AQA化学 离子键 共价键 金属键 分子间力
AQA 的 A-Level 化学把成键单独列成一节，3.1.3 下面从离子键一直排到分子间作用力，七个小题的内容落在 Paper 1 和 Paper 2 的物化部分。这一块概念不算多，失分点却高度集中：定义里少写一个词，解释题就只剩过程分。下面按考纲顺序把七个小节拆开，把每个知识点在答案里应该出现的措辞和常见的错法一起写清楚，配了数值和例子，可以直接对着真题用。
Bonding sits in section 3.1.3 of the AQA A-level Chemistry specification 7405, and its seven subsections run from ionic bonding to forces between molecules. Material from this block appears in the physical chemistry parts of both Paper 1 and Paper 2. The ideas are not numerous, yet the marks lost each year cluster in the same places: a definition missing one word, or an explanation that describes a trend without naming the force responsible. What follows follows the specification order, pairing each point with the wording an examiner expects and the mistakes that keep appearing in scripts.
离子键与晶格能的数量关系
考纲 3.1.3.1 把离子键定义成带相反电荷的离子在晶格中的静电吸引，in a lattice 这几个词不能省，它把离子键和单纯的"两个离子互相吸引"区分开。晶格由正负离子交替排列构成，整体电中性，所以化学式要按电荷配平：镁离子是 2+，氯离子是 1-，氯化镁写成 MgCl2 而不是 MgCl。
熔点高低取决于拆开晶格需要多少能量。离子电荷越大、半径越小，静电吸引越强，熔点越高。氧化镁由 2+ 和 2- 离子构成，熔点约 2852 摄氏度；氯化钠是 1+ 和 1-，熔点约 801 摄氏度。电荷翻倍让吸引力上升约四倍，同时 2+ 离子半径更小、间距更短，两个因素同向叠加。导电只在熔融或溶于水时出现，固态晶格中离子被固定在格点上，加电压也不能迁移。
AQA writes ionic bonding as the electrostatic attraction between oppositely charged ions in a lattice. Dropping the word lattice drops a mark, because an extended array of alternating charge is what separates ionic bonding from any two ions attracting each other. The array alternates positive and negative ions, so the solid stays neutral overall. Formulas follow charge: magnesium is 2+ and chloride is 1-, hence MgCl2 and not MgCl.
Melting point reflects the energy needed to pull that lattice apart. Larger ionic charge and smaller ionic radius both raise the attraction, so magnesium oxide, built from 2+ and 2- ions, melts near 2852 C while sodium chloride, built from 1+ and 1-, melts near 801 C. Doubling the charge lifts the attraction roughly fourfold, and the smaller 2+ ion shortens the distance at the same time. Conduction happens only when molten or dissolved, since the ions must be free to move; in the solid they sit fixed on lattice sites.
共价键与配位键的电子来源
共价键是一对共享电子，双键和三键对应两对和三对。考纲 3.1.3.2 单独把配位键拎出来：一对电子由同一个原子提供，两个原子共用。铵根离子 NH4+ 是标准例子，氨分子上的孤对电子进入氢离子的空轨道；水合氢离子 H3O+ 同理。画结构式时共价键用一条线，配位键用箭头，箭头从提供电子的一端指向接受的一端。
很多学生以为配位键形成之后和普通共价键不一样，实际上铵根里四个 N-H 键完全等价，键长键角都一样。箭头的意义只在于说明电子来源，不说明键的强弱。另一个高频错误是把双键当成两条分开的单键去加键能，数据题里这样算会偏高，因为双键比单键强，但不到两倍。
A covalent bond is a shared pair of electrons, with double and triple bonds holding two and three pairs. Section 3.1.3.2 treats the dative covalent bond separately: one atom supplies both electrons and the two atoms share them. The ammonium ion is the standard case, where the lone pair on ammonia enters the empty orbital of a hydrogen ion, and the hydroxonium ion works in the same way. Structural formulas draw a covalent bond as a line and a dative bond as an arrow running from the donor to the acceptor.
Once formed, a dative bond behaves like any other covalent bond: the four N-H bonds in ammonium are identical in length and angle. The arrow records where the electrons came from, not how strong the bond is. A second common slip is treating a double bond as two separate single bonds when adding up bond enthalpies, which overestimates the total because a double bond is stronger than two singles but not twice as strong.
金属键中的离域电子与延展性
金属键被描述为离域电子与按晶格排列的正离子之间的吸引。这些电子不属于任何一个原子，在整个金属中自由移动，这也是金属导电和导热的来源：加电压时电子定向移动，一端受热时电子把动能传向另一端。
延展性来自结构本身。离子晶体受力时同种电荷会彼此靠近，静电排斥让晶体沿解理面裂开；金属里正离子浸在电子海中，一层离子滑过另一层之后仍然被同样的离域电子拉住，所以能压成薄片或拉成丝。熔点随金属键强度变化，镁约 650 摄氏度，钠只有约 98 摄氏度，差别来自离域电子的数量和离子电荷密度。
Metallic bonding is the attraction between delocalised electrons and positive ions arranged in a lattice. Those electrons belong to no single atom and move freely through the metal, which explains conduction of both charge and heat: a voltage drives them in one direction, and heating one end transfers kinetic energy along the structure.
Malleability follows from the same picture. In an ionic crystal, stress brings ions of like charge together and the repulsion splits the crystal along a cleavage plane; in a metal, layers of positive ions slide past one another and stay held by the same sea of electrons, so the metal flattens or draws into wire. Melting point tracks the strength of the metallic bond: magnesium melts near 650 C, sodium near 98 C, and the gap comes from the number of delocalised electrons and the charge density of the ions.
四类晶体结构与导电熔点的对应
考纲列了四种晶体结构：离子型、金属型、巨型共价型、分子型，并要求用金刚石、石墨、冰、碘、镁、氯化钠这六个例子来说明。判断熔点先看结构类型：巨型共价要拆开大量共价键，熔点最高；离子型和金属型要破坏晶格；分子型只需克服分子间作用力，熔点最低。
导电性按载流子判断。金属有离域电子，石墨层内也有离域电子，两者固态就导电；离子晶体固态不导电，熔融或溶液里离子自由移动才导电；分子晶体和金刚石既没有自由电子也不含可移动离子，通常不导电。石墨的两个性质来自同一层内的电子，层与层之间靠弱的色散力，所以它摸起来发滑，能当润滑剂。
The specification names four crystal structures, ionic, metallic, macromolecular and molecular, and asks for diamond, graphite, ice, iodine, magnesium and sodium chloride as the examples. Melting point follows the structure type: a macromolecular solid requires many covalent bonds to be broken, an ionic or metallic solid requires its lattice to be dismantled, and a molecular solid requires only its intermolecular forces to be overcome.
Conduction depends on what carries the charge. Metals and graphite both conduct as solids because they contain delocalised electrons, whereas an ionic solid conducts only when molten or in solution, and molecular solids such as iodine do not conduct at all. Graphite combines both features: electrons delocalise within each layer, while the layers themselves are held by weak dispersion forces, which is why graphite feels slippery.
电子对排斥决定键角的顺序
电子对排斥的顺序是孤对与孤对之间最强，孤对与成键对之间次之，成键对与成键对之间最弱。所以甲烷 109.5 度，氨 107 度，水 104.5 度：中心原子周围的电子对数都是四，孤对从零增加到两对，每多一对就把剩下的键角压小一点。
判断形状先数中心原子的电子对数，再数孤对。两对是直线型 180 度，三对是平面三角 120 度，四对是四面体 109.5 度，五对是三角双锥 90 度和 120 度，六对是八面体 90 度。硫酸根和硝酸根用同样的方法，把双键当作一对电子处理。常见的错误是拿整个分子的电子对数去数而不看中心原子，二氧化碳里氧上也有孤对，但形状只由碳周围的两对成键电子决定。
Repulsion is not equal in every direction. A lone pair spreads out more than a bonding pair, so lone pair with lone pair repulsion ranks highest, lone pair with bonding pair comes next, and bonding pair with bonding pair sits lowest. Any central atom carrying four pairs starts from the tetrahedral angle of 109.5 degrees. Ammonia loses 2.5 degrees to its single lone pair and water loses another 2.5, which is why the two values read 107 and 104.5.
To assign a shape, count the electron pairs around the central atom and then count the lone pairs. Two pairs give linear at 180 degrees, three give trigonal planar at 120, four give tetrahedral at 109.5, five give trigonal bipyramidal at 90 and 120, and six give octahedral at 90. Nitrate and sulfate follow the same counting once a double bond is treated as one pair. Counting every electron in the molecule rather than around the central atom is a frequent slip: oxygen in carbon dioxide carries lone pairs, yet the shape is set by the two bonding pairs on carbon.
电负性与分子是否有永久偶极
电负性是一个原子在共价键中吸引成键电子对的能力。鲍林标度里氟 4.0，氧 3.5，氯和氮都是 3.0，碳 2.5，氢 2.1。差值足够大时电子云偏向电负性大的原子，键上出现部分正电荷和部分负电荷，写成 delta 加和 delta 减。
有极性键不等于分子有永久偶极。四氯化碳里四根 C-Cl 键都有极性，但四面体对称排列让四个键的偶极相互抵消，分子整体没有净偶极；三氯甲烷里一个氯被氢取代，对称性破坏，净偶极就出现了。二氧化碳是直线型 180 度，两个 C=O 反向，净偶极为零；水是弯曲的 104.5 度，两个 O-H 不能抵消，所以水是极性分子。判断题要同时写键的极性和分子的几何形状。
Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond. On the Pauling scale fluorine is 4.0, oxygen 3.5, chlorine and nitrogen both 3.0, carbon 2.5 and hydrogen 2.1. Where the difference is large, the electron density shifts towards the more electronegative atom, leaving partial charges shown as delta plus and delta minus.
A polar bond does not guarantee a permanent dipole. Carbon tetrachloride has four polar C-Cl bonds, yet the tetrahedral arrangement cancels them and the molecule carries no net dipole, while replacing one chlorine with hydrogen in trichloromethane breaks that symmetry and a dipole appears. Carbon dioxide is linear at 180 degrees and its two C=O dipoles point in opposite directions, so the net is zero; water is bent at 104.5 degrees, the two O-H dipoles fail to cancel, and the molecule is polar. An answer needs both facts, the polarity of the bonds and the geometry that decides whether they add up.
色散力、偶极作用与氢键的强度排序
三种分子间作用力里，色散力最弱但无处不在，它来自瞬时电子分布不均诱导出的偶极，强度随电子数增加而增大。碘在室温下是固体，溴是液体，氯和氟是气体，这个顺序就是电子数顺序；烷烃的沸点随碳链变长而升高，理由相同。
永久偶极之间的作用比色散力强一些，但比氢键弱。氢键只发生在氢与氮、氧、氟相连的分子之间，能量大约 10 到 40 千焦每摩，比共价键弱一个数量级，却足以把水的沸点抬到 100 摄氏度，而电子数相同的硫化氢在零下 60 摄氏度左右就沸腾了。
Dispersion acts between every pair of molecules, including noble gas atoms, yet it is the weakest of the three interactions. An uneven distribution of electrons at one instant creates a small dipole, and that dipole distorts the electron cloud of a neighbour. More electrons mean a larger induced dipole, so the force grows down a group and along a chain. The familiar sequence of iodine as a solid, bromine as a liquid and chlorine and fluorine as gases follows that trend, as does the rise in boiling point along an alkane series.
Permanent dipole-dipole attraction is stronger than dispersion but weaker than hydrogen bonding. Hydrogen bonding requires hydrogen attached to nitrogen, oxygen or fluorine, and releases roughly 10 to 40 kilojoules per mole, an order of magnitude below a covalent bond yet enough to lift the boiling point of water to 100 C, while hydrogen sulfide with the same number of electrons boils near minus 60 C.
氢键判据与冰的密度反常
氢键的判据只有两条：氢必须直接连在氮、氧、氟上，并且要有孤对电子可以接受。氯的电负性和氮一样是 3.0，但氯原子半径大，电子云分散，氢键弱到可以忽略，所以氯化氢在室温下是气体，而氟化氢的沸点是 19.5 摄氏度。
冰的密度比水小，原因在排列方式。每个水分子在冰里通过氢键连到四个相邻分子，形成敞开的四面体骨架，氢键把分子撑得比液态水中平均间距更远，骨架之间留下空腔，单位体积内的分子数反而比液态水少，所以冰浮在水面上。水在接近 4 摄氏度时密度最大，继续降温时部分氢键开始固定，体积又回涨。
Two conditions identify a hydrogen bond: hydrogen must be bonded directly to nitrogen, oxygen or fluorine, and a lone pair must be available to accept it. Chlorine matches nitrogen at 3.0 on the Pauling scale, yet its larger radius spreads the electron density and any hydrogen bond is negligible, so hydrogen chloride is a gas at room temperature while hydrogen fluoride boils at 19.5 C.
Ice floats because of how the molecules pack. Every water molecule hydrogen bonds to four neighbours, producing an open tetrahedral framework, and the hydrogen bonds hold the molecules further apart than the average separation in liquid water, leaving cavities that lower the number of molecules per unit volume. Water reaches its maximum density near 4 C, and cooling beyond that point starts to lock the framework in place, so the volume expands again.
简答题里最容易丢分的三种表述
第一种是解释沸点时写"破坏共价键"。分子晶体沸腾只克服分子间作用力，共价键不断；巨型共价物质如金刚石熔化时才需要断开大量共价键。第二种是把氢键说成分子内的化学键，或者写成"氢键让水分子更紧密"，后一句在冰的问题上正好说反。
第三种是只给趋势不给原因。写"氧化镁熔点比氯化钠高因为离子键更强"只能拿到部分分，要把电荷和半径两个因素写出来。阅卷时最常看到的一句是 bonding is stronger，没有说明哪种作用力、为什么更强。题目动词也决定答案长度：要求 state 时不要展开解释，要求 explain 时不要只写名称，写多了不加分，写错了倒扣不了分但会挤掉别的时间。
Two phrases cost marks more often than any others. The first is boiling breaks covalent bonds; boiling a molecular substance overcomes intermolecular forces only, while a macromolecular solid such as diamond does need many covalent bonds broken. The second is hydrogen bond treated as a chemical bond inside a molecule, or as a force that pulls water molecules closer together. That last claim reverses what happens in ice, where the hydrogen bonds hold the molecules apart.
The third is a trend with no reason attached. Writing that magnesium oxide melts higher than sodium chloride because its bonding is stronger earns part of the marks; naming the charge and the radius earns the rest. The phrase bonding is stronger appears in scripts constantly without saying which force is meant or why it is stronger. The command word also sets the length: state asks for a name, explain asks for the mechanism.
把上面九节压成一句话：先判断结构类型，再判断粒子之间是什么作用力，最后用电荷、半径、电子对数这些可以量的因素解释强弱。三步走完，成键这一节的解释题基本不会跑偏。
Compressed into one line: identify the structure type, identify which force holds the particles together, then explain the strength with quantities such as charge, radius and electron pair count. Those three steps cover most of the explanation marks in this section.
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