📚 S2 Models of Bonding and Structure | 化学键合与结构模型
Why is diamond hard but graphite soft? Why does salt dissolve in water while cooking oil does not? The answers lie in the models of bonding and structure that chemists use to explain and predict the properties of substances. In this IB Chemistry topic (S2), we explore ionic, covalent, and metallic bonding, the different structures they form, and the intermolecular forces that hold molecules together. By linking bonding to observable behaviour – melting point, conductivity, solubility – we build a powerful toolkit for understanding the material world around us.
为什么金刚石坚硬而石墨柔软？为什么食盐在水中溶解而食用油不能？答案就在于化学家用来解释和预测物质性质的键合与结构模型。在这个 IB 化学专题（S2）中，我们将探索离子键、共价键和金属键，它们所形成的不同结构，以及将分子聚集在一起的分子间作用力。通过将键合与可观察到的行为——熔点、导电性、溶解度——联系起来，我们构建了一个理解周围物质世界的强大工具包。
1. Introduction to Bonding and Structure | 键合与结构导论
Chemical bonding is the glue that holds atoms together. The type of bonding and the resulting three-dimensional structure determine nearly all physical and chemical properties of a substance. We classify bonding into three main types: ionic, covalent, and metallic. In addition, the forces between particles – whether ions, simple molecules, or polymer chains – further influence macroscopic behaviour. Understanding the models allows us to make predictions, even about unfamiliar compounds.
化学键是将原子聚在一起的胶水。键合的类型以及由此产生的三维结构几乎决定了物质的所有物理和化学性质。我们将键合分为三种主要类型：离子键、共价键和金属键。此外，粒子之间的作用力——无论是离子、简单分子还是聚合物链——进一步影响宏观行为。理解这些模型使我们能够做出预测，甚至对陌生物质也是如此。
2. Ionic Bonding and Giant Ionic Lattices | 离子键与巨型离子晶格
Ionic bonding occurs between metals (which lose electrons to form positive cations) and non-metals (which gain electrons to form negative anions). The strong electrostatic attraction between oppositely charged ions holds them in a regular, repeating giant ionic lattice. Each cation is surrounded by anions and vice versa, maximising attractions and minimising repulsions. This structure explains high melting and boiling points: a great deal of energy is needed to overcome the many strong ionic bonds.
离子键发生在金属（失去电子形成正阳离子）和非金属（获得电子形成负阴离子）之间。带相反电荷的离子之间强大的静电吸引力将它们固定在一个规则的、重复的巨型离子晶格中。每个阳离子被阴离子包围，反之亦然，使吸引力最大化、排斥力最小化。这种结构解释了高熔点和高沸点：需要大量能量来克服众多强大的离子键。
Ionic compounds do not conduct electricity when solid because the ions are locked in place. However, when molten or dissolved in water, the ions become mobile, and the substance conducts electricity. Brittleness is another typical property: a sharp blow shifts layers of ions, aligning like charges and causing the crystal to shatter.
离子化合物在固态时不导电，因为离子被固定在位置上。然而，当熔化或溶于水时，离子变得可移动，物质便能导电。脆性是另一个典型性质：一个突然的敲击会使离子层发生位移，使相同电荷对齐，导致晶体碎裂。
- Example: NaCl, MgO, CaCl₂ | 示例：NaCl、MgO、CaCl₂
- High lattice energy correlates with smaller ion size and higher charge. | 离子半径越小、电荷越高，晶格能越大。
3. Covalent Bonding: Simple Molecular and Giant Covalent | 共价键：简单分子与巨型共价结构
Covalent bonding involves the sharing of electron pairs between non-metal atoms. This sharing can occur in small, discrete molecules (simple molecular substances) or in extensive networks (giant covalent structures). In a simple molecular substance such as iodine (I₂) or water (H₂O), the atoms within each molecule are held by strong covalent bonds, but separate molecules are attracted only by weak intermolecular forces. As a result, these substances have low melting and boiling points.
共价键涉及非金属原子之间共享电子对。这种共享可以发生在小的离散分子中（简单分子物质）或大型网络（巨型共价结构）中。在简单分子物质如碘（I₂）或水（H₂O）中，每个分子内的原子由强共价键连接，但分子之间仅由弱的分子间作用力吸引。因此，这些物质具有低熔点和低沸点。
In giant covalent structures, such as diamond and quartz (SiO₂), all atoms are linked by a continuous network of covalent bonds. These substances are extremely hard and have very high melting points because melting requires breaking strong covalent bonds throughout the entire structure. They do not conduct electricity (except graphite, which has delocalised electrons).
在巨型共价结构中，如金刚石和石英（SiO₂），所有原子通过一个连续的共价键网络连接在一起。这些物质非常坚硬，熔点极高，因为熔化需要打破整个结构中的强共价键。它们不导电（石墨除外，石墨具有离域电子）。
4. Metallic Bonding | 金属键
Metallic bonding is described as a lattice of positive metal ions surrounded by a 'sea' of delocalised valence electrons. These electrons are free to move throughout the structure, which explains the electrical and thermal conductivity of metals. The strong electrostatic attraction between cations and the electron sea gives metals their malleability and ductility: when layers of ions slide past one another, the delocalised electrons can readjust, keeping the metallic bonds intact rather than causing the material to fracture.
金属键被描述为由正金属离子组成的晶格，周围是离域价电子的“海洋”。这些电子可以在整个结构中自由移动，这解释了金属的导电性和导热性。阳离子与电子海之间的强大静电吸引力赋予了金属延展性和韧性：当离子层相互滑动时，离域电子可以重新调整，保持金属键完整，而不会导致材料断裂。
- Alloys: mixing elements can disrupt the regular lattice, making the alloy harder and less malleable than the pure metal. | 合金：混合元素可以破坏规则晶格，使合金比纯金属更硬、韧性更差。
- Trends: melting point generally increases with ionic charge and decreasing ion size (e.g., Na < Mg < Al). | 趋势：熔点通常随离子电荷增加和离子半径减小而升高（如 Na < Mg < Al）。
5. Intermolecular Forces: London Dispersion, Dipole-Dipole, Hydrogen Bonding | 分子间作用力：伦敦色散力、偶极-偶极作用、氢键
Intermolecular forces (IMFs) are attractive forces between molecules. They are much weaker than ionic, covalent, or metallic bonds, yet they dictate boiling points, solubility, and other properties of molecular substances. There are three key IMFs: London dispersion forces (present in all molecules), dipole-dipole interactions (polar molecules), and hydrogen bonding (a special strong dipole-dipole interaction when H is bonded to N, O, or F).
分子间作用力是分子之间的吸引力。它们比离子键、共价键或金属键弱得多，但它们决定了分子物质的沸点、溶解度和其他性质。关键的分子间作用力有三种：伦敦色散力（所有分子中都存在）、偶极-偶极相互作用（极性分子）和氢键（当 H 与 N、O 或 F 键合时的一种特殊强偶极-偶极相互作用）。
| Intermolecular Force | 分子间作用力 | Origin | 起源 | Relative Strength | 相对强度 |
|---|---|---|
| London dispersion forces | 伦敦色散力 | Instantaneous and induced dipoles | 瞬时偶极与诱导偶极 | Weakest, increases with greater electron cloud polarisability and molecular surface area |
| Dipole-dipole | 偶极-偶极 | Attraction between oppositely charged ends of permanent dipoles | Stronger than London forces for comparable molar mass |
| Hydrogen bonding | 氢键 | Highly polarised H–N/O/F bond + lone pair on adjacent molecule | Strongest IMF; responsible for anomalously high boiling points of H₂O, NH₃, HF |
When comparing molecules of similar molar mass, hydrogen bonding leads to the highest boiling points, followed by dipole-dipole, then London forces. However, for very large non-polar molecules, London forces can become dominant due to increased polarisability.
当比较摩尔质量相似的分子时，氢键导致最高的沸点，其次是偶极-偶极，然后是伦敦色散力。然而，对于非常大的非极性分子，由于极化率增加，伦敦色散力可能占据主导地位。
6. 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 minimise repulsion. This arrangement dictates the three-dimensional shape of a molecule or ion. Both bonding pairs (BP) and lone pairs (LP) count as regions of electron density; the repulsion order is: LP–LP > LP–BP > BP–BP. This explains why molecular geometries can deviate from ideal angles when lone pairs are present.
价层电子对互斥理论（VSEPR）指出，中心原子周围的电子对会排列以使排斥力最小化。这种排列决定了分子或离子的三维形状。键合电子对和孤电子对都视为电子区域；排斥力顺序为：孤对-孤对 > 孤对-键对 > 键对-键对。这解释了当存在孤对电子时，分子几何构型为何会偏离理想角度。
| Electron Domains (BP + LP) | 电子域（键对+孤对） | Example | 示例 | Shape Name | 形状名称 | Bond Angle | 键角 |
|---|---|---|---|
| 2 (2 BP, 0 LP) | BeCl₂, CO₂ | Linear | 直线形 | 180° |
| 3 (3 BP, 0 LP) | BF₃, SO₃ | Trigonal planar | 平面三角形 | 120° |
| 3 (2 BP, 1 LP) | SO₂, O₃ | Bent (V-shaped) | 弯曲形 | < 120° |
| 4 (4 BP, 0 LP) | CH₄, NH₄⁺ | Tetrahedral | 四面体 | 109.5° |
| 4 (3 BP, 1 LP) | NH₃ | Trigonal pyramidal | 三角锥形 | 107° |
| 4 (2 BP, 2 LP) | H₂O | Bent (V-shaped) | 弯曲形 | 104.5° |
| 5 (5 BP, 0 LP) | PF₅ | Trigonal bipyramidal | 三角双锥 | 90°, 120°, 180° |
| 6 (6 BP, 0 LP) | SF₆ | Octahedral | 八面体 | 90°, 180° |
Remember: the shape is named based on the arrangement of atoms (bonding pairs) only, not the lone pairs. However, lone pairs influence the bond angles because they exert greater repulsion.
记住：形状是根据原子（键对）的排列来命名的，而不是孤对电子。然而，孤对电子会影响键角，因为它们施加更大的排斥力。
7. Bond Polarity and Molecular Polarity | 键的极性与分子极性
A covalent bond is polar when the two atoms have different electronegativities, creating a partial charge separation: δ⁺ on the less electronegative atom and δ⁻ on the more electronegative one. The greater the electronegativity difference, the more polar the bond. A bond becomes ionic when the difference is very large (usually > 1.7 on the Pauling scale), but the boundary is a continuum; many compounds have significant covalent character.
当两个原子的电负性不同时，共价键是极性的，从而产生部分电荷分离：电负性较小的原子带 δ⁺，电负性较大的原子带 δ⁻。电负性差越大，键的极性越强。当差异非常大时（通常在 Pauling 标度上 > 1.7），键成为离子键，但边界是连续的；许多化合物具有显著的共价特性。
A molecule may be non-polar overall even if it contains polar bonds, provided the dipole moments cancel vectorially. For example, CO₂ is linear and non-polar, while H₂O is bent and polar. Symmetry is key; tetrahedral CCl₄ is non-polar because the four C–Cl dipoles cancel, whereas CHCl₃ is polar because one C–H bond differs, breaking the symmetry.
如果偶极矩在向量上相互抵消，即使含有极性键，分子整体也可能是非极性的。例如，CO₂ 是直线形且非极性，而 H₂O 是弯曲形且极性。对称性至关重要；四面体 CCl₄ 是非极性的，因为四个 C–Cl 偶极相互抵消，而 CHCl₃ 是极性的，因为有一个不同的 C–H 键打破了对称性。
8. Resonance and Delocalisation | 共振与离域
Some molecules or ions cannot be accurately described by a single Lewis structure because the electrons are delocalised over multiple atoms. This phenomenon is called resonance. The actual structure is a resonance hybrid, a blend of the possible contributing structures. Classic examples include the carbonate ion (CO₃²⁻) and the nitrate ion (NO₃⁻), where all bond lengths are identical and intermediate between single and double bonds. Delocalisation stabilises the ion, lowering its energy.
有些分子或离子无法用单个路易斯结构准确描述，因为电子在多个原子上离域。这种现象称为共振。实际结构是一个共振杂化体，是可能贡献结构的混合物。典型例子包括碳酸根离子（CO₃²⁻）和硝酸根离子（NO₃⁻），其中所有键长都相同，介于单键和双键之间。离域作用使离子稳定，降低了其能量。
In benzene (C₆H₆), the six π electrons are delocalised in a ring above and below the plane of carbon atoms, giving identical C–C bond lengths (139 pm) and exceptional stability. The delocalised model also explains the electrical conductivity of graphite and the unique properties of conjugated organic molecules.
在苯（C₆H₆）中，六个 π 电子在碳原子平面的上方和下方形成离域环，使得所有 C–C 键长相同（139 pm）并具有卓越的稳定性。离域模型还解释了石墨的导电性以及共轭有机分子的独特性质。
9. Allotropes of Carbon: Diamond, Graphite, Graphene, Fullerenes | 碳的同素异形体：金刚石、石墨、石墨烯、富勒烯
Carbon forms several allotropes with strikingly different properties, all arising purely from bonding and structure. Diamond: each carbon is tetrahedrally bonded to four other carbons in a rigid 3D network; it is the hardest known natural substance, an electrical insulator, and a superb thermal conductor. Graphite: carbon atoms are arranged in flat layers of hexagonal rings; within layers, strong covalent bonds exist, but between layers only weak London forces operate, allowing layers to slide – hence its lubricating property and use in pencils. Delocalised electrons between layers enable electrical conductivity parallel to the layers.
碳形成多种同素异形体，它们有着截然不同的性质，这些都完全源于键合与结构。金刚石：每个碳与另外四个碳形成四面体键，构成刚性的三维网络；它是已知最硬的天然物质，电绝缘体，却是极好的热导体。石墨：碳原子排列成六边形环的平面层；层内存在强共价键，但层间只有弱的伦敦色散力，允许层与层之间滑动——因此具有润滑性，可用于铅笔。层间的离域电子使得平行于层的方向具有导电性。
Graphene is a single layer of graphite, just one atom thick. It is incredibly strong, transparent, and an excellent conductor of electricity and heat. Fullerenes, such as buckminsterfullerene (C₆₀), are discrete molecular cages that can act as simple molecular solids with relatively low sublimation points. Each allotrope demonstrates how identical atoms can produce vastly different macroscopic properties through variation in bonding and arrangement.
石墨烯是单层的石墨，厚度仅为一个原子。它强度极高、透明，是极佳的电和热导体。富勒烯，如巴克敏斯特富勒烯（C₆₀），是离散的分子笼，其行为类似于简单分子固体，具有相对较低的升华点。每一种同素异形体都展示了相同的原子如何通过键合与排列方式的变化，产生截然不同的宏观性质。
10. Predicting Physical Properties from Bonding and Structure | 从键合与结构预测物理性质
Once we identify the bonding type and structure, we can predict physical properties reliably. High melting point substances usually have either an ionic lattice, a giant covalent network, or a metallic lattice. Low melting point substances are typically simple molecular solids with weak IMFs. Electrical conductivity is expected in metals (solid and liquid), molten/dissolved ionic compounds, and graphite (in specific directions). Solubility follows the 'like dissolves like' rule: polar and ionic substances dissolve in polar solvents such as water, while non-polar substances dissolve in non-polar solvents.
一旦我们确定了键合类型和结构，就可以可靠地预测物理性质。高熔点物质通常具有离子晶格、巨型共价网络或金属晶格。低熔点物质通常是具有弱分子间作用力的简单分子固体。金属（固态和液态）、熔融/溶解的离子化合物以及石墨（在特定方向上）预期具有导电性。溶解度遵循“相似相溶”规则：极性和离子物质溶于水等极性溶剂，而非极性物质溶于非极性溶剂。
A systematic comparison can be made using the following categories: ionic, giant covalent, simple molecular, and metallic. For each, ask: What particles are present? What forces hold them together? How much energy is needed to separate them? Applying this logic consistently will help you explain observations or deduce the type of bonding in an unknown sample from its properties.
可以使用以下类别进行系统比较：离子型、巨型共价型、简单分子型和金属型。对于每一种，问自己：存在什么样的粒子？什么力将它们保持在一起？需要多少能量才能将它们分开？始终如一地应用这种逻辑，将帮助你解释观察结果，或根据性质推断未知样品的键合类型。
11. Common Pitfalls and Key Tips | 常见误区与关键提示
- Misidentifying forces: Do not confuse intermolecular forces with covalent bonds. When a simple molecular substance melts, it is the IMFs that break, not the covalent bonds within the molecule. | 混淆作用力：不要将分子间作用力与共价键混淆。当简单分子物质熔化时，断裂的是分子间作用力，而不是分子内的共价键。
- VSEPR memorisation: Learn the shapes by counting electron domains and lone pairs, not by rote for each molecule. | VSEPR 记忆：通过数电子域和孤对电子来学习形状，而不是死记硬背每个分子。
- Polarity vs. shape: Always draw a 3D representation and consider vector addition of bond dipoles to decide overall polarity. | 极性与形状：始终绘制三维表示图，并考虑键偶极的向量叠加来决定整体极性。
- Alloy strength: Remember that adding a second element to a metal usually increases hardness due to lattice distortion, which hinders layer sliding. | 合金强度：记住向金属中加入第二种元素通常会因晶格畸变而增加硬度，这阻碍了层间滑动。
- Use precise language: 'Weak intermolecular forces' rather than 'weak bonds'. | 使用精确语言：“弱的分子间作用力”而不是“弱键”。
12. Linking Models to the IB Assessment | 模型与 IB 评估的联系
In IB examinations, questions on bonding and structure often ask you to 'explain' or 'suggest' based on data such as melting points, electrical conductivities, or solubility. You are expected to choose the appropriate model and use it to justify the property. Data-based questions may require you to differentiate between substances of similar formula mass but different types of bonding, or to discuss trends across a period. Strengthen your answers by using key terms like delocalised, lattice, electrostatic attraction, and IMFs, and always connect the microscopic to the macroscopic.
在 IB 考试中，关于键合与结构的问题常常要求你根据熔点、导电性或溶解度等数据来“解释”或“提出建议”。你需要选择合适的模型并用它来证明该性质。基于数据的问题可能要求你区分化学式量相似但键合类型不同的物质，或讨论周期内的变化趋势。通过使用“离域”、“晶格”、“静电吸引”和“分子间作用力”等关键术语来强化你的答案，并始终将微观与宏观联系起来。
Published by TutorHao | IB Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图