AQA化学 ALevel 离子键 共价键 氢键 电负性
AQA 的 A-level 化学(7405)把 bonding 单独列成 3.1.3 一节，下面分七个子节，从离子晶格一直排到分子间作用力。这一节的内容考在 Paper 1(7405/1)上，那张卷子 2 小时、105 分、占整个 A-level 的 35%，范围包括 3.1.1 到 3.1.4、3.1.6 到 3.1.8 与 3.1.10 到 3.1.12 的物理化学、全部无机化学以及相关的实验技能(AQA 官方 spec，2015 年 9 月起施教，2017 年首次 A-level 考试)。很多学生能把离子键背成静电吸引，却答不出氧化镁的熔点为什么比氯化钠高两千摄氏度。这篇按考纲顺序走一遍三种化学键、键的极性和四类分子间作用力，每处都给出可以写进答题卷的表述与失分位置。
The 7405 specification gives bonding its own section, 3.1.3, and it runs from ionic lattices down to intermolecular forces across seven sub-topics. It is examined in Paper 1, a two-hour paper worth 105 marks and 35% of the A-level covering the physical chemistry of sections 3.1.1 to 3.1.4, 3.1.6 to 3.1.8 and 3.1.10 to 3.1.12 alongside all of the inorganic content and the related practical skills; first teaching began in September 2015 and the first A-level exams followed in 2017. Many students can recite that ionic bonding is electrostatic attraction yet cannot explain why magnesium oxide melts about 2000 degrees C higher than sodium chloride, so the article works through the three bond types, bond polarity and the four kinds of intermolecular force in specification order.
离子键为什么必须写出晶格
离子键的定义是带相反电荷的离子之间的静电吸引，这些离子排列成规则的晶格。答题时只写 attraction between ions 会丢掉晶格那一个给分点，因为离子既不配对也不成团，它们在整个晶体里按固定位置排下去。氯化钠的晶格中每个钠离子周围有六个氯离子，配位数记作 6:6，所以它的化学式写成 NaCl 而不是 NaCl2。考纲还要求会写常见多原子离子的化学式：硫酸根 SO4 2-、氢氧根 OH-、硝酸根 NO3-、碳酸根 CO3 2-、铵根 NH4+，写错一个电荷就会连带影响离子方程式。判断简单离子的电荷看元素在周期表中的位置，第一主族失一个电子成正一价，第六主族得两个电子成负二价，过渡金属不遵守这条规律。阅卷时错得最多的是把硫酸根写成 SO4-，整道题的电荷平衡跟着作废。
Ionic bonding is the electrostatic attraction between oppositely charged ions arranged in a lattice; the word lattice carries a mark of its own, because the ions neither pair up nor cluster but sit at fixed positions throughout the crystal. Sodium chloride places each Na+ ion among six Cl- ions, described as 6:6 coordination, which is why the formula is NaCl and not NaCl2; you also need the formulas of compound ions such as sulfate, hydroxide, nitrate, carbonate and ammonium. Students lose most often by writing sulfate as SO4- and taking charge balance down with it.
电荷与半径决定离子晶体的熔点
氧化镁的熔点 2852 摄氏度，氯化钠只有 801 摄氏度，差距来自离子电荷与离子半径两件事。电荷越高，离子之间的静电吸引越强，拆开晶格所需要的能量就越多；半径越小，两个电荷中心靠得越近，吸引同样变强。把这两点合成一句话，就是电荷密度越高，晶格越难破坏，这是解释题里最省字也最准确的表述。Mg2+ 的半径约 72 pm，Na+ 约 102 pm，氧离子又带两个负电荷，所以氧化镁在电荷与距离两项上都占优。真题常问为什么氟化钠的熔点 993 摄氏度高于氯化钠的 801 摄氏度，答案落在 F- 半径约 133 pm、小于 Cl- 的约 181 pm 上，电荷相同而距离更近。很多学生以为离子越大吸引越强，把半径的方向写反，这道题就白写。
Magnesium oxide melts at 2852 degrees C against 801 for sodium chloride, and the difference comes from ionic charge and ionic radius; a higher charge strengthens the electrostatic attraction, while a smaller radius brings the charge centres closer together. Combine the two and you get charge density, the phrase to use when an explanation question asks you to compare lattice strengths, as with Mg2+ at roughly 72 pm against 102 pm for Na+. A common question asks why sodium fluoride melts at 993 degrees C rather than 801, and the answer is fluoride's 133 pm radius against 181 pm for chloride at equal charge.
共价键与配位键的电子来源
共价键是一对共用电子，双键共用两对，三键共用三对，画结构式时用一条线代表一对电子。配位键的不同只在电子的来源：两个电子都由同一个原子提供，画法是从提供电子的原子画一个箭头指向接受电子的原子。铵根是必会的例子，氨分子上的孤对电子进入氢离子空的 1s 轨道，写成 H3N -> H+，产物四个氮氢键完全等价，离子呈正四面体，键角 109.5 度。水合氢离子同理，水分子提供孤对电子给氢离子，写成 H2O -> H+。这一步最容易出的错是在成品结构里还留着箭头，AQA 只要求在描述成键过程的那一步用箭头，结构式照旧画普通短线。三氟化硼与氨的反应是配位键的另一种考法，硼原子缺电子，接受氨的孤对电子。
A covalent bond is a shared pair of electrons, a double bond shares two pairs and a triple bond three; a dative or co-ordinate bond differs only in that one atom supplies both electrons of the pair. Draw it as an arrow from the donor to the acceptor, as in H3N -> H+, and note that the finished ammonium ion has four identical N-H bonds with a tetrahedral shape and angles of 109.5 degrees. Examiners expect the arrow in the step that describes bond formation rather than in the final structure, so leaving it in costs a mark; boron trifluoride reacting with ammonia is the other standard case, since boron is electron deficient.
金属键与离域电子
金属键的定义是离域电子与排列成晶格的正离子之间的吸引，这个模型一次解释三件事。金属导电，因为离域电子能在电场下移动；金属导热，因为电子把能量从高温处带到低温处；金属能锤成薄片，因为一层正离子在另一层上滑动之后，周围的离域电子仍然把它们黏在一起，所以金属不会像离子晶体那样一敲就碎。钠的熔点只有 97.8 摄氏度，每个钠原子只贡献一个离域电子；镁的熔点 650 摄氏度，因为它贡献两个；铝贡献三个，用镁铝合金做飞机结构件就是这个道理。学生最常把金属键说成金属原子之间的键，漏掉离域电子与正离子这两个主体，两分的解释题只能拿一分。
Metallic bonding is the attraction between delocalised electrons and positive ions arranged in a lattice, and that single model explains three properties at once; the electrons move under an electric field so the metal conducts, they carry energy from hot regions to cold ones so it conducts heat, and they keep holding the layers of ions together after one layer slides over another, which is why a metal bends instead of shattering like an ionic crystal. Sodium melts at only 97.8 degrees C because each atom releases one delocalised electron, magnesium reaches 650 with two, and aluminium with three gives the strength behind magnesium-aluminium aircraft parts. A frequent error is describing metallic bonding as bonds between metal atoms, which leaves out both the electrons and the ions.
平均键焓为什么只是平均值
键焓是把一摩尔气态分子中的某个键完全断开、生成气态原子所需的能量，单位是 kJ/mol。氢气的氢氢键焓 436，氯气的氯氯键焓 243，这两个数值可以直接测量，因为都来自同一种双原子分子。氯化氢的氢氯键焓 432 却是平均值，原因是同一对原子在不同分子里的环境不同，CH3Cl 里的碳氯键与 CHCl3 里的碳氯键断键所需能量并不相等，把各种分子的测量值平均才得到 432 这个数。用平均值算反应焓会带来几个 kJ/mol 的误差，AQA 的评分方案接受这种误差，前提是你在答案里说明用的是平均键焓。反应 H2(g) + Cl2(g) -> 2HCl(g) 的焓变可以这样算：断键吸热 436 加 243 等于 679，成键放热 2 乘 432 等于 864，相减得到 -185 kJ/mol。
A bond enthalpy is the energy needed to break one mole of a particular bond in gaseous molecules to give gaseous atoms, and it is measured in kJ/mol; H-H at 436 and Cl-Cl at 243 are direct measurements because each comes from a single diatomic molecule. The 432 for H-Cl is a mean value instead, since the same pair of atoms sits in different surroundings in different molecules, so the C-Cl bond in CH3Cl and in CHCl3 does not need identical energy to break. Examiners accept the small error a mean introduces provided you state that a mean was used; for H2 + Cl2 -> 2HCl, breaking gives 436 + 243 = 679 and forming gives 2 x 432 = 864, so the enthalpy change is -185 kJ/mol.
四种晶体类型与熔点导电性
考纲把晶体分成四类：离子晶体如氯化钠与氧化镁，金属晶体如钠与镁，巨分子晶体即巨型共价结构如金刚石与石墨，分子晶体如碘与冰。碘在 114 摄氏度熔化，金刚石要到约 3550 摄氏度，两者内部都有强共价键，差别在于熔化时必须破坏什么，碘只需要克服分子之间的范德华力，金刚石必须断开整片共价键网络。这类对比题要写分子间作用力与巨型共价结构，写成化学键强弱会吃亏。石墨是同一类里的特例，层内是共价键，层间靠范德华力，所以它软、熔点高、而且能导电，因为每个碳原子还剩一个离域电子可以在层内移动。导电性判断只问一件事，电荷能不能移动：金属与石墨可以，离子晶体只有在熔融或溶解之后才可以，固态氯化钠不导电。学生常把石墨的层间作用力写成共价键，这样解释不了它为什么软。
The specification sorts crystals into four types; ionic such as sodium chloride and magnesium oxide, metallic such as sodium and magnesium, macromolecular or giant covalent such as diamond and graphite, and molecular such as iodine and ice. Iodine melts at 114 degrees C while diamond needs about 3550, and both contain strong covalent bonds inside their units, so the difference lies in what melting actually breaks: iodine only has to overcome van der Waals forces between molecules, whereas diamond has to break the whole covalent network. Graphite is the awkward case, with covalent bonds inside each layer and van der Waals forces between layers, which explains why it is soft, melts at a high temperature and still conducts, since every carbon keeps one delocalised electron.
电负性与键的极性
电负性的定义是一个原子吸引共价键中那对电子的能力，AQA 用鲍林标度，氟 4.0 最高，氧 3.5，氮与氯都是 3.0，碳 2.5，氢 2.1。两个原子的电负性不同，共用电子就偏向电负性大的那一端，偏负的一端标 δ-，偏正的一端标 δ+，这样的键叫极性共价键。极性键不等于极性分子，因为分子的形状会把键偶极加和或者互相抵消。二氧化碳是直线形，两个碳氧偶极方向相反、大小相等，合偶极为零，所以整个分子没有永久偶极；四氯化碳的四个碳氯键指向正四面体的四个顶点，同样抵消。水就不一样，两个氧氢键的夹角 104.5 度，偶极无法抵消，分子有永久偶极，所以水在电场里会转向。常见的错误是看到极性键就断定分子有极性，漏掉形状这一步。
Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond, and AQA uses the Pauling scale, where fluorine is 4.0, oxygen 3.5, nitrogen and chlorine 3.0, carbon 2.5 and hydrogen 2.1. When the two atoms differ, the shared pair sits closer to the more electronegative one, so that end is labelled δ- and the other δ+, giving a polar covalent bond; symmetry then decides whether the molecule as a whole carries a permanent dipole. Carbon dioxide is linear and its two equal C=O dipoles point in opposite directions, so they cancel and the molecule is non-polar, exactly as the four C-Cl bonds of tetrachloromethane cancel across a tetrahedron; water with its 104.5 degree angle cannot cancel, which is why it turns in an electric field.
孤对电子把键角压小
AQA 用价层电子对互斥理论判断形状，规则只有一句：中心原子外层的电子对排到彼此距离最远的位置，使排斥降到最小。排斥的强弱顺序是孤对与孤对最大，其次是孤对与成键对，成键对与成键对最小。甲烷的四个电子对全部成键，排斥均等，键角 109.5 度，形状正四面体。氨有四个电子对，其中一个是孤对，孤对的电子云更靠近中心原子、占据更大空间，把剩下三个氮氢键压过来，键角降到 107 度，形状三角锥。水有两个孤对，键角进一步降到 104.5 度，形状折线形。这一步最容易只写键角数字不写理由，分数停在结果分那一档，拿不到解释分。
VSEPR in AQA is one rule; pairs of electrons in the outer shell sit as far apart as possible so that repulsion is minimised, and lone pair-lone pair repulsion exceeds lone pair-bond pair, which in turn exceeds bond pair-bond pair. Methane has four bonding pairs with equal repulsion, giving a tetrahedral shape and angles of 109.5 degrees, while ammonia carries one lone pair whose cloud sits nearer the central atom and pushes the three N-H bonds to 107 degrees in a trigonal pyramidal shape. Water holds two lone pairs, so the angle drops again to 104.5 degrees and the shape is bent; roughly 2.5 degrees per lone pair is the working figure for comparison questions.
诱导偶极与范德华力的强弱判据
范德华力也叫诱导偶极-偶极力、色散力或者伦敦力，来源是电子云的瞬时不对称。某一瞬间电子偏到分子的一侧，产生瞬时偶极，这个偶极把邻近分子的电子云推开，诱导出一个方向相同的偶极，两个偶极之间于是出现微弱吸引。单个作用很弱，但它在所有分子之间都存在，累积起来才显出效果，所以分子越大越明显。强弱主要由电子数目决定，电子越多、电子云越容易变形，力越强，碘的 Mr 254、沸点 184 摄氏度，氯气 Mr 71、沸点 -34 摄氏度，差距就在电子数上。分子的接触面积同样重要，直链的正戊烷沸点 36 摄氏度高于支链的新戊烷 9.5 摄氏度，因为支链把分子收成球状、接触面变小。学生最常把范德华力与共价键混为一谈，写成断开了分子内的键。
The weakest of the intermolecular forces goes by several names, including induced dipole-dipole, dispersion and London forces, and it starts with the momentary asymmetry of an electron cloud; an instant of uneven electron distribution creates a temporary dipole, which then distorts the cloud of a neighbouring molecule and induces a dipole in the same direction. Individually the attraction is tiny, yet it exists between all molecules and only shows its effect when added up, which is why bigger molecules feel it more, so iodine with Mr 254 boils at 184 degrees C against -34 for chlorine with Mr 71. Contact area matters too, and straight-chain pentane boils at 36 degrees C while its branched isomer neopentane boils at 9.5.
永久偶极与氢键的三个条件
永久偶极-偶极力出现在极性分子之间，氯化氢与氯化氢之间就是例子，带部分正电的氢端吸引另一个分子带部分负电的氯端。氢键比它更强，出现条件有三条，氢原子直接连在氮、氧或氟上；附近有一个电负性大而且带孤对电子的原子；两个原子之间的距离足够近。氢键的能量大约在 10 到 40 kJ/mol 之间，比范德华力大一个数量级，却远小于共价键，所以它改变的是熔沸点这类物理性质，不会把分子拆散。氟化氢的 Mr 只有 20，比氯化氢的 36.5 小，按范德华力的规律本该沸点更低，实测却是 19.5 摄氏度对 -85 摄氏度，差距全部来自氢键。数量也要交代清楚，冰里每个水分子最多与四个邻居形成氢键，氟化氢只有一个氢可以给出，链状排布限制了每个分子的氢键数目。阅卷时得分的写法是先点明氢键存在，再指出破坏它需要额外能量，最后给出两个分子之间的比较结果。
Permanent dipole-dipole forces act between polar molecules, as between two hydrogen chloride molecules, where the partially positive hydrogen of one attracts the partially negative chlorine of the other. Hydrogen bonding is stronger and needs three conditions at once; a hydrogen atom bonded directly to nitrogen, oxygen or fluorine, a nearby electronegative atom carrying a lone pair, and a short enough distance between the two. The energy involved runs from about 10 to 40 kJ/mol, an order of magnitude above van der Waals forces yet far below a covalent bond, so it changes melting and boiling points without breaking molecules apart; HF with Mr 20 boils at 19.5 degrees C while the heavier HCl, Mr 36.5, boils at -85.
冰的密度与水的反常沸点
冰在 0 摄氏度时的密度约 0.917 g/cm3，小于液态水的 1.000 g/cm3，所以冰浮在水面上，湖面的冰层反而保护下面的水不至于全部冻住。原因在冰的晶格，每个水分子用四个氢键与邻居相连，排成六边形的开放骨架，分子之间的平均距离比液态水更大。冰熔化时一部分氢键断开，分子可以挤得更近，密度因此上升，这一点在 4 摄氏度附近达到最大值。第二处反常是水的沸点，水的相对分子质量只有 18，比硫化氢的 34 还小，按范德华力的规律应该更容易挥发，实际水的沸点是 100 摄氏度，硫化氢是 -60 摄氏度，中间那 160 摄氏度全部来自氢键。AQA 的解释题要求把两件事分开写，氢键需要额外的能量才能破坏，而共价键并没有断。很多学生在这一点上写反，把沸腾说成断开氧氢共价键。
Ice has a density near 0.917 g/cm3 at 0 degrees C against 1.000 for liquid water, so it floats and the surface layer insulates the water below; the reason sits in the lattice, where each water molecule hydrogen bonds to four neighbours and builds an open hexagonal framework with a larger average separation than the liquid. Melting breaks some of those hydrogen bonds so the molecules pack closer and the density rises, peaking near 4 degrees C. The second oddity is the boiling point, since water with a relative molecular mass of 18 should be more volatile than hydrogen sulfide at 34, yet water boils at 100 degrees C and hydrogen sulfide at -60; examiners want the two ideas kept apart, because boiling breaks hydrogen bonds and not the O-H covalent bonds.
阅卷口径与三周复习安排
bonding 的解释题一般 2 到 4 分，评分方案按点给分，每一句话都要对应一个物理原因。写解释可以套用三步，先说清楚相互作用是什么，再说明它与哪种性质相关，最后给出比较结果，例如熔点高低、沸点高低或者能否导电。实验技能在 7405 里单独认证，报告为通过或不通过，不计入 A-level 总分，所以晶格与分子间作用力这部分主要靠卷面题拿分。复习可以按三周安排，第一周把三种键与四类晶体的定义和代表物质各写一遍，第二周整理电负性与分子形状的判断流程，第三周每天限时做两道解释题，写完对照 mark scheme 数自己踩中几个给分点。错得最多的是解释题的最后一分，也就是把两个比较对象点明的那一句。阅卷时也常看到单位写错，把 pm 与摄氏度混用，分数就这么丢掉了。
Explanation questions on bonding are usually worth two to four marks and the mark scheme awards them point by point, so every sentence needs a chemical reason behind it. A reliable three-step pattern is to name the interaction, link it to the property being asked about, and finish with the comparison itself, whether that means melting point, boiling point or conductivity. Practical skills are reported separately in 7405 as a pass or fail endorsement and do not enter the A-level grade, so lattice and intermolecular content is won on paper; a three-week plan of definitions in week one, electronegativity and molecular shape in week two and timed explanation questions in week three covers the topic.
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