📚 Edexcel A-Level Combined 021: Chemical Bonding and Molecular Shape | 爱德思 A-Level 综合 021：化学键与分子形状
In Edexcel A-Level Combined Science 021, bonding and molecular shape bring together the language of electrons, the architecture of molecules, and the physical properties of substances. A secure grasp of ionic, covalent and metallic bonding, together with VSEPR theory, allows you to predict shape, polarity and intermolecular forces systematically.
在爱德思 A-Level 综合科学 021 中，化学键与分子形状把电子语言、分子空间结构以及物质物理性质联系在一起。熟练掌握离子键、共价键、金属键和价层电子对互斥理论，可以让你系统地预测分子形状、极性和分子间作用力。
1. The Big Picture: Why Bonding Determines Properties | 总体图景：键合如何决定性质
Bonding determines melting points, solubility, conductivity and reactivity. When exam questions ask you to explain a property, they are usually testing whether you can move from particle-level structure to macroscopic behaviour.
化学键类型决定了熔点、溶解度、导电性和反应活性。当考题要求解释某个性质时，通常考查你是否能从微观粒子结构过渡到宏观行为。
- Giant structures — 巨型结构：strong forces throughout the lattice give high melting points; examples include diamond, silicon and sodium chloride. | 整个晶格中作用力强，因此熔点高；例如金刚石、硅和氯化钠。
- Simple molecular structures — 简单分子结构：weak intermolecular forces give low melting and boiling points; examples include I₂, CH₄ and H₂O. | 分子间作用力弱，因此熔点和沸点低；例如碘、甲烷和水。
- Charged particles — 带电粒子：mobile ions or delocalised electrons allow electrical conductivity in molten or aqueous states and in metals. | 可移动的离子或离域电子使熔融态、水溶液中的离子化合物以及金属能够导电。
2. Ionic Bonding and Lattice Energy | 离子键与晶格能
Ionic bonding is the electrostatic attraction between oppositely charged ions in a giant lattice. The ions form when a metal loses electrons and a non-metal gains them, often achieving a noble-gas electron configuration.
离子键是巨型晶格中带相反电荷离子之间的静电吸引力。金属失去电子、非金属得到电子后形成离子，通常达到稀有气体电子排布。
You should be careful with the term "ionic bond": it is not a shared pair, but an attraction acting in all directions around each ion. Lattice energy becomes more exothermic with higher charge and smaller ionic radius, so MgO has a much stronger lattice than NaF.
使用“离子键”一词时要小心：它不是共用电子对，而是在每个离子周围所有方向上都存在的静电吸引。离子电荷越高、离子半径越小，晶格能越负（放热越多），因此 MgO 的晶格比 NaF 强得多。
- High melting point | 熔点高：giant ionic lattice requires a lot of energy to break. | 巨型离子晶格需要大量能量才能破坏。
- Brittle | 脆性：when layers shift, like-charge ions repel and the lattice shatters. | 当离子层发生滑动时，同号离子相互排斥，晶体破碎。
- Conducts when molten or aqueous | 熔融态或水溶液导电：ions become mobile; solid does not conduct because ions cannot move. | 离子可以自由移动；固态不导电是因为离子被固定在晶格中。
3. Covalent Bonding: The Octet Rule and Exceptions | 共价键：八隅体规则与例外
Covalent bonding involves shared electron pairs. The octet rule works for many period 2 elements, but there are important exceptions: expanded octets in PCl₅ and SF₆, and incomplete octets in BF₃.
共价键涉及共用电子对。八隅体规则适用于许多第二周期元素，但也有重要例外：PCl₅ 和 SF₆ 具有扩展八隅体，BF₃ 具有不完全八隅体。
Use dot-and-cross diagrams to show valence electrons. Dative covalent bonds, such as in NH₄⁺ and H₃O⁺, occur when both shared electrons come from one atom. In the exam, label the dative bond clearly with an arrow from the donor atom.
用点叉图表示价电子。配位共价键（如 NH₄⁺ 和 H₃O⁺ 中）由同一个原子提供两个共用电子。考试时要清楚地从给予体原子画箭头标注配位键。
4. Metallic Bonding and Delocalised Electrons | 金属键与离域电子
In metallic bonding, positive metal ions sit in a sea of delocalised electrons. The strong attraction accounts for high melting points and electrical conductivity; the lack of fixed bonds allows layers to slide, making metals malleable and ductile.
在金属键中，正金属离子位于离域电子的“海洋”中。强烈的吸引力解释了金属的高熔点和导电性；没有固定方向性的键使金属层可以滑动，因此金属具有延展性和可塑性。
Trends across a period and down a group can be explained by ionic radius and charge density: Mg²⁺ has stronger metallic bonding than Na⁺ because it has a higher charge and smaller ion, and Al³⁺ is stronger still. This explains the melting point trend Na < Mg < Al in period 3.
周期内和族内的趋势可以用离子半径和电荷密度来解释：Mg²⁺ 的金属键比 Na⁺ 强，因为它电荷更高、离子更小；Al³⁺ 又更强。这解释了第三周期熔点 Na < Mg < Al 的趋势。
5. VSEPR Theory: Five Basic Electron-Pair Geometries | 价层电子对互斥理论：五种基本电子对几何构型
VSEPR theory states that electron pairs around a central atom repel each other and arrange themselves to minimise repulsion. Lone pairs repel more strongly than bonding pairs, so they compress bond angles.
价层电子对互斥理论认为，中心原子周围的电子对相互排斥，并排列成使排斥最小的构型。孤电子对的排斥力大于成键电子对，因此会压缩键角。
Start by counting the total number of electron domains: each single, double or triple bond counts as one domain, and each lone pair counts as one domain. The basic geometry
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