📚 A-Level Chemical Bonding and Molecular Structure: Core Exam Points | A-Level化学键与分子结构核心考点
Chemical bonding and molecular structure sit at the centre of both AS and A2 Chemistry. Almost every later topic - energetics, kinetics, equilibria, periodicity, organic mechanisms and transition metal chemistry - assumes that you can explain why atoms join together and why the resulting shapes matter. In exams the question is rarely just 'define a covalent bond'; it is far more often 'explain why the melting point of X is higher than that of Y'. This guide works through the core content in specification order and highlights the wording that actually earns marks.
化学键与分子结构是 AS 与 A2 化学的核心。后面的能量学、动力学、平衡、周期性、有机反应机理、过渡金属化学等几乎所有专题，都默认你能够解释原子为什么结合、以及由此形成的空间形状为什么重要。考试中很少只考 "共价键的定义"，更常见的是 "解释 X 的熔点为什么高于 Y"。本文按考纲顺序梳理核心内容，并标出真正能拿分的关键表述。
1. Ionic Bonding and Lattice Structure | 离子键与晶格结构
Ionic bonding involves the transfer of one or more electrons from a metal atom to a non-metal atom, leaving a cation and an anion whose outer shells resemble a noble gas configuration. The bond itself is the electrostatic attraction between these oppositely charged ions. Because that attraction is non-directional - it acts equally in every direction - ions build up a giant three-dimensional lattice instead of discrete pairs. The formula NaCl therefore shows the ratio of Na⁺ to Cl⁻ ions in the lattice, not a molecule.
离子键是电子从金属原子完全转移到非金属原子上，形成外层电子构型类似稀有气体的阳离子和阴离子。化学键本身是这些带相反电荷离子之间的静电引力。由于这种引力没有方向性，在各个方向上都一样，离子会堆积成巨大的三维晶格，而不是独立的原子对。所以 NaCl 这个式子表示晶格中 Na⁺ 与 Cl⁻ 的个数比，并不代表一个分子。
Lattice enthalpy is the enthalpy change when one mole of a solid ionic lattice is formed from its gaseous ions. It becomes more exothermic as ionic charge increases and as ionic radius decreases, because both changes strengthen the electrostatic attraction. This is why MgO, with 2+ and 2− ions, has a lattice enthalpy of about −3889 kJ mol⁻¹ and melts near 2852 °C, while NaCl melts at only 801 °C.
晶格焓是指由气态离子生成 1 摩尔固态离子晶格时的焓变。离子电荷越高、离子半径越小，晶格焓越放热，因为这两个因素都会增强静电引力。所以带 2+ 与 2− 电荷的 MgO 晶格焓约为 −3889 kJ mol⁻¹，熔点接近 2852 °C，而 NaCl 的熔点只有 801 °C。
Na⁺(g) + Cl⁻(g) → NaCl(s) ΔH = −787 kJ mol⁻¹
Ionic solids are hard, brittle and high melting because the whole lattice is held together by strong electrostatic forces. They do not conduct electricity when solid, since the ions cannot move, but they do conduct when molten or dissolved in water because the ions become mobile. Dissolution depends on a balance: the energy released when water molecules hydrate the ions must be enough to overcome the lattice enthalpy.
离子晶体坚硬、易碎、熔点高，因为整个晶格靠强静电作用维持。固态时不导电，因为离子不能移动；熔融或溶于水时能导电，因为离子可以自由移动。溶解能不能发生，取决于水合离子释放的能量能否补偿破坏晶格所需的能量。
2. Covalent Bonding and Bond Strength | 共价键与键的强弱
A covalent bond is a shared pair of electrons between two atoms, arranged so that each atom counts the shared pair towards a noble gas electron count. Dot-and-cross diagrams show this clearly: dots represent the outer electrons of one atom, crosses those of the other, and the overlap region is the shared pair. A single bond shares one pair, a double bond two pairs, and a triple bond three pairs.
共价键是两个原子之间共用的一对电子，使双方都达到稀有气体的电子数。点叉图能清楚表示这一点：点表示一个原子的外层电子，叉表示另一个原子的外层电子，重叠区域就是共用电子对。单键共用一对电子，双键共用两对，三键共用三对。
sideways overlap of p orbitals and has electron density above and below the bond axis, where the electrons are more exposed. A pi bond is therefore weaker than the corresponding sigma bond, which is one reason why molecules containing C=C readily undergo addition reactions.
从轨道角度看，σ 键来自轨道 "头碰头" 重叠，电子密度集中在两个原子核连线上；π 键来自 p 轨道 "肩并肩" 侧面重叠，电子密度分布在键轴上下方，电子暴露程度更高。因此 π 键比相应的 σ 键弱，这也是含 C=C 的分子容易发生加成反应的原因之一。
length and bond enthalpy move in opposite directions. Hydrogen halides illustrate this: H–F is 568 kJ mol⁻¹, H–Cl about 432, H–Br 366 and H– 299, because the overlap becomes less effective as the halogen gets larger. For carbon, C–C is 346 kJ mol⁻¹, C=C is 614 and C≡C is 839, so a double bond is stronger than a single bond but less than twice as strong.
键长与键能的变化方向相反。卤化氢就是典型例子：H–F 为 568 kJ mol⁻¹，H–Cl 约 432，H–Br
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