant covalent structure with delocalised electrons between layers, making it the only non-metal that conducts electricity, and its layers slide easily.

石墨是一种特殊的巨型共价结构，层间存在离域电子，使其成为唯一导电的非金属单质，且层与层之间容易滑动。

11. Shapes of Molecules and Bond Angles | 分子构型与键角

According to VSEPR theory, electron pairs around a central atom repel each other and arrange themselves as far apart as possible. This determines the molecular geometry and bond angles.

按VSEPR理论，中心原子周围的电子对相互排斥并尽可能远离，从而决定分子几何构型和键角。

Linear: BeCl₂, CO₂ – 180°
直线形：BeCl₂、CO₂ – 180°
Trigonal planar: BF₃ – 120°
平面三角形：BF₃ – 120°
Tetrahedral: CH₄ – 109.5°
正四面体：CH₄ – 109.5°
Pyramidal: NH₃ – 107° (one lone pair)
三角锥形：NH₃ – 107°（有一对孤对电子）
Bent: H₂O – 104.5° (two lone pairs)
V形或角形：H₂O – 104.5°（有两对孤对电子）

Lone pairs repel more strongly than bonding pairs, which compresses bond angles in NH₃ and H₂O.

孤对电子的排斥力大于成键电子对，因此NH₃和H₂O的键角被压缩。

12. Exam Focus Summary | 考点聚焦总结

For Cambridge A-Level Chemistry, focus on how bonding models relate to macroscopic properties, the definitions of ionic, covalent and metallic bonding, and the nature of hydrogen bonds.

剑桥A-Level化学中，重点掌握各类键型的模型定义、结构与宏观性质之间的关联，以及氢键的本质。

Be able to draw dot-and-cross and displayed formulae.
能够绘制点叉图和结构式。
Use electronegativity differences to predict ionic versus covalent behaviour.
用电负性差异判断离子键或共价键特征。
Use VSEPR to predict shapes and bond angles.
用VSEPR理论判断分子构型和键角。
Understand why melting points, boiling points and solubility vary with structure.
理解熔点、沸点和溶解度随结构变化的原因。

Revise across tables and past papers to see how bonding concepts are tested in multiple-choice and structured questions.

通过表格对比和历年真题练习，熟悉选择题与结构题中如何考查化学键概念。

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Chemical Bonding | 化学键