📚 S2.2 The Covalent Model | 共价模型
Covalent bonding lies at the heart of molecular chemistry. It describes how atoms share electrons to achieve greater stability, forming the vast array of molecules that make up the world around us. IB Chemistry S2.2 explores not just the Lewis structures you see on paper, but the deeper ideas that govern bond polarity, molecular shape, resonance, and the quantum-mechanical picture of sigma and pi bonds. Understanding this model is essential for predicting physical properties, reactivity, and the behaviour of substances from simple diatomic gases to complex biomolecules.
共价键是分子化学的核心。它描述了原子如何通过共用电子来实现更稳定的电子排布，并形成构成我们周围世界的无数分子。IB 化学 S2.2 不仅探索纸上所画的路易斯结构，还深入研究控制键的极性、分子形状、共振以及 σ 键与 π 键的量子力学图像等更深层的概念。理解这一模型对于预测物理性质、反应活性以及从简单双原子气体到复杂生物分子的物质行为至关重要。
1. The Nature of the Covalent Bond | 共价键的本质
A covalent bond forms when two atoms share one or more pairs of electrons. The shared electrons are attracted to the nuclei of both atoms, creating an electrostatic 'glue' that holds them together. This typically occurs between two non-metal atoms that have similar electronegativities. By sharing, each atom effectively fills its outer shell, moving toward the stable electron configuration of a noble gas.
当两个原子共用一对或多对电子时，便形成了共价键。共用的电子被两个原子的核所吸引，产生一种静电“胶水”，将二者结合在一起。这通常发生在电负性相近的两个非金属原子之间。通过共用电子，每个原子都能有效地填满其外层电子层，趋向于惰性气体的稳定电子排布。
- Electrons are shared in a region of high probability between the nuclei.
- 电子在原子核之间高概率出现的区域中被共用。
- The attraction is directional, which gives molecules specific shapes.
- 这种吸引力具有方向性，赋予了分子特定的形状。
2. Lewis Structures and the Octet Rule | 路易斯结构与八隅律
Lewis structures are diagrams that show the bonding between atoms and the lone pairs of electrons present in a molecule. Valence electrons are represented as dots or lines. The guiding principle is the octet rule: atoms tend to form bonds until each atom (except hydrogen) is surrounded by eight valence electrons. Hydrogen follows the 'duet' rule, seeking two electrons.
路易斯结构是表示分子中原子间键合以及存在的孤对电子的图解。价电子用点或线来表示。其指导原则是八隅律：原子倾向于形成键，直到每个原子（氢除外）被八个价电子包围。氢遵循“二偶律”，仅求达到两个电子。
- Total number of valence electrons is counted first; for an ion, add or subtract electrons.
- 首先计算总的价电子数；对于离子，需加上或减去电子。
- Single, double, and triple bonds correspond to sharing 2, 4, or 6 electrons respectively.
- 单键、双键和三键分别对应共用 2 个、4 个和 6 个电子。
Exceptions to the octet rule include electron-deficient molecules like BF₃ (boron has 6 electrons) and expanded octet species like SF₆ (sulfur has 12 electrons).
八隅律的例外包括缺电子分子如 BF₃（硼周围有 6 个电子）和八隅体扩展的物种如 SF₆（硫周围有 12 个电子）。
3. Coordinate Covalent Bonds | 配位共价键
A coordinate covalent bond (or dative bond) is a covalent bond in which both electrons originate from the same atom. Once formed, a coordinate bond is identical in strength and length to any other covalent bond. It commonly appears in the formation of adducts and transition metal complexes.
配位共价键（或称为配价键）是一种共价键，其中两个电子都来自同一个原子。一旦形成，配位键在强度和长度上与任何其它共价键完全相同。它常出现在加合物和过渡金属配合物的形成过程中。
Example: the ammonium ion, NH₄⁺, is formed when a proton H⁺ (with an empty 1s orbital) accepts a lone pair from the nitrogen in NH₃.
例子：铵根离子 NH₄⁺，是当一个质子 H⁺（带有一个空的 1s 轨道）接受来自 NH₃ 中氮的孤对电子时形成的。
NH₃ + H⁺ → NH₄⁺
4. Formal Charge | 形式电荷
Formal charge is a bookkeeping tool used to evaluate Lewis structures and predict the most stable arrangement of atoms. It is calculated assuming that electrons in a bond are shared equally, regardless of electronegativity. The most plausible Lewis structure generally has formal charges closest to zero, with any negative formal charge residing on the more electronegative atom.
形式电荷是一种用于评估路易斯结构和预测最稳定原子排布的计算工具。在计算时，假定键中的电子均等共用，而不考虑电负性。最合理的路易斯结构通常具有最接近零的形式电荷，并且任何负的形式电荷都位于电负性更大的原子上。
- Formal charge = valence electrons - (non-bonding electrons + ½ bonding electrons).
- 形式电荷 = 价电子数 - (非键电子数 + ½ 键合电子数)。
For the cyanate ion OCN⁻, the structure with the negative charge on oxygen is more stable than one with the charge on nitrogen, owing to oxygen's higher electronegativity.
对于氰酸根离子 OCN⁻，负电荷在氧上的结构比负电荷在氮上的更稳定，这是因为氧的电负性更高。
5. Resonance Structures | 共振结构
When a single Lewis structure cannot fully describe a molecule's bonding, resonance structures are used. The true structure is a resonance hybrid—a weighted average of all valid Lewis structures. Electrons are delocalised across the molecule, which generally increases stability. This is evident in molecules like ozone (O₃) and the carbonate ion (CO₃²⁻).
当单一的路易斯结构无法完全描述分子的键合情况时，就会使用共振结构。真实的结构是一个共振杂化体——所有有效路易斯结构的加权平均。电子在整个分子中离域，这通常会提高稳定性。这在臭氧 (O₃) 和碳酸根离子 (CO₃²⁻) 等分子中非常明显。
- All bonds in the carbonate ion are equivalent, with a bond order of 1⅓.
- 碳酸根离子中的所有碳氧键都是等效的，键级为 1⅓。
- Resonance in benzene (C₆H₆) explains its equal C–C bond lengths and unusual stability.
- 苯 (C₆H₆) 中的共振解释了其相等的 C–C 键长和异常的稳定性。
6. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity (EN) is the ability of an atom to attract the shared pair of electrons in a covalent bond. When two atoms have different EN values, the electron cloud is drawn closer to the more electronegative atom, creating a polar covalent bond. This results in partial positive (δ⁺) and partial negative (δ⁻) charges.
电负性 (EN) 是原子在共价键中吸引共用电子对的能力。当两个原子的 EN 值不同时，电子云会被拉向电负性更强的原子，形成极性共价键。这就产生了部分正电荷 (δ⁺) 和部分负电荷 (δ⁻)。
| EN Difference (ΔEN) | Bond Type | 键型 |
|---|---|
| 0.0 – 0.4 | Non-polar covalent | 非极性共价键 |
| 0.5 – 1.7 | Polar covalent | 极性共价键 |
| ≥ 1.8 | Ionic | 离子键 |
The covalent–ionic continuum means bonding is rarely purely ionic or purely covalent. Even in a salt like LiCl, there is some covalent character.
共价–离子连续统意味着键合极少是纯离子键或纯共价键。即使在 LiCl 这样的盐中，也存在一些共价性。
7. VSEPR Theory and Molecular Geometry | VSEPR 理论与分子几何构型
Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the three-dimensional shape of molecules. It states that electron pairs (bonding and non-bonding) around a central atom arrange themselves to minimise repulsion, which determines the bond angles. Non-bonding (lone) pairs exert greater repulsion than bonding pairs, thereby compressing bond angles.
价层电子对互斥 (VSEPR) 理论可预测分子的三维形状。该理论指出，中心原子周围的电子对（成键和非成键）会自行排布，使排斥力最小化，从而决定键角。非成键（孤对）电子对的排斥力大于成键电子对，因此会使键角压缩。
- 2 electron pairs: linear, 180° (BeCl₂)
- 2 对电子：直线形，180° (BeCl₂)
- 3 electron pairs: trigonal planar, 120° (BF₃); bent for 2 bonds + 1 lone pair (SO₂)
- 3 对电子：平面三角形，120° (BF₃)；2 成键 + 1 孤对时为 V 形 (SO₂)
- 4 electron pairs: tetrahedral, 109.5° (CH₄); pyramidal with 1 lone pair (NH₃); bent with 2 lone pairs (H₂O)
- 4 对电子：四面体形，109.5° (CH₄)；1 孤对为三角锥形 (NH₃)；2 孤对为 V 形 (H₂O)
- 5 electron pairs: trigonal bipyramidal, 90°, 120° (PCl₅)
- 5 对电子：三角双锥形，90°, 120° (PCl₅)
- 6 electron pairs: octahedral, 90° (SF₆)
- 6 对电子：八面体形，90° (SF₆)
8. Molecular Polarity | 分子极性
A molecule is polar if it has an overall dipole moment— e linear shape cancels the dipoles, making the molecule non-polar. Water (H₂O) has polar O–H bonds and a bent shape, giving it a net dipole and making it a polar solvent.
如果分子具有总偶极矩，那么它就是极性分子——这就要求极性键呈不对称排列，以使键偶极不能相互抵消。VSEPR 预测的形状对于确定净偶极矩至关重要。二氧化碳 (CO₂) 有两个极性的 C=O 键，但直线形结构使偶极相互抵消，因此分子是非极性的。水 (H₂O) 具有极性的 O–H 键，且为 V 形，导致净偶极存在，使其成为极性溶剂。
Symmetry is key: tetrahedral CCl₄ is non-polar because the four C–Cl dipoles cancel, whereas CHCl₃ is polar because the dipoles no longer cancel.
对称性是关键：四面体的 CCl₄ 是非极性的，因为四个 C–Cl 偶极彼此抵消；而 CHCl₃ 是极性的，因为偶极无法完全抵消。
9. 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 enthalpy) is the energy required to break one mole of a bond in the gaseous state. These two properties are inversely related: shorter bonds tend to be stronger. Multiple bonds are shorter and stronger than single bonds between the same pair of atoms.
键长是两个成键原子核之间的平均距离。键能（键焓）是断裂气态下 1 摩尔键所需的能量。这两个性质呈反比关系：键越短，往往越强。在同对原子之间，多重键比单键更短、更强。
- C–C single bond: 154 pm, 348 kJ mol⁻¹
- C–C 单键：154 pm，348 kJ mol⁻¹
- C=C double bond: 134 pm, 614 kJ mol⁻¹
- C=C 双键：134 pm，614 kJ mol⁻¹
- C≡C triple bond: 120 pm, 839 kJ mol⁻¹
- C≡C 三键：120 pm，839 kJ mol⁻¹
Average bond enthalpies allow chemists to estimate enthalpy changes in reactions, though actual values vary with molecular environment.
平均键焓使化学家能够估算反应中的焓变，尽管实际数值会因分子环境而有所不同。
10. Sigma and Pi Bonds | σ 键与 π 键
According to quantum mechanics, covalent bonds are classified by the symmetry of the electron cloud overlap. A sigma (σ) bond forms by head-on overlap of orbitals, with electron density concentrated along the internuclear axis. It is the first bond formed between any two atoms and can be formed by s–s, s–p, or p–p overlap. A pi (π) bond forms by side-on overlap of parallel p orbitals; electron density lies above and below the internuclear axis. π bonds are weaker than σ bonds and occur in double and triple bonds in combination with a σ bond.
根据量子力学，共价键按电子云重叠的对称性进行分类。σ 键由轨道的迎头重叠形成，电子密度集中在核间轴上。它是任意两个原子之间形成的第一个键，可以由 s–s、s–p 或 p–p 重叠形成。π 键则由平行 p 轨道的侧向重叠形成，电子密度分布在核间轴的上方和下方。π 键比 σ 键弱，并与 σ 键共同存在于双键和三键中。
- A double bond contains one σ and one π bond (e.g., C₂H₄).
- 双键包含一个 σ 键和一个 π 键（如 C₂H₄）。
- A triple bond contains one σ and two π bonds (e.g., C₂H₂).
- 三键包含一个 σ 键和两个 π 键（如 C₂H₂）。
11. Hybridisation | 杂化
Hybridisation is the mixing of atomic orbitals to form new hybrid orbitals suitable for the observed bonding geometries. It reconciles VSEPR shapes with the orbital model. The number of hybrid orbitals equals the number of electron domains (bonding + lone pairs) around the central atom.
杂化是原子轨道混合形成新杂化轨道的过程，这些轨道适用于已观测到的成键几何构型。它将 VSEPR 形状与轨道模型统一起来。杂化轨道的数量等于中心原子周围的电子域（成键 + 孤对）的数量。
- sp hybridisation: 2 domains, linear, 180° (BeCl₂, C₂H₂).
- sp 杂化：2 个电子域，直线形，180° (BeCl₂, C₂H₂)。
- sp² hybridisation: 3 domains, trigonal planar, 120° (BF₃, C₂H₄).
- sp² 杂化：3 个电子域，平面三角形，120° (BF₃, C₂H₄)。
- sp³ hybridisation: 4 domains, tetrahedral, 109.5° (CH₄, NH₃, H₂O).
- sp³ 杂化：4 个电子域，四面体形，109.5° (CH₄, NH₃, H₂O)。
In ethene (C₂H₄), each carbon is sp² hybridised; the unhybridised p orbital forms the π bond. In ammonia, nitrogen is sp³ hybridised, with one hybrid occupied by a lone pair, accounting for the pyramidal shape.
在乙烯 (C₂H₄) 中，每个碳都是 sp² 杂化的；未杂化的 p 轨道形成 π 键。在氨中，氮是 sp³ 杂化的，其中一个杂化轨道被孤对电子占据，从而解释了三角锥形的结构。
12. The Covalent Model in Extended Structures | 扩展结构中的共价模型
The covalent model extends beyond discrete molecules to giant covalent (network) structures. Diamond, graphite, graphene, silicon dioxide, and silicon are classic examples. In diamond, each carbon atom is sp³ hybridised, forming a rigid tetrahedral network that makes it extremely hard. In graphite, carbon atoms are sp² hybridised in planar sheets with delocalised π electrons between layers, allowing electrical conductivity. Graphene is a single layer of graphite with extraordinary strength and conductivity.
共价模型不仅适用于离散分子，还延伸到巨型共价（网络）结构中。金刚石、石墨、石墨烯、二氧化硅和硅是典型的例子。在金刚石中，每个碳原子都是 sp³ 杂化的，形成刚性的四面体网络，使其极为坚硬。在石墨中，碳原子采用 sp² 杂化，形成平面片层，层间存在离域 π 电子，因而能够导电。石墨烯是单层石墨，具有非凡的强度和导电性。
- Silicon dioxide (SiO₂) has a tetrahedral network; each silicon is bonded to four oxygens, each oxygen to two silicons.
- 二氧化硅 (SiO₂) 具有四面体网络；每个硅原子与四个氧原子键合，每个氧原子与两个硅原子键合。
- The properties of allotropes like diamond and graphite differ sharply due to the bonding arrangement, not the element itself.
- 同素异形体如金刚石和石墨的性质差异很大，这归因于键合方式而非元素本身。
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