📚 IGCSE Chemistry: Atomic Structure Exam Essentials | IGCSE 化学：原子结构考点精讲
Atomic structure is a foundational topic in IGCSE Chemistry, underpinning your understanding of bonding, reactivity, and the periodic table. Exam questions frequently target subatomic particles, isotopes, electron configurations, and relative atomic mass calculations. This bilingual revision guide breaks down the most important concepts in clear, paired English-Chinese explanations to help you revise efficiently and score higher.
原子结构是 IGCSE 化学的基础课题，为您理解化学键、反应性和元素周期表奠定了基础。考试题目经常涉及亚原子粒子、同位素、电子排布和相对原子质量的计算。这份双语复习指南用清晰的英汉对照讲解分解最重要的概念，帮助您高效复习并提高分数。
1. Subatomic Particles | 亚原子粒子
All atoms consist of three subatomic particles: protons, neutrons, and electrons. Protons are positively charged and found in the nucleus. Neutrons have no charge and are also located in the nucleus. Electrons are negatively charged and move around the nucleus in specific energy levels, often called shells.
所有原子都由三种亚原子粒子组成：质子、中子和电子。质子带正电，位于原子核内。中子不带电，也存在于原子核中。电子带负电，在特定的能级（通常称为电子层）上绕核运动。
The table below summarises the properties of these particles. Notice how electrons contribute almost nothing to the mass of an atom, yet their number and arrangement determine chemical behaviour.
下表总结了这些粒子的性质。请注意，电子对原子质量的贡献几乎为零，但它们的数量和排布决定了化学行为。
| Particle | Relative Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1840 (negligible) | Shells |
2. Atomic Number and Mass Number | 原子序数与质量数
The atomic number (Z) of an element is the number of protons in the nucleus of one atom. It is unique to each element and determines the element’s identity. The mass number (A) is the total number of protons and neutrons in the nucleus. It represents the approximate mass of the atom in atomic mass units.
元素的原子序数 (Z) 是单个原子核内质子的数目。每种元素都有唯一的原子序数，决定了元素本身。质量数 (A) 是核内质子和中子的总数，以原子质量单位表示原子的近似质量。
IGCSE papers often ask you to interpret nuclide notation. The symbol is written with the mass number as a superscript on the left and the atomic number as a subscript on the left: AZX. For example, a sodium atom with 11 protons and 12 neutrons is ²³₁₁Na. In this notation, you can easily calculate the number of neutrons by subtracting the atomic number from the mass number.
IGCSE 试卷常常要求解释核素符号。写法是将质量数写在左上标，原子序数写在左下标：AZX。例如，一个拥有 11 个质子和 12 个中子的钠原子表示为 ²³₁₁Na。通过这个符号，您只需用质量数减去原子序数，就能轻松算出中子数目。
3. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Therefore, they have the same atomic number but different mass numbers. For instance, chlorine has two stable isotopes: chlorine-35 (³⁵Cl) with 18 neutrons, and chlorine-37 (³⁷Cl) with 20 neutrons. Both have 17 protons.
同位素是相同元素的不同原子，它们具有相同的质子数，但中子数不同。因此，它们的原子序数相同而质量数不同。例如，氯有两种稳定同位素：氯-35 (³⁵Cl) 有 18 个中子，氯-37 (³⁷Cl) 有 20 个中子。两者都有 17 个质子。
Because chemical properties are determined by the electron arrangement, isotopes of the same element have identical chemical behaviour. However, their physical properties, such as mass and density, differ slightly. This is a common exam point: isotopes react in the same way.
因为化学性质由电子排布决定，同一元素的同位素具有完全相同的化学行为。然而，它们的物理性质（如质量、密度）略有不同。这是一个常见的考点：同位素的化学反应方式相同。
4. Relative Atomic Mass Calculation | 相对原子质量的计算
Relative atomic mass (Ar) is defined as the weighted average mass of an element’s isotopes relative to 1/12th the mass of a carbon-12 atom. It is calculated using the percentage abundance of each isotope. The formula is:
相对原子质量 (Ar) 定义为元素各同位素相对于碳-12 原子质量的 1/12 的加权平均质量。它通过各同位素的丰度百分比进行计算。公式如下：
Ar = (mass₁ × %abundance₁ + mass₂ × %abundance₂ + …) ÷ 100
Let’s calculate the Ar of chlorine: ³⁵Cl (75 %) and ³⁷Cl (25 %). Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5. Therefore, the relative atomic mass of chlorine is 35.5, which appears on the periodic table. Remember to divide by 100 when percentages are given, or by the sum if using ratios.
我们来计算氯的相对原子质量：³⁵Cl (75%) 和 ³⁷Cl (25%)。Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5。因此，氯的相对原子质量是 35.5，这也是元素周期表上显示的值。若给出的是百分比，记得除以 100；若给出的是比值，则除以比值之和。
5. Electron Configuration | 电子排布
Electrons occupy shells (energy levels) around the nucleus. The lowest energy shells fill first. For the first 20 elements, the maximum number of electrons each shell can hold follows a simple pattern: first shell: 2; second shell: 8; third shell: 8. For example, sodium (atomic number 11) has the electronic configuration 2,8,1. Oxygen (atomic number 8) is 2,6.
电子占据原子核周围的电子层（能级）。能量最低的电子层最先被填满。对于前 20 种元素，每个电子层可容纳的最大电子数遵循一个简单规律：第一层：2；第二层：8；第三层：8。例如，钠（原子序数 11）的电子排布为 2,8,1。氧（原子序数 8）为 2,6。
You must be able to draw ‘dot-and-cross’ diagrams or write the numerical configuration. Remember that when the third shell begins to fill after argon (18 electrons), the fourth shell starts before the third is fully occupied (e.g., potassium is 2,8,8,1, not 2,8,9). This is a common trap in IGCSE exams.
您必须能够绘制“点叉”图或写出数字排布。请记住，在氩（18 个电子）之后，第三层未满时第四层便开始填充（例如钾的排布是 2,8,8,1，而不是 2,8,9）。这是 IGCSE 考试中常见的陷阱。
6. Valence Electrons and the Periodic Table | 价电子与元素周期表
The electrons in the outermost shell are called valence electrons. The number of valence electrons determines an element’s group in the periodic table (for Groups 1–2 and 13–18). For example, lithium (2,1), sodium (2,8,1), and potassium (2,8,8,1) all have one valence electron and are in Group 1.
最外层中的电子称为价电子。价电子数决定了元素在周期表中的族数（适用于第 1–2 和 13–18 族）。例如，锂 (2,1)、钠 (2,8,1) 和钾 (2,8,8,1) 都有 1 个价电子，位于第 1 族。
Elements in the same group have similar chemical properties because they have the same number of valence electrons. The period number tells you how many occupied electron shells an atom has. Carbon (2,4) has two shells, so it is in Period 2.
同一族的元素具有相似的化学性质，因为它们拥有相同的价电子数。周期序数表示原子中已占据的电子层数。碳 (2,4) 有两个电子层，因此它位于第 2 周期。
7. Formation of Ions | 离子的形成
Atoms become ions by gaining or losing electrons to achieve a full outer shell – the stable configuration of a noble gas. Metals typically lose their valence electrons to form positive ions (cations). Non-metals gain electrons to form negative ions (anions).
原子通过获得或失去电子使最外层达到满壳层（即稀有气体的稳定构型）而变成离子。金属通常失去价电子形成阳离子。非金属获得电子形成阴离子。
Consider sodium: Na (2,8,1) loses one electron to form Na⁺ (2,8). Magnesium (2,8,2) loses two electrons to form Mg²⁺ (2,8). Meanwhile, chlorine (2,8,7) gains one electron to form Cl⁻ (2,8,8). Oxide ion O²⁻ (2,8) comes from oxygen gaining two electrons. The charge equals the number of electrons lost or gained.
以钠为例：Na (2,8,1) 失去一个电子变成 Na⁺ (2,8)。镁 (2,8,2) 失去两个电子形成 Mg²⁺ (2,8)。而氯 (2,8,7) 获得一个电子变成 Cl⁻ (2,8,8)。氧离子 O²⁻ (2,8) 由氧获得两个电子形成。所带电荷数等于失去或获得的电子数。
8. History of Atomic Models | 原子模型的历史
IGCSE often includes a question on how the atomic model developed. John Dalton proposed atoms as solid spheres. J.J. Thomson discovered the electron and suggested the ‘plum pudding’ model – electrons embedded in a positive sphere. Ernest Rutherford’s gold foil experiment showed atoms have a small, dense, positively charged nucleus, with electrons around it. Niels Bohr refined this by introducing fixed electron shells (energy levels).
IGCSE 经常包含关于原子模型发展的问题。约翰·道尔顿提出原子是实心球体。J.J.汤姆逊发现了电子，提出了“葡萄干布丁”模型——电子嵌在正电球体中。欧内斯特·卢瑟福的金箔实验表明原子有一个微小、致密、带正电的原子核，电子绕其运动。尼尔斯·玻尔进一步完善，引入了固定的电子层（能级）。
Key experimental evidence: Rutherford’s experiment – most alpha particles passed straight through, some were deflected, and a few bounced back, suggesting a concentrated nucleus. You should be able to compare these models and explain why each was replaced.
关键实验证据：卢瑟福实验——大多数 α 粒子直线穿过，一些发生偏转，极少数被反弹回来，这表明存在一个集中的原子核。您应能够比较这些模型并解释每个模型被取代的原因。
9. Calculating Subatomic Particles | 亚原子粒子的计算
For a neutral atom: number of protons = atomic number, number of electrons = atomic number, number of neutrons = mass number – atomic number. For a charged ion, protons stay the same, electrons = atomic number – (charge for cation) or + (charge for anion). For example, ²⁷₁₃Al³⁺: protons = 13, neutrons = 27 – 13 = 14, electrons = 13 – 3 = 10.
对于中性原子：质子数 = 原子序数，电子数 = 原子序数，中子数 = 质量数 – 原子序数。对于带电离子，质子数不变，电子数 = 原子序数 – 阳离子电荷数（或 + 阴离子电荷数）。例如，²⁷₁₃Al³⁺：质子数 = 13，中子数 =
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