WJEC化学 离子键 共价键 金属键 结构性质 | WJEC Chemistry: Ionic, Covalent and Metallic Bonding, Structure and Properties

WJEC化学 离子键 共价键 金属键 结构性质

化学键是WJEC GCSE化学的核心考点，集中在Unit 2.1（Bonding, structure and properties）。Unit 2整卷占总分45%，考试时长1小时15分钟、满分80分，结构性质类题目几乎每年必考。很多同学觉得这部分知识点零散，其实只要抓住"结构决定性质"这一条主线，把三种化学键和四种巨型结构放进同一张对比表里，考题就变得很有规律。

Chemical bonding is the core of WJEC GCSE Chemistry, covered in Unit 2.1 (Bonding, structure and properties). Unit 2 carries 45% of the final grade: 1 hour 15 minutes and 80 marks, with structure-and-properties questions appearing in almost every paper. Many students find this topic scattered, but once you follow the single thread that structure determines properties and lay the three bond types and four giant structures out in one comparison table, the questions become highly predictable.

一、为什么原子会成键：八隅体与能量最低

原子成键的根本原因是追求稳定：除了稀有气体，大多数原子的最外层电子都不满，通过得失或共享电子达到八隅体（helium例外，只需2个电子）。从能量角度看，成键后的体系能量更低、更稳定，这就是化学键形成的动力。WJEC考纲要求你能用"full outer shell"和"stable"这两个关键词解释成键原因，答题时先写电子排布，再写稳定性。

Atoms bond because they seek stability: apart from noble gases, most atoms have incomplete outer shells and reach an octet (helium needs only 2 electrons) by losing, gaining or sharing electrons. Energetically, the bonded system sits at a lower, more stable state, which is the driving force for bond formation. The WJEC specification expects you to explain bonding using the ideas of a full outer shell and stability; in an answer, describe the electron arrangement first, then link it to stability.

判断成键类型的第一步是看元素种类：金属与非金属之间形成离子键，非金属与非金属之间形成共价键，金属原子之间则形成金属键。考试中最常见的失分点是把"transfer"和"share"搞混，离子键是电子转移，共价键是电子共享，这两个词必须分清楚。

The first step in identifying bond type is looking at the elements: metal plus non-metal gives ionic bonding, non-metal plus non-metal gives covalent bonding, and metal atoms together give metallic bonding. The most common mark-losing error in exams is confusing transfer with share: ionic bonding transfers electrons, covalent bonding shares them, so keep these two words distinct.

二、离子键：电子转移与巨型离子晶格

离子键发生在活泼金属（如Na、Mg）与活泼非金属（如Cl、O）之间。金属原子失去最外层电子形成阳离子，非金属原子得到电子形成阴离子，正负离子之间巨大的静电引力把无数离子排列成巨型离子晶格（giant ionic lattice）。画dot-and-cross图时，只画最外层电子，必须标出离子电荷，比如NaCl是1:1，MgCl2是1:2，MgO是1:1。

Ionic bonding occurs between reactive metals such as sodium and magnesium and reactive non-metals such as chlorine and oxygen. Metal atoms lose outer electrons to form positive ions, non-metal atoms gain electrons to form negative ions, and the strong electrostatic attraction between oppositely charged ions arranges countless ions into a giant ionic lattice. When drawing dot-and-cross diagrams, show only outer-shell electrons and label the ionic charges: NaCl is 1:1, MgCl2 is 1:2 and MgO is 1:1.

离子化合物的性质全部来自晶格结构：由于静电引力很强，离子化合物熔点沸点高；固态时离子被固定在晶格中不能移动，所以不导电；熔融或溶于水后离子自由移动，才能导电。敲击时晶格层错位，同号离子靠近互相排斥，因此离子化合物脆而易碎。记住这条因果链，六分"explain"题就能拿满。

All the properties of ionic compounds follow from the lattice. Because the electrostatic forces are strong, ionic compounds have high melting and boiling points. In the solid state the ions are locked in place and cannot move, so they do not conduct; once molten or dissolved in water the ions are free to move and conduction begins. When struck, the lattice layers slip so that like charges come together and repel, which is why ionic compounds are brittle. Follow this cause-and-effect chain and you can score full marks on six-mark explain questions.

离子电荷可以从元素周期表的族数直接推出：第1族形成+1，第2族形成+2，第6族形成-2，第7族形成-1。题目常考过渡金属离子（如Fe2+、Cu2+）需要记忆。写离子符号时，数字在前、正负号在后，这个细节在dot-and-cross题里经常扣分。

Ionic charges can be read straight from the periodic table: group 1 forms +1, group 2 forms +2, group 6 forms -2 and group 7 forms -1. Transition metal ions such as Fe2+ and Cu2+ simply need to be memorised. When writing ion symbols, put the number before the sign, a small detail that regularly costs marks in dot-and-cross questions.

三、共价键：电子共享与简单分子

共价键由两个非金属原子共享电子对形成，可以是单键、双键或三键：H2是单键，O2是双键，N2是三键。画共价分子的dot-and-cross图时同样只画最外层电子，常见考点包括H2O、CH4、NH3和CO2，其中H2O中O提供两个电子与其他原子的单电子配对，CH4中C需要四个电子达到八隅体。

A covalent bond forms when two non-metal atoms share a pair of electrons, and the sharing can be single, double or triple: H2 has a single bond, O2 a double bond and N2 a triple bond. Dot-and-cross diagrams for covalent molecules also show only outer-shell electrons; common exam molecules are H2O, CH4, NH3 and CO2. In H2O the oxygen contributes two electrons to pair with single electrons from the hydrogen atoms, while in CH4 carbon needs four shared pairs to reach an octet.

由共价键形成的简单分子物质（如CO2、H2O、CH4）是分子晶体：分子内部共价键很强，但分子之间只有微弱的分子间作用力，所以熔点沸点很低，常温多为气体或液体。它们通常不导电，因为分子整体呈电中性、没有自由移动的离子或电子。写原因时务必区分"分子内共价键"和"分子间作用力"，这是WJEC最爱的表述陷阱。

Simple molecular substances built from covalent bonds, such as CO2, H2O and CH4, form molecular structures: the covalent bonds within each molecule are strong, but only weak intermolecular forces act between molecules, so melting and boiling points are low and most are gases or liquids at room temperature. They usually do not conduct because the molecules are electrically neutral with no free-moving ions or electrons. When explaining, always distinguish the strong covalent bonds inside molecules from the weak forces between molecules, a distinction WJEC loves to test.

注意"diatomic"概念：H2、O2、N2、Cl2都是双原子分子，元素符号右下角的2表示一个分子里有两个原子。考纲还要求知道共价化合物可以形成巨型结构（如diamond、SiO2），这与简单分子完全不同，下面单独展开。

Note the idea of diatomic molecules: H2, O2, N2 and Cl2 all exist as two-atom molecules, with the subscript 2 showing two atoms per molecule. The specification also expects you to know that covalent bonding can build giant structures such as diamond and silicon dioxide, which behave completely differently from simple molecules and are covered separately below.

四、金属键：正离子与电子海

金属键描述金属原子之间的结合：金属原子失去最外层电子后变成正离子，排列在晶格中，这些失去的电子形成"电子海"（sea of delocalised electrons），可以在正离子之间自由移动。正是这些自由电子让金属在固态和熔融态都能导电，也让金属成为良好的导热体。

Metallic bonding describes how metal atoms hold together: each atom releases its outer electrons to become a positive ion sitting in a lattice, and the released electrons form a sea of delocalised electrons free to move between the ions. These free electrons are why metals conduct electricity in both the solid and molten states, and why they are also good conductors of heat.

金属具有延展性和可锻性（malleable and ductile），因为晶格层在受力时可以滑动而不破坏金属键；金属通常熔点较高，因为金属键较强。合金（alloy）是把不同大小的原子混入晶格，层间滑动受阻，所以合金比纯金属更硬。WJEC常在2.3金属提取章节再次考到合金，但在bonding题里也偶尔出现对比问法。

Metals are malleable and ductile because the lattice layers can slide over one another without breaking the metallic bonds, and melting points are generally high because the metallic bonds are strong. An alloy mixes atoms of different sizes into the lattice, which blocks layer sliding and makes the alloy harder than the pure metal. WJEC revisits alloys in the metal extraction unit, but comparison questions can also appear here in the bonding context.

五、巨型共价结构：钻石、石墨与富勒烯

巨型共价结构（giant covalent structures）是WJEC Unit 2.1的重头戏，diamond和graphite的对比是历年必考。钻石中每个碳原子与另外4个碳原子形成共价键，呈正四面体网状排列，结构极其坚硬、熔点极高，且没有自由电子，不导电。石墨中每个碳原子只与3个碳成键，形成六边形层状结构，层间靠微弱作用力连接，每层还有一个离域电子，所以石墨能导电、质软可作润滑剂。

Giant covalent structures are the centrepiece of WJEC Unit 2.1, and comparing diamond with graphite is a guaranteed question. In diamond every carbon atom bonds covalently to four others in a tetrahedral network, making it extremely hard with a very high melting point and no free electrons, so it does not conduct. In graphite each carbon bonds to only three others, forming hexagonal layers held by weak forces, with one delocalised electron per layer, which is why graphite conducts electricity and is soft enough to act as a lubricant.

考纲还要求认识graphene和fullerenes：graphene是单层石墨，是目前已知最薄最强的材料之一，能导电，可用于电子屏幕和复合材料；fullerenes是碳的笼状分子，如C60，中空结构可用于药物输送，也可作润滑剂。SiO2（silicon dioxide）是另一种常见巨型共价结构，每个硅原子与4个氧原子成键，石英和沙子的主要成分，熔点高、不导电。

The specification also covers graphene and fullerenes: graphene is a single layer of graphite, among the thinnest and strongest materials known, conducts electricity and appears in screens and composites; fullerenes are cage-like carbon molecules such as C60, whose hollow structure suits drug delivery and lubrication. Silicon dioxide, found in quartz and sand, is another giant covalent structure in which each silicon bonds to four oxygens; it has a high melting point and does not conduct.

考试常把四种结构放在一起让你比较熔点、导电性、硬度并解释原因。答题模板：先写结构（原子排列方式），再写作用力类型，最后把性质连接到作用力强度上。比如"graphite熔点高因为碳原子间有大量强共价键，需要大量能量破坏"，注意别答成"分子间作用力"。

Exams often ask you to compare the four structures on melting point, conductivity and hardness with explanations. Use this answer template: describe the structure first, then the type of force, then connect the property to the force strength. For example, graphite melts at a high temperature because its carbon atoms are joined by many strong covalent bonds that need huge amounts of energy to break, so never answer with intermolecular forces for a giant structure.

六、WJEC考试技巧：从结构写到性质

WJEC的bonding题目有三类高频考法。第一类是dot-and-cross图，阅卷按点给分：正确离子/原子、正确电荷、正确配对，缺一不可。第二类是六分explain题，要求你完整写出"强静电引力＋破坏需要大量能量"的结构化答案，并明确区分固态不导电与熔融导电。第三类是结构对比表题，把diamond、graphite、ionic compound、simple molecule的性质填表，注意每个空格都要写完整理由而非只写结论。

WJEC bonding questions come in three high-frequency formats. First, dot-and-cross diagrams, marked point by point: correct species, correct charges and correct pairing are all required. Second, six-mark explain questions that need a structured answer covering strong electrostatic forces and the large amount of energy needed to break them, while clearly separating the solid non-conducting state from the molten conducting state. Third, comparison-table questions on diamond, graphite, ionic compounds and simple molecules, where every box needs a full reason, not just a conclusion.

答题时注意命令词：state只需一句话，describe要写出结构细节，explain必须给出"因为"的因果链，suggest允许结合已学知识合理推测。Unit 2考试时间1小时15分钟，建议给六分题留足8到10分钟，先列关键词再成文。平时把2019年以来的WJEC past paper Unit 2都做一遍，bonding题重复率很高。

Watch the command words: state needs one sentence, describe requires structural detail, explain demands a because-based causal chain, and suggest allows reasoned speculation using what you know. Unit 2 runs for 1 hour 15 minutes, so leave 8 to 10 minutes for each six-mark question and jot keywords before writing. Working through every WJEC Unit 2 past paper from 2019 onwards is the fastest way to raise marks, because bonding questions repeat heavily.

七、常见错误与避坑清单

错误1：混淆转移与共享。离子键是电子transfer（转移），共价键是electron pair sharing（共享），答题用词错了直接丢分。

Mistake 1: confusing transfer with sharing. Ionic bonding transfers electrons, covalent bonding shares electron pairs; the wrong verb loses marks immediately.

错误2：把离子化合物说成"分子"。离子化合物是巨型晶格，不存在独立分子，只有共价简单物质才有molecule的说法。

Mistake 2: calling ionic compounds molecules. Ionic compounds form a giant lattice with no separate molecules; only simple covalent substances exist as molecules.

错误3：石墨导电的原因写错。石墨导电是因为层内每个碳原子提供一个离域电子（delocalised electron），不是因为有"自由离子"；钻石不导电是因为4个电子全部成键。

Mistake 3: giving the wrong reason for graphite conduction. Graphite conducts because each carbon in a layer contributes a delocalised electron, not because of free ions; diamond does not conduct because all four electrons are used in bonds.

错误4：dot-and-cross漏标电荷或比例。离子图必须写电荷，分子图必须数对共享电子对数，收尾时逐项核对一遍。

Mistake 4: missing charges or ratios in dot-and-cross diagrams. Ionic diagrams need charges labelled and molecular diagrams need the shared pairs counted correctly, so check each item at the end.

八、五步学习建议

建议1：默画所有dot-and-cross图。闭卷画NaCl、MgO、H2O、CH4、CO2、NH3六张图，画完对照课本订正，直到零错误。

Tip 1: draw every dot-and-cross diagram from memory. Without notes, draw NaCl, MgO, H2O, CH4, CO2 and NH3, then correct each against the textbook until you reach zero errors.

建议2：做一张结构性质对比表。把离子化合物、简单分子、金属、巨型共价四类按熔点、导电性、硬度、原因四列整理，贴在书桌前每天看一遍。

Tip 2: build one structure-property comparison table. List ionic compounds, simple molecules, metals and giant covalent structures across melting point, conductivity, hardness and the reason, and review it daily.

建议3：族数推导电荷。练习从元素周期表快速说出第1、2、6、7族的离子电荷，这是所有离子题的基础。

Tip 3: derive charges from group numbers. Practise giving the ionic charge for groups 1, 2, 6 and 7 instantly, since every ionic question builds on it.

建议4：限时刷Unit 2真题。每套卷子按1小时15分钟计时，做完对照mark scheme逐条批改，把漏掉的关键词记进错题本。

Tip 4: do timed Unit 2 past papers. Complete each paper in 1 hour 15 minutes, mark every line against the mark scheme and copy missed keywords into an error notebook.

建议5：用"讲给别人听"检验理解。合上书，用中英双语把"为什么石墨导电而钻石不导电"讲给同学或家长听，讲不通的地方就是你的薄弱点。

Tip 5: test understanding by teaching. Close the book and explain in Chinese and English why graphite conducts while diamond does not; anywhere you stumble is a weak point to fix.

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