📚 Atomic Structure & Bonding: IGCSE Edexcel Science Revision | 原子结构与化学键：IGCSE Edexcel 科学复习
Understanding atomic structure and bonding is essential for IGCSE Science. This guide breaks down the key concepts exactly as required by the Edexcel specification, from subatomic particles to ionic, covalent and metallic bonding.
理解原子结构和化学键是 IGCSE 科学的关键。本指南严格按照 Edexcel 考纲要求，从亚原子粒子到离子键、共价键和金属键，为你逐一拆解核心概念。
1. Subatomic Particles | 亚原子粒子
All matter is made of atoms. Each atom contains protons, neutrons and electrons. Protons and neutrons form the nucleus, while electrons move in energy levels (shells) around the nucleus. The relative mass and charge of each particle must be memorised for the exam.
所有物质都由原子构成。每个原子包含质子、中子和电子。质子和中子构成原子核，电子在核外的能层（电子层）中运动。每种粒子的相对质量和相对电荷必须牢记，考试中经常直接考查。
| Particle | Relative Mass | Relative Charge |
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | 1/1840 (negligible) | -1 |
The atomic number is the number of protons. In a neutral atom, the number of electrons equals the number of protons. The mass number is the total number of protons plus neutrons. You can calculate the number of neutrons by subtracting the atomic number from the mass number.
原子序数等于质子数。在电中性的原子中，电子数等于质子数。质量数等于质子数加中子数。中子数可以通过质量数减去原子序数得出。
2. Atomic Number and Mass Number | 原子序数与质量数
For an element represented as ᴬᵣX, where A is the mass number and Z is the atomic number, the number of neutrons = A − Z. For example, a sodium atom ²³₁₁Na has 11 protons, 11 electrons and 12 neutrons.
对于元素符号 ᴬᵣX，其中 A 为质量数，Z 为原子序数，中子数 = A − Z。例如，钠原子 ²³₁₁Na 有 11 个质子、11 个电子和 12 个中子。
Neutrons = Mass number − Atomic number
Ions are formed when atoms lose or gain electrons. A positive ion (cation) forms when electrons are lost, while a negative ion (anion) forms when electrons are gained. For instance, the sodium ion Na⁺ has 11 protons but only 10 electrons.
离子是原子失去或获得电子后形成的。失去电子形成正离子（阳离子），获得电子形成负离子（阴离子）。例如，钠离子 Na⁺ 有 11 个质子，但只有 10 个电子。
3. Isotopes | 同位素
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Because they have the same number of protons and electrons, they show identical chemical properties. However, their physical properties may differ slightly, such as density.
同位素是同一元素中具有相同质子数但不同中子数的原子。由于质子数和电子数相同，它们的化学性质完全相同。但物理性质可能略有不同，例如密度。
Carbon-12 and carbon-14 are well-known isotopes. Carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons. Both are carbon because they each have 6 protons.
碳-12 和碳-14 是常见的同位素。碳-12 有 6 个质子和 6 个中子，碳-14 有 6 个质子和 8 个中子。两者都是碳元素，因为它们都有 6 个质子。
4. Electron Shells and Electronic Configuration | 电子层与电子排布
Electrons are arranged in shells (energy levels). The first shell can hold a maximum of 2 electrons, the second and third shells can hold up to 8 electrons each. For the first 20 elements, you can write the electronic configuration as numbers, for example: sodium is 2.8.1.
电子排列在电子层（能级）中。第一层最多容纳 2 个电子，第二层和第三层最多各容纳 8 个电子。对于前 20 号元素，可以用数字写出电子排布，例如钠的排布是 2.8.1。
You may also use the shell diagram. In the Edexcel exam, you must be able to draw the electron arrangement for the first 20 elements, showing electrons as dots or crosses in circles.
你也可以使用电子层示意图。在 Edexcel 考试中，你必须能够画出前 20 号元素的电子排布图，用点或叉号在圆圈中表示电子。
The number of electrons in the outermost shell determines the chemical reactivity of an element. Group number equals the number of outer-shell electrons (for Groups 1, 2 and 3), and the number of shells equals the period number.
最外层电子数决定元素的化学活泼性。对于第 1、2、3 主族元素，族序数等于最外层电子数；电子层数等于周期数。
5. Why Atoms Bond | 原子为什么要成键
Atoms bond to achieve a full outer electron shell, which is the most stable arrangement. Noble gases already have full shells and therefore are unreactive. Other atoms can lose, gain or share electrons to obtain a noble-gas configuration.
原子通过成键达到最外层电子充满的稳定结构。稀有气体已经具有满电子层，因此不活泼。其他原子可以通过失去、获得或共用电子来获得稀有气体的电子构型。
There are three main types of chemical bonding: ionic bonding, covalent bonding and metallic bonding. The type of bonding that occurs depends on whether the atoms are metals or non-metals.
化学键主要有三种类型：离子键、共价键和金属键。形成何种类型的键取决于原子是金属还是非金属。
6. Ionic Bonding | 离子键
Ionic bonding occurs between a metal and a non-metal. The metal atom loses electrons to form a positive ion, while the non-metal atom gains electrons to form a negative ion. The oppositely charged ions are held together by strong electrostatic forces of attraction.
离子键形成于金属和非金属之间。金属原子失去电子形成正离子，非金属原子获得电子形成负离子。带相反电荷的离子通过强烈的静电引力结合在一起。
For example, in sodium chloride, sodium (2.8.1) loses one electron to become Na⁺ (2.8), and chlorine (2.8.7) gains one electron to become Cl⁻ (2.8.8). The formula NaCl shows a 1:1 ratio.
例如，在氯化钠中，钠（2.8.1）失去一个电子成为 Na⁺（2.8），氯（2.8.7）获得一个电子成为 Cl⁻（2.8.8）。化学式 NaCl 表示 1:1 的比例。
Na → Na⁺ + e⁻ Cl + e⁻ → Cl⁻
Ionic compounds form giant lattice structures with high melting and boiling points. They conduct electricity when molten or dissolved in water because ions are free to move, but do not conduct when solid.
离子化合物形成巨大的晶格结构，熔点和沸点高。在熔融状态或溶于水时能导电，因为离子可以自由移动；固态不导电。
7. Covalent Bonding | 共价键
Covalent bonding occurs between non-metals. Atoms share pairs of electrons so that each atom achieves a full outer shell. The shared pair of electrons is called a covalent bond.
共价键形成于非金属原子之间。原子通过共用电子对使各自达到满电子层。共享的电子对称为共价键。
Simple molecular substances, such as H₂, O₂, HCl, H₂O and CH₄, have strong covalent bonds inside the molecule but weak intermolecular forces between molecules. This explains their low melting and boiling points, and why they do not conduct electricity.
简单分子物质，如 H₂、O₂、HCl、H₂O 和 CH₄，分子内共价键强，但分子间作用力弱。这解释了它们熔沸点低、不导电的特性。
In the exam, you may be asked to draw dot-and-cross diagrams. For methane (CH₄), carbon shares four electrons with four hydrogen atoms, giving carbon a full outer shell of 8 electrons and each hydrogen a full shell of 2.
考试中可能要求你画电子点叉图。对于甲烷（CH₄），碳与四个氢原子分别共享一对电子，使碳达到 8 电子满壳层，每个氢达到 2 电子满壳层。
8. Giant Covalent Structures | 巨型共价结构
Some covalent substances form giant structures. Diamond and graphite are two forms (allotropes) of carbon. Each carbon atom forms four covalent bonds in diamond, making it extremely hard. Graphite has layers of carbon atoms; each carbon is bonded to three others, leaving one delocalised electron, which allows graphite to conduct electricity.
某些共价物质形成巨型结构。金刚石和石墨是碳的两种同素异形体。金刚石中每个碳原子形成四个共价键，因此极其坚硬。石墨以层状结构存在，每个碳原子与另外三个碳原子成键，剩余一个自由电子，使石墨能够导电。
Silicon dioxide (SiO₂) is another giant covalent structure. It has a high melting point and is very hard, similar to diamond. These structures do not conduct electricity (except graphite) and are insoluble in water.
二氧化硅（SiO₂）是另一种巨型共价结构。它熔点高、硬度大，与金刚石类似。这些结构通常不导电（石墨除外），也不溶于水。
9. Metallic Bonding | 金属键
Metallic bonding occurs in metals. Positive metal ions are arranged in a regular lattice, and the outermost electrons are delocalised, forming a ‘sea of electrons’ that move freely throughout the structure.
金属键存在于金属中。正金属离子有规则地排列在晶格中，最外层电子发生离域，形成贯穿整个结构的“电子海”自由移动。
The strong electrostatic attraction between the positive ions and the delocalised electrons is the metallic bond. This explains why metals have high melting and boiling points, are good conductors of electricity and heat, are malleable and ductile, and have high density.
正离子和离域电子之间的强静电引力就是金属键。这解释了金属具有高熔沸点、良好导电导热性、延展性和高密度的原因。
10. Comparing Bonding Types | 三类化学键对比
The table below summarises the key differences between ionic, covalent and metallic bonding. You should be able to relate structure to properties in the exam.
下表总结了离子键、共价键和金属键之间的主要区别。考试中你需要能将结构与性质联系起来。
| Property | Ionic | Simple Covalent | Giant Covalent | Metallic |
| Elements | Metal + non-metal | Non-metal + non-metal | Non-metals (e.g. C, Si) | Metals |
| Melting point | High | Low | Very high | High (variable) |
| Conducts electricity | When molten/dissolved | No | Only graphite | Yes |
| Solubility in water | Often soluble | Often insoluble | Insoluble | Insoluble |
Remember: ionic compounds require the movement of ions to conduct electricity; metals and graphite use delocalised electrons; simple molecules have no free charge carriers.
请记住：离子化合物导电依赖离子的移动；金属和石墨依靠自由电子导电；简单分子没有可自由移动的电荷载体。
11. Common Exam Tips | 常见考试技巧
Edexcel IGCSE questions often ask you to (1) determine the number of protons, neutrons and electrons from the mass number and atomic number, (2) draw electronic configurations, (3) identify the type of bonding based on elements, and (4) explain properties using structure and bonding.
Edexcel IGCSE 题目经常要求你：(1) 根据质量数和原子序数判断质子、中子、电子数目；(2) 画出电子排布；(3) 根据元素种类判断键型；(4) 用结构与键型解释物质性质。
Always use scientific vocabulary accurately. Say “electrostatic forces of attraction” for ionic and metallic bonds; say “strong covalent bonds” for diamond; say “intermolecular forces” for molecular substances. Marks are awarded for precise language.
务必准确使用科学术语。离子键和金属键用“静电引力”；金刚石用“强共价键”；分子物质用“分子间作用力”。准确表达才能得分。
12. Quick Review Questions | 快速检测题
Test yourself before the exam. Try these five questions without looking back at the notes.
考前自测一下。试着不看笔记回答下面五个问题。
- 1. How many protons, neutrons and electrons are in ⁴⁰₂₀Ca?
- 2. Write the electronic configuration of oxygen (atomic number 8).
- 3. Is MgO ionic or covalent? Explain.
- 4. Why does graphite conduct electricity but diamond does not?
- 5. Why do metals have high melting points?
Answers: (1) 20 protons, 20 neutrons, 20 electrons. (2) 2.6. (3) Ionic, because magnesium is a metal and oxygen is a non-metal. (4) Graphite has delocalised electrons; diamond has all four electrons localised in covalent bonds. (5) Strong electrostatic forces between positive ions and delocalised electrons require a large amount of energy to overcome.
答案：(1) 20 个质子、20 个中子、20 个电子。(2) 2.6。(3) MgO 是离子化合物，因为镁是金属、氧是非金属。(4) 石墨存在离域电子；金刚石的 4 个电子都固定参与共价键。(5) 正离子与离域电子之间的强静电引力需要大量能量才能克服。
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