AQA化学 GCSE 离子键 共价键 金属键 石墨
AQA 的 GCSE 化学把结构决定性质这条主线放在 Topic 2，也就是 4.2 Bonding, structure, and the properties of matter。这一章不靠背方程式拿分，靠的是把键的类型、粒子的排列方式与物质的熔点、硬度、导电性一一对上。Paper 1 覆盖 Topic 1 到 Topic 5，时长 1 小时 45 分钟、满分 100 分、占整科 50%，三种化学键的分辨题几乎每年都出现。很多学生以为这一章只要记住离子键、共价键、金属键三个名字，实际上答题时要写出静电引力，以及电子是转移还是共用。下面按 spec 的编号顺序，把这一章的得分点拆成十二块。
The whole of Topic 2 rests on one idea: what the particles are and how they are arranged decides how a substance behaves. AQA asks you to link every structure, from a small molecule to a giant lattice, to its melting point, its hardness and whether it conducts electricity; recalling the three names on their own earns very little. This article follows the specification order from 4.2.1 to 4.2.4 and points out where the marks are usually lost.
三种键的分辨与 Paper 1 的位置
spec 的 4.2.1.1 把三种强化学键一次性摆出来：离子键的粒子是带相反电荷的离子，共价键的粒子是共用电子对的原子，金属键的粒子是共用离域电子的原子。判断用哪种键只看组成元素的种类，金属与非金属组成的化合物走离子键，大多数非金属单质与非金属化合物走共价键，金属元素与合金走金属键。氯化钠里是钠离子与氯离子，甲烷里是碳与氢共用电子对，铜与青铜里是离域电子把原子连在一起。三种键都属于强键，区别在于电子是被转移了、被共用了，还是被整个结构共用。这一章在 Paper 1 里很少单独占一整道大题，通常和原子结构或周期表的数据题合并出现。答题时把静电引力这四个字写出来，答案才落到得分点上。
The specification lists three strong bonds in 4.2.1.1: ionic bonding between oppositely charged ions, covalent bonding between atoms that share pairs of electrons, and metallic bonding between atoms that share delocalised electrons. The type of bond follows the elements involved, so a metal combined with a non-metal gives an ionic compound while non-metals on their own give covalent substances; alloys and pure metals are held together by delocalised electrons. Examiners look for the phrase electrostatic forces of attraction, because transfer and sharing are the two words that separate a full-mark answer from a vague one.
离子键里的电子转移与电荷规律
4.2.1.2 描述的过程很直接：金属原子外层的电子转移到非金属原子上，金属失电子变成带正电的离子，非金属得电子变成带负电的离子。钠原子失去一个电子后剩下 10 个电子，构型与氖相同，得到的离子都带上了稀有气体的电子排布。离子电荷可以直接从族数读出，第 1 族金属形成 1+ 离子，第 2 族形成 2+ 离子，第 6 族非金属形成 2- 离子，第 7 族形成 1- 离子。氧化镁里是镁离子与氧离子，电荷都是 2，离子之间的引力比氯化钠里的 1+ 与 1- 强得多，熔点因此高得多。常见的错误是把电荷写成 Na1+ 或 Mg+，或者把金属写成得到电子的一方。举个例子，钾与溴反应生成溴化钾，钾给电子、溴收电子，产物是钾离子与溴离子。
Metals lose the electrons in their outer shell and non-metals gain them, which leaves both ions with the electronic structure of a noble gas in Group 0. The charge follows the group number, so Group 1 gives 1+ ions and Group 7 gives 1- ions; magnesium oxide holds 2+ and 2- ions, and those stronger attractions explain why its melting point sits far above that of sodium chloride. A common error is writing a charge that ignores the outer electrons, such as Mg+ instead of Mg2+.
点叉图、化学式与图示的局限
spec 要求会画第 1、2 族金属与第 6、7 族非金属形成的离子化合物的点叉图，画法是把转移的电子用叉与点区分来源，再把离子写成带电荷的方括号。写化学式时用交叉法配平电荷，钠离子是 1+、氧离子是 2-，所以氧化钠写作 Na2O；镁离子是 2+、氯离子是 1-，所以氯化镁写作 MgCl2。图示都有局限，点叉图只画外层电子，看不出离子的相对大小；二维图看不出晶格在三个方向上的排列；离子化合物里的离子数目极多，图上只能画出一小部分。阅卷时这类解释题要求指出具体局限，只写图不完整通常不给分。点叉图里漏掉方括号与电荷，是这一类题最常见的扣分动作。
Dot and cross diagrams are expected for compounds made from Group 1 or 2 metals with Group 6 or 7 non-metals, and the charges have to balance, which gives Na2O when sodium joins oxygen and MgCl2 when magnesium joins chlorine. Diagrams are only models, so a dot and cross diagram shows the outer electrons but not the relative sizes of the ions; a two-dimensional drawing cannot show how the lattice is arranged in three dimensions. Marks are lost when the brackets and charges are missing, because the diagram then shows atoms rather than ions.
巨离子晶格的高熔点与熔融导电
4.2.1.3 把离子化合物定义成离子的巨结构，离子之间在空间各个方向上都有强静电引力，这种引力就是离子键。4.2.2.3 接着解释性质，离子化合物是巨型离子晶格，熔点与沸点都高，因为破坏大量强键需要很多能量；熔融或溶于水后能导电，因为离子可以自由移动、电荷可以流动。常见数据表给出氯化钠的熔点 801 摄氏度、氧化镁的熔点 2852 摄氏度，差别正来自离子电荷的高低。固态离子化合物不导电，离子被固定在晶格位置上，动不了。很多学生以为含有离子就等于能导电，忽略了离子必须能自由移动这一条前提。
An ionic compound is a giant lattice of ions held in every direction by strong electrostatic forces, and those many strong bonds explain the high melting and boiling points, because a large amount of energy is needed to break them. Sodium chloride melts at 801 °C while magnesium oxide melts at 2852 °C, and the gap comes from the higher charges on the magnesium and oxide ions; melting or dissolving in water frees the ions, so the liquid or the solution conducts while the solid does not. Saying that a substance contains ions is not enough for the mark, because the ions must be free to move and carry charge.
共价键与小分子的辨认
4.2.1.4 说得很清楚，原子共用电子对形成共价键，键很强。共价键物质的形态差别很大，氢气、氯气、氧气、氮气、氯化氢、水、氨与甲烷都是小分子，聚乙烯与聚氯乙烯是大分子聚合物，金刚石与二氧化硅属于巨型共价结构。从化学式辨认小分子是常见的起步题，H2O 与 CH4 一眼能看出，写成重复单元加 n 的就是聚合物。用一条直线表示一对共用电子是最常用的画法，聚合物画重复单元，巨型共价结构只画其中一部分。这一步最容易出错的地方，是把化学式里的数字当成分子个数，其实它表示一个分子里原子的数目。把三类结构放在一起对照，比单独背特征更容易记牢。
A covalent bond forms when two atoms share a pair of electrons, and the same bond type holds together substances as different as hydrogen, methane, poly(ethene) and diamond. Small molecules are recognised from their formulae; a polymer is written as a repeating unit with the letter n, while a giant covalent structure such as silicon dioxide has no separate molecules at all. The number in a formula tells you how many atoms are in one molecule, not how many molecules are present, and that confusion costs marks in the first question of the topic.
分子间力与小分子的低沸点
4.2.2.4 的解释只有两句话，却每年都考，小分子物质通常是气体或液体，熔点与沸点较低；熔化或沸腾时被克服的只是分子之间的弱力，分子内部的共价键没有被破坏。小分子不导电，因为分子整体不显电性，也没有可以移动的带电粒子。卤素是最好用的例子，氯气在常温下是气体，溴是液体，碘是固体，分子变大后分子间力变强，熔点与沸点依次升高。水的沸点是 100 摄氏度，氯化钠的熔点却有 801 摄氏度，这一对比能说明分子间力与离子键的强弱差别。推理题问的通常就是这条链条，分子变大，分子间力变强，熔点升高。很多学生把破坏分子间力写成破坏共价键，这是这一章丢分最多的写法之一。写方程式时四态符号要用对，固体写 s、液体写 l、气体写 g、水溶液写 aq，这是 4.2.2.2 的考点。
Small molecules are usually gases or liquids with low melting and boiling points, and the forces overcome when they melt or boil are the weak ones between the molecules rather than the strong covalent bonds inside them. Chlorine is a gas, bromine a liquid and iodine a solid at room temperature, which follows from larger molecules having stronger forces between them; water boils at 100 °C while sodium chloride melts at 801 °C, and the comparison shows the difference in strength between intermolecular forces and ionic bonding. Writing that covalent bonds break during melting is one of the most common ways students lose marks here.
聚合物的分子内强键与室温固体
4.2.2.5 只考一条对比，聚合物分子很大，分子内原子之间由强共价键相连，分子与分子之间的分子间力相对较强，所以聚合物在室温下是固体。聚乙烯、聚丙烯与聚氯乙烯都属于这一类，它们的分子链很长，分子间力累加起来足以把材料固定住。聚合物的重复单元可以写成方括号加 n 的形式，n 是一个很大的数；从给出的重复单元反推单体也是常见题型，重复单元是 CH2-CH2 时单体就是乙烯。很多学生以为 n 代表分子个数，其实 n 只表示分子里重复单元的数量。对照小分子那一条，就能把分子间力弱、熔点低与分子间力较强、室温固体这两组结论分开记住。整个 Topic 2 的性质题，基本都是这三类结构的排列组合。
Polymers are large molecules in which the atoms are joined by strong covalent bonds, and the forces between the polymer molecules are strong enough to make most of them solids at room temperature. Poly(ethene), poly(propene) and PVC behave in this way, and a repeating unit such as CH2-CH2 can be traced back to the monomer ethene; the letter n simply means a large number of repeats. The contrast with small molecules is what examiners test: weak forces between molecules give low melting points, stronger forces give a solid.
金刚石与二氧化硅的巨共价网络
4.2.2.6 的结论是巨型共价结构的熔点非常高，因为结构里每一个原子都靠强共价键与其他原子相连，熔化或沸腾必须破坏这些键；金刚石、石墨与二氧化硅是三个常见例子。4.2.3.1 补充金刚石的细节，每个碳原子与另外四个碳原子形成四条共价键，整块材料就是一个巨大的三维网络，因此金刚石很硬、熔点极高，而且不导电。不导电的原因是结构里没有可以自由移动的带电粒子，金刚石里的电子都被锁在共价键中。二氧化硅的骨架与金刚石类似，只是换成硅与氧交替排列，常见数据表给出二氧化硅的熔点约 1710 摄氏度。常见的错误是写金刚石不导电是因为它属于非金属，答案要落到没有自由移动的带电粒子，才给满分。
Giant covalent structures melt at very high temperatures because every atom is joined to its neighbours by strong covalent bonds, and those bonds must be broken before the substance can melt or boil. Diamond is the classic case, with each carbon atom forming four covalent bonds in a three-dimensional network, which is why it is hard, has a very high melting point and does not conduct electricity; silicon dioxide is built in the same way and melts at about 1710 °C. Diamond fails to conduct because no charged particles are free to move, not because it is a non-metal.
石墨的层状结构与离域电子
4.2.3.2 把石墨的结构说得很细，每个碳原子与三个碳原子形成三条共价键，排成六元环连成的层，层与层之间没有共价键；每个碳原子有一个电子离域，spec 也明说石墨在这一点上与金属相似。这三条结构信息正好解释三个性质，层内共价键很强，所以石墨熔点很高；层与层之间只有弱力，所以层可以滑动，石墨因此很软、能留下痕迹、可以做润滑剂；离域电子可以在层内移动，所以石墨导电、导热。石墨与金刚石都是碳单质，差别只在原子的排列方式。很多学生以为石墨软是共价键弱，其实层内的键一点也不弱，软的是层与层之间的作用。答题时把层之间没有共价键写出来，比写结构松散更容易得分。
Graphite has layers of hexagonal rings in which each carbon atom forms three covalent bonds, and there are no covalent bonds between the layers; one electron from each carbon atom is delocalised, which is why the specification compares graphite to a metal. Because the forces between the layers are weak they can slide, so graphite feels soft and works as a lubricant, while the strong covalent bonds inside each layer keep the melting point very high and the delocalised electrons let it conduct electricity and heat. Saying that graphite is soft because its covalent bonds are weak misses the point, since the weak part is the interaction between the layers.
石墨烯、富勒烯与碳纳米管
4.2.3.3 有三个需要记住的结构，石墨烯是石墨的单层，厚度只有一个原子，强度高、导电性好、比表面积大，因此用于电子器件与复合材料；富勒烯是碳原子组成的中空分子，结构以六元环为主，也可能出现五元环或七元环；碳纳米管是圆柱形的富勒烯，长径比非常大，用于纳米技术、电子器件与材料。第一个被发现的富勒烯是 C60，形状接近球体，通常叫巴基球。用途题常要求举出富勒烯与碳纳米管的具体例子，催化剂、润滑剂与药物输送都是可接受的答案。阅卷时常见答案是石墨烯就是石墨，漏掉单层这个关键词，分数往往只给一半。把三个结构放在一起对比，比单独背特征更容易记牢。
Graphene is a single layer of graphite, one atom thick, and its strength, conductivity and large surface area make it useful in electronics and composites; fullerenes are hollow molecules of carbon built mainly from hexagonal rings that may also contain five- or seven-membered rings. Carbon nanotubes are cylindrical fullerenes with a very high length to diameter ratio, used in nanotechnology, electronics and materials, and C60, the first fullerene discovered, is the spherical one. When a question asks for uses, name them concretely, such as catalysts, lubricants and drug delivery, rather than describing the shape again.
金属键、合金硬度与导电导热
4.2.1.5 的定义是金属由规则排列的原子巨结构组成，外层电子离域后可以在整个结构中自由移动，离域电子的共用形成了强的金属键。4.2.2.7 讲合金，纯金属里原子大小相同，受力时层与层可以滑动，所以金属有延展性；加入尺寸不同的原子后层被撑歪，滑动变得困难，合金因此比纯金属硬。铜合金是常考的例子，青铜是铜与锡，黄铜是铜与锌，金饰里掺入其他金属也是同样的道理。4.2.2.8 把导电与导热都归到离域电子身上，导电是电子携带电荷穿过金属，导热是能量由电子传递。很多学生以为合金变硬是因为掺进去的原子更大，真正的原因是原子尺寸不同让层无法滑动。
Metals are giant structures of atoms in a regular pattern, and the outer electrons are delocalised and free to move, so the shared electrons form strong metallic bonds. Pure metals can be bent because layers of same-sized atoms slide over each other, while alloying with atoms of a different size distorts the layers and makes sliding harder, which is why bronze, brass and gold jewellery are harder than the pure metals; the delocalised electrons also carry charge and thermal energy through the metal. Alloying works because the layers cannot slide, not simply because the added atoms are larger.
纳米粒子的尺寸、比表面积与用途
4.2.4 标注为 chemistry only，考组合科学的同学不考这一节，考化学单科的同学要注意它同样出现在 Paper 1。4.2.4.1 给出尺寸范围，纳米粒子的直径在 1 到 100 纳米之间，含有几百个原子，性质可能与同种材料的大块形态不同；1 纳米的粒子只有几个原子宽，所以它比单个原子大得多，却比常见的粉末颗粒小得多。比表面积是最常考的计算，边长为 1 纳米的立方体表面积 6 平方纳米、体积 1 立方纳米，比表面积是 6；边长换成 100 纳米后比表面积只有 0.06，尺寸缩小到百分之一，比表面积就放大一百倍。用途题要写具体，银纳米粒子抗菌，用在敷料与抗菌涂层里；二氧化钛纳米粒子用在防晒霜里，既挡紫外线又不留白痕；纳米粒子还用在化妆品与除臭剂中，也作为催化剂、润滑剂与涂层。风险同样要提，纳米粒子可能对健康有影响，长期影响目前仍在评估，所以使用需要受到监管。
Nanoparticles are 1 to 100 nm across, contain a few hundred atoms and may behave differently from the same material in bulk; a cube of side 1 nm has a surface area of 6 nm2 and a volume of 1 nm3, giving a surface area to volume ratio of 6, while a cube of side 100 nm has a ratio of only 0.06. That ratio is what makes them useful, since silver nanoparticles act as antibacterial agents in dressings, titanium dioxide nanoparticles go into sun creams, and nanoparticles also appear in cosmetics, deodorants, catalysts, lubricants and coatings. The risks have to be named as well, because nanoparticles may affect health and the long-term evidence is still being collected.
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