WJEC GCSE化学 离子键 共价键 金属键 导电性
化学键在 WJEC 的 GCSE Chemistry 里不是孤立的一章，它更像一套解释工具：熔沸点为什么高或低，固体导不导电，物质能不能溶于水，答案最后都要落到原子之间怎么连接。规格把离子键、共价键、金属键三种连接方式放在同一个单元里讲，后面的定量计算与反应速率会反复用到它们。WJEC 从 2016 年 9 月开始按现行规格教学，2018 年夏季首次按 9 到 1 授证，官网已经挂出更新版本，新增的 Unit 3 从 2026 年 9 月起教学、2028 年首次授证，Unit 1 里化学键与结构的内容没有变化。下面按离子键、共价键、金属键、分子间力、巨分子结构、性质判断、卷面答法七块展开，每块都给判断依据与失分点。
Bonding in WJEC GCSE Chemistry works as a set of explanations rather than a stand-alone chapter, because melting points, conductivity and solubility all come back to how atoms are joined. The specification groups ionic, covalent and metallic bonding in one unit and reuses them later in quantitative work and rates of reaction. WJEC has taught the current specification since September 2016, with first awards in summer 2018 on the 9 to 1 scale, and the updated version on the WJEC website adds a new Unit 3 for teaching from September 2026 and first award in 2028, while the bonding and structure content of Unit 1 stays as it is. The seven parts below run through ionic, covalent and metallic bonding, forces between molecules, giant structures, how to read properties off a structure, and how the questions are marked.
离子键：电子转移与离子晶格
金属原子与非金属原子相遇时，金属一侧失去最外层电子，非金属一侧得到电子，双方都变成带电荷的离子。钠原子丢掉一个电子成为 Na+，排布从 2,8,1 变成 2,8；氯原子得到一个电子成为 Cl-，排布从 2,8,7 变成 2,8,8，两者都达到相邻稀有气体的稳定结构。正负离子靠静电引力彼此吸引，并把周围反号的离子一起拉过来，最后堆成有规则的离子晶格。氯化钠就是这张晶格里无数个 Na+ 与 Cl- 交替排列，不存在单独的 NaCl 分子。高熔点、固态不导电、熔融或水溶液导电，这三条性质都能从晶格结构直接读出来。很多学生把 NaCl 说成分子，这类表述在阅卷时也经常出现，句子写对了一半也没分。符号的电荷写在右上角，画点叉图要给每个离子加方括号，电子转移方向写反是最常见的失分点。
Metals lose their outer electrons and non-metals gain them, so both sides end up as charged ions: sodium becomes Na+, moving from 2,8,1 to 2,8, and chlorine becomes Cl-, moving from 2,8,7 to 2,8,8, which matches the arrangement of the nearest noble gas. Electrostatic attraction pulls opposite charges together and gathers more ions of the other sign around them, and the result is a regular ionic lattice in which every Na+ sits among Cl- and the other way round, so sodium chloride contains no separate NaCl units. A high melting point, no conduction when solid, and conduction when molten or dissolved all follow straight from that lattice. Writing NaCl as a molecule is a familiar mistake in mark schemes, and reversing the direction of electron transfer costs marks in dot and cross diagrams, where each ion needs its own brackets and charge.
共价键：电子共用与简单分子
非金属原子之间谁都不肯把电子完全交出去，于是各自拿出电子共用，形成共价键。氢分子里两个氢原子共用一对电子，两个都达到 2 个电子的稳定结构；氯分子、氧气同理，氮气共用三对电子，所以氮气非常不活泼，空气里的氮气在常温下几乎不参与反应。共价键把原子绑成一个个独立的分子，分子与分子之间只剩下弱得多的引力，这一点决定了简单分子物质的物理性质。甲烷、水、二氧化碳、氯化氢在室温下是气体、液体、气体、气体，熔沸点都低，原因在分子之间那层弱引力，而不是共价键本身太弱。拆开分子内的共价键需要很多能量，加热熔化时先被攻破的却是分子之间的力，很多学生以为熔沸点低就等于化学键弱，概念正好弄反。常见的错误是拿元素符号当分子式写，氢气要写 H2 而不是 H。考试常要求用点叉图表示共用电子对，把电子画在两个原子的重叠区域里，才能看出这对电子属于双方共有。
Non-metal atoms share electrons instead of handing them over, and the shared pair is the covalent bond: the two hydrogen atoms in a hydrogen molecule share one pair so both reach the stable two-electron arrangement, nitrogen shares three pairs and is correspondingly unreactive, which is why nitrogen in air hardly reacts at room temperature. Covalent bonds tie atoms into discrete molecules, while the forces between molecules stay much weaker, and that gap decides how simple molecular substances behave. Methane, water, carbon dioxide and hydrogen chloride are gases or volatile liquids at room temperature and boil far below the temperature needed to break the covalent bonds inside them; a low boiling point therefore does not mean a weak chemical bond, and the usual error writes H instead of H2 for hydrogen. Dot and cross diagrams only score when the shared pair is drawn in the overlap between the two atoms, showing that both atoms hold it.
金属键：自由电子与延展性
金属原子最外层电子少，容易脱离原子。在金属晶体里，这些电子不再属于某一个离子，而是整块金属共有，形成常说的电子海，留下的正离子规则地排在晶格上。自由电子可以在整块金属里移动，所以金属导电导热都很快，铜和铝做成导线就用这个性质。正离子层与层之间可以滑动，滑过之后电子海仍然把两侧黏在一起，所以金属能锤成薄片、拉成细丝而不断裂。大多数金属的熔点很高，钨的熔点超过 3400 摄氏度，用来做灯丝；汞在常温下是液体，说明不同金属之间的差别也很大。合金把大小不同的原子混进排列整齐的晶格，层与层不再容易滑动，因此比纯金属更硬，钢与黄铜都属于这一类。常见的错误是把金属的导电性说成电子在原子之间来回跳，其实电子在整个晶格里都是自由的。
Metal atoms hold their outer electrons loosely, and in a metal crystal those electrons leave their atoms and belong to the whole block, forming a sea of delocalised electrons around a regular grid of positive ions. Because the electrons can move anywhere in the metal, heat and charge travel through it quickly, which is why copper and aluminium serve as conductors. The rows of positive ions can slide over one another while the electron sea keeps holding them together, so metals can be hammered into sheets or drawn into wires without cracking, and most of them melt only above a thousand degrees, tungsten beyond 3400 degrees Celsius, though mercury is already liquid at room temperature. Alloys mix differently sized atoms into the lattice so the layers stop sliding easily, which makes steel and brass harder than the pure metals.
分子间力与卤素熔沸点趋势
把氯气、溴、碘摆在一条线上看，分子的相对分子质量依次变大，分子之间的引力随之变强，常温下的状态从黄绿色气体变成深红棕色液体，再变成紫黑色固体。卤素这一族的变化在 WJEC 的卷子里反复出现，题面通常给一张状态表，让你从分子间引力解释，背结论不如把推理说清楚。同类分子之间还有一条规律：分子越大、电子越多、形状越接近长条，分子之间接触越充分，熔沸点越高。烷烃从甲烷到丁烷都是气体，到戊烷开始变成液体，正是因为碳链变长后分子间接触面积增加。分子间力与共价键完全不是一回事，前者只有后者的几十分之一量级，所以简单分子物质的熔沸点远远低于巨分子结构。很多学生以为分子量大就一定熔点高，遇到金刚石与简单分子的对比就翻车，金刚石靠的是整块共价键网络，与分子间力无关。
Chlorine, bromine and iodine line up with increasing molecular mass and stronger forces between the molecules, which is why the room-temperature state moves from a yellow-green gas to a dark red-brown liquid and then to a purple-black solid. WJEC questions often print a table of states and ask for this explanation, so the reasoning matters more than the names. Within a family the larger molecules, with more electrons and longer shapes, touch each other over greater areas and so boil higher, which is why the alkanes start as gases at methane and reach liquids by pentane. Intermolecular forces belong to a different scale from covalent bonds, roughly tens of times weaker, so simple molecular substances melt and boil far below giant covalent structures, and diamond melts above any simple molecule because its whole structure is one covalent network rather than a heap of molecules.
巨分子结构：金刚石 石墨 二氧化硅
金刚石里每个碳原子与另外四个碳原子成键，排成三维骨架，整块晶体可以看作一个巨大的分子。要熔化它就得拆开大量共价键，所以熔点超过 3500 摄氏度，硬度也是自然界里最高的，切割与钻探都靠它。石墨同样是碳，每个碳原子只与三个邻居成键，排成一层层的六角环，剩下的那个电子可以在层内自由移动，所以石墨能导电。层与层之间只有弱引力，层容易滑开，铅笔写字和做润滑剂都是这个原因。二氧化硅的骨架与金刚石类似，每个硅原子与四个氧原子成键，沙子、石英与玻璃的主要成分都是它，熔点高而且不导电。金刚石与石墨都由碳原子组成却性质相差很远，题面常拿它当结构决定性质的例子，答题要把每个碳原子的成键数目与层状结构写清楚。常见的错误是笼统写两者结构不同，评分方案要看的是每个碳原子与几个邻居成键。
Every carbon atom in diamond bonds to four neighbours in a three-dimensional framework, so the crystal behaves as one enormous molecule: melting it means breaking a great many covalent bonds, which is why the melting point is above 3500 degrees Celsius and the hardness is the highest found in nature. Graphite is also carbon, but each atom bonds to only three neighbours in hexagonal layers, leaving one electron free to move within a layer, and that is what makes graphite conduct. The layers themselves are held by weak forces and slide over one another, so graphite writes on paper and works as a lubricant; silicon dioxide mirrors the diamond framework with each silicon bonded to four oxygens and forms the bulk of sand, quartz and glass, melting high and conducting nothing. Diamond and graphite differ so sharply in behaviour although both are pure carbon, and the marks turn on stating how many neighbours each carbon atom bonds to.
结构决定性质：从熔沸点与导电性反推结构
遇到陌生的物质要判断性质，先看它属于哪一类结构，再看粒子之间靠什么连接，性质自然推得出来。离子晶格里离子被静电引力锁住，熔化要打断整张网，所以熔点高；固态离子不能移动，因此不导电，熔融或者溶于水之后离子可以移动，导电性就出现了。金属靠自由电子导电，固体状态就能导电，这一点把它与离子化合物直接分开。简单分子物质分子内是共价键、分子间是弱引力，所以熔沸点低、大多不导电，酸的水溶液要单独看，导电的其实是电离出来的离子。巨分子结构整块由共价键连成，熔点极高，除了石墨这种层内带自由电子的情况之外都不导电。溶解性可以记两条粗线：多数离子化合物溶于水，简单分子物质里糖与乙醇易溶，碘与油类难溶，依据是分子能不能与水形成足够强的相互作用。卷面上常见的问法是给一张性质表让你反推结构类型，把导电性与熔点两条合起来看，答案基本唯一。
Given an unfamiliar substance, start from its structural class and the way its particles are joined, and the properties follow; ionic lattices lock their ions in place, so melting means breaking the whole arrangement and the melting point is high, and the solid cannot conduct while the melt and the solution can, because the ions are then free to move. Metals conduct as solids through delocalised electrons, which separates them from ionic compounds at once, whereas simple molecular substances keep covalent bonds inside their molecules and weak forces between them, melting and boiling low and mostly not conducting, with acid solutions needing care since the current is carried by the ions formed when the acid ionises. Giant structures are held by covalent bonds throughout and melt very high, conducting only where free electrons exist inside the layers, as in graphite; on solubility, most ionic compounds dissolve in water, while among simple molecular substances sugar and ethanol dissolve easily and iodine or oils do not, depending on how well the molecules interact with water.
卷面答法：点叉图与解释题
点叉图有一套固定画法：先画原子核与电子层，再用两种符号区分失去、得到或共用的电子，离子要逐个加方括号并在右上角标电荷，最后数一遍最外层电子是否守恒。解释题通常两到三分，因和果都要落到纸面上。问为什么氯化钠熔点高，答离子被强静电引力锁在晶格里、拆开需要大量能量，只写离子键很强通常拿不到满分。问为什么石墨能导电，答每个碳原子只用三个电子成键、剩下的电子在层内自由移动，两句话各占一分。考试时把物质先归类，再套用上面对应的说法，比每道题重新推理快得多。WJEC 官网公布的规格与评分方案可以直接下载，做近两年的真题时对着评分方案核对用词，把自己漏掉的关键词圈出来，比盲目刷题有效。规格的 Unit 1 已经把化学键与结构放在一起，复习时把离子、共价、金属、巨分子四类结构的熔沸点、导电性、溶解性列成一张对照表，考前看这一张就够。
A reliable dot and cross diagram follows a fixed order: draw the shells, mark lost, gained or shared electrons with two different symbols, add brackets and charges for ions, then check that the outer electrons balance. Explanation questions carry two or three marks, so cause and effect must both appear: for the high melting point of sodium chloride, the ions are locked in a lattice by strong electrostatic attraction and separating them takes a great deal of energy, and writing only that the ionic bond is strong usually misses the mark. For graphite, each carbon atom bonds with three of its outer electrons and the remaining one moves freely within the layer. Classifying the substance before answering beats reasoning from scratch each time, and the specification and mark schemes on the WJEC website are free to download, so checking two recent papers line by line against the scheme shows which keywords you keep dropping.
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