📚 IGCSE WJEC Chemistry: Atomic Structure Revision | IGCSE WJEC 化学：原子结构 考点精讲
Understanding atomic structure is the foundation of IGCSE WJEC Chemistry. It explains how elements differ, how atoms bond, and why the periodic table is organised as it is. This article focuses on the essential revision points you need to master, from subatomic particles to electron configurations and isotopes.
理解原子结构是 IGCSE WJEC 化学的基础。它解释了元素为什么不同、原子如何结合以及周期表为何如此排列。本文聚焦你需要掌握的核心考点，涵盖亚原子粒子、电子排布和同位素等关键内容。
1. The Basic Structure of the Atom | 原子的基本结构
Atoms are the smallest particles of an element that can take part in chemical reactions. Each atom consists of a central nucleus containing protons and neutrons, surrounded by electrons moving in shells.
原子是能参与化学反应的元素最小粒子。每个原子由一个包含质子和中子的中心原子核以及在其周围分层运动的电子组成。
The nucleus is tiny compared with the overall size of the atom, yet it contains nearly all the mass. Most of the atom is empty space.
原子核相对于整个原子来说极小，却几乎集中了全部质量。原子的大部分空间是空的。
2. Subatomic Particles | 亚原子粒子
There are three key subatomic particles: protons, neutrons and electrons. Their relative masses and charges are summarised below.
有三种关键亚原子粒子：质子、中子和电子。它们的相对质量与电荷总结如下。
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton (p⁺) | 1 | +1 | Nucleus |
| Neutron (n⁰) | 1 | 0 | Nucleus |
| Electron (e⁻) | 1/1840 (negligible) | −1 | Shells around nucleus |
The number of protons defines the element. In a neutral atom, the number of electrons equals the number of protons and the charges balance.
质子数目决定了元素种类。在中性原子中，电子数等于质子数，电荷互相抵消。
3. Atomic Number and Mass Number | 原子序数与质量数
The atomic number (Z) is the number of protons in the nucleus. The mass number (A) is the total number of protons plus neutrons.
原子序数（Z）是原子核中质子的数目。质量数（A）是质子数与中子数之和。
We represent these numbers using standard notation: mass number as a superscript to the left and atomic number as a subscript, e.g. ²³₁₁Na for sodium.
我们用标准符号来表示：质量数写在左侧上标位置，原子序数写在左侧下标位置，例如钠表示为 ²³₁₁Na。
To find the number of neutrons, subtract the atomic number from the mass number: neutrons = A − Z.
要计算中子数，用质量数减去原子序数：中子数 = A − Z。
4. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.
同位素是同一种元素中质子数相同而中子数不同的原子。它们具有相同的原子序数但不同的质量数。
For example, carbon has two main stable isotopes: carbon-12 (¹²₆C) with 6 protons and 6 neutrons, and carbon-14 (¹⁴₆C) with 6 protons and 8 neutrons.
例如，碳有两种主要稳定同位素：碳-12（¹²₆C）有6个质子、6个中子，碳-14（¹⁴₆C）有6个质子、8个中子。
Isotopes of an element have identical chemical properties because they have the same electron configuration. Their physical properties such as density may differ slightly.
同一元素的同位素具有完全相同的化学性质，因为它们电子排布相同；但物理性质（如密度）会有细微差别。
5. Relative Atomic Mass (Aᵣ) | 相对原子质量（Aᵣ）
Relative atomic mass (Aᵣ) is the average mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom. It has no units.
相对原子质量（Aᵣ）是元素一个原子的平均质量与一个碳-12原子质量的 1/12 相比的比值，没有单位。
The definition is always tied to carbon-12. Since most elements consist of a mixture of isotopes, Aᵣ is a weighted average.
此定义始终与碳-12 相关。由于多数元素是同位素的混合物，Aᵣ 是加权平均值。
WJEC may ask you to explain why Aᵣ is not a whole number – it is because of the existence of isotopes with different masses.
WJEC 可能会要求解释为什么 Aᵣ 不是整数——这是因为存在不同质量的同位素。
6. Calculating Relative Atomic Mass from Isotopic Abundance | 从同位素丰度计算相对原子质量
You need to be able to calculate Aᵣ given the percentage abundances and mass numbers of isotopes. The formula is:
Aᵣ = (mass₁ × %₁ + mass₂ × %₂ + …) / 100
你需要能够根据同位素的丰度百分比和质量数计算 Aᵣ。公式为：
Aᵣ = (质量₁ × 丰度%₁ + 质量₂ × 丰度%₂ + …) / 100
For example, chlorine has two isotopes: ³⁵Cl (75% abundance) and ³⁷Cl (25% abundance). Its Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5.
例如，氯有两种同位素：³⁵Cl（丰度 75%）和 ³⁷Cl（丰度 25%），其 Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5。
Always show your working clearly and check that your answer is between the smallest and largest mass numbers of the isotopes.
务必清晰展示计算过程，并检查答案是否介于同位素的最小和最大质量数之间。
7. Electron Shells | 电子层
Electrons are arranged in shells (energy levels) around the nucleus. The first shell can hold up to 2 electrons, the second up to 8, and the third up to 8 for the first 20 elements.
电子在原子核周围分层（能级）排布。第一层最多容纳 2 个电子，第二层最多 8 个，对于前 20 号元素第三层最多也是 8 个。
Electrons fill the shells starting from the one closest to the nucleus. You will be expected to draw or state the electronic configuration for the first 20 elements.
电子从最靠近原子核的壳层开始填充。你需要会画或写出前 20 号元素的电子排布。
8. Electronic Configuration of the First 20 Elements | 前 20 号元素的电子排布
The electronic configuration can be written in the form 2,8,1 for sodium (Na) or as a diagram showing the shells. The table below lists the patterns for key elements.
电子排布可以写成 2,8,1（钠 Na）的形式，或用壳层图表示。下表列出了一些关键元素的排布规律。
| Element | Atomic number | Electronic configuration |
|---|---|---|
| Hydrogen (H) | 1 | 1 |
| Carbon (C) | 6 | 2,4 |
| Oxygen (O) | 8 | 2,6 |
| Sodium (Na) | 11 | 2,8,1 |
| Chlorine (Cl) | 17 | 2,8,7 |
| Calcium (Ca) | 20 | 2,8,8,2 |
Notice how the group number in the periodic table often matches the number of electrons in the outer shell for Groups 1, 2 and 13-18.
注意周期表中的族数常常与最外层电子数相对应，适用于第 1、2 和 13-18 族。
9. Valence Electrons and the Periodic Table | 价电子与周期表
The electrons in the outermost shell are called valence electrons. They determine the chemical properties of the element and how it bonds.
最外层电子称为价电子。它们决定了元素的化学性质及其结合方式。
Atoms of elements in the same group have the same number of valence electrons. For instance, lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) all have one outer electron and belong to Group 1.
同族元素具有相同数量的价电子。例如锂（2,1）、钠（2,8,1）和钾（2,8,8,1）最外层都只有1个电子，属于第 1 族。
The period number tells you how many occupied shells are present. Sodium is in period 3 and has three shells.
周期数表示已占用的电子层数。钠在第三周期，有三个电子层。
10. Formation of Ions and the Octet Rule | 离子形成与八隅体规则
Atoms become more stable by gaining or losing electrons to achieve a full outer shell, usually 8 electrons (or 2 for the very small atoms). This is called the octet rule.
原子通过得到或失去电子达到全满最外层（通常为8个电子，很小原子为2个）而变得更加稳定，这称为八隅体规则。
Metals tend to lose electrons and form positive ions (cations). For example, a sodium atom loses one electron to form Na⁺ with configuration 2,8.
金属倾向于失电子形成正离子（阳离子）。例如钠原子失去一个电子形成 Na⁺，排布为 2,8。
Non-metals tend to gain electrons and form negative ions (anions). A chlorine atom gains one electron to form Cl⁻ with configuration 2,8,8.
非金属倾向于得电子形成负离子（阴离子）。氯原子得到一个电子形成 Cl⁻，排布变为 2,8,8。
The charge on an ion is the difference between the number of protons and electrons. In an exam, be able to predict the ionic charge from the group number.
离子的电荷等于质子数与电子数之差。考试中要能根据族序数预测离子所带电荷。
11. Key Definitions Summary | 关键定义总结
Make sure you can write concise definitions for these terms, as they often appear in WJEC papers:
- Atomic number: number of protons in an atom of an element.
- Mass number: total number of protons and neutrons in an atom.
- Isotopes: atoms with the same atomic number but different mass numbers.
- Relative atomic mass: the average mass of an atom compared to 1/12 the mass of carbon-12.
确保能写出下列术语的简明定义，它们在 WJEC 试卷中经常出现：
- 原子序数：元素一个原子中的质子数。
- 质量数：原子中质子数与中子数之和。
- 同位素：原子序数相同但质量数不同的原子。
- 相对原子质量：一个原子的平均质量与碳-12 原子质量的 1/12 之比。
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