📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
Understanding the structure of the atom and how the periodic table is organised is fundamental to IGCSE Science. This revision guide explains the key concepts required for Edexcel IGCSE, including subatomic particles, isotopes, electron arrangements and periodic trends.
理解原子结构和元素周期表的组织方式是 IGCSE 科学的基础。本复习指南讲解 Edexcel IGCSE 所要求的关键概念，包括亚原子粒子、同位素、电子排布及周期趋势。
1. The Atom and Its Subatomic Particles | 原子及其亚原子粒子
An atom is the smallest particle of an element that retains the chemical properties of that element. It consists of a central nucleus containing protons and neutrons, surrounded by electrons in shells.
原子是保持元素化学性质的最小粒子。它由一个包含质子和中子的原子核以及周围电子壳层中的电子组成。
- Proton – positively charged, relative mass 1, found in the nucleus.
- Neutron – neutral, relative mass 1, found in the nucleus.
- Electron – negatively charged, relative mass 1/1836, found in shells.
- 质子 – 带正电，相对质量 1，位于原子核内。
- 中子 – 不带电，相对质量 1，位于原子核内。
- 电子 – 带负电，相对质量约为 1/1836，位于电子壳层中。
Charge of nucleus = number of protons
原子核所带电荷数 = 质子数
2. Atomic Number and Mass Number | 原子序数与质量数
The atomic number (Z) is the number of protons in an atom. The mass number (A) is the total number of protons and neutrons in the nucleus.
原子序数（Z）是原子中的质子数。质量数（A）是原子核中质子数与中子数之和。
Number of neutrons = A − Z
中子数 = 质量数 − 原子序数
For example, a chlorine atom with atomic number 17 and mass number 35 has 17 protons, 17 electrons and 18 neutrons.
例如，一个原子序数为 17、质量数为 35 的氯原子有 17 个质子、17 个电子和 18 个中子。
3. Isotopes | 同位素
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Therefore they have the same atomic number but different mass numbers.
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。因此它们具有相同的原子序数但不同的质量数。
For example, carbon-12 and carbon-14 are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.
例如，碳-12 和碳-14 是碳的同位素。两者都有 6 个质子，但碳-12 有 6 个中子，碳-14 有 8 个中子。
Isotopes of the same element have the same chemical properties because they have the same electron arrangement. However, their physical properties, such as density, may differ.
同一元素的同位素具有相同的化学性质，因为它们具有相同的电子排布。然而，它们的物理性质（如密度）可能不同。
4. Electron Arrangement | 电子排布
Electrons occupy shells (energy levels) around the nucleus. The first shell can hold up to 2 electrons, the second and third shells can each hold up to 8 electrons.
电子占据原子核周围的壳层（能级）。第一壳层最多可容纳 2 个电子，第二和第三壳层各最多可容纳 8 个电子。
To draw an atom, write the electrons in energy levels. For example, sodium has 11 electrons: 2,8,1.
在画出原子结构时，按能级写出电子。例如，钠有 11 个电子：2,8,1。
Electronic configuration of sodium: 2,8,1
钠的电子排布：2,8,1
5. The Periodic Table | 元素周期表
The periodic table arranges elements in order of increasing atomic number. Elements with similar properties are placed in vertical columns called groups.
元素周期表按原子序数递增的顺序排列元素。具有相似性质的元素位于称为族的垂直列中。
Each horizontal row is called a period. The period number corresponds to the number of occupied electron shells.
每个水平行称为一个周期。周期数与已占据的电子壳层数相对应。
6. Groups and Periods | 族与周期
Elements in the same group have the same number of electrons in their outermost shell. This gives them similar chemical reactions.
同一族的元素具有相同的最外层电子数。这使它们具有相似的化学反应。
For example, Group 1 elements have one outer electron, Group 7 elements have seven outer electrons. The group number often helps predict the valency.
例如，第 1 族元素有一个外层电子，第 7 族元素有七个外层电子。族号通常有助于预测化合价。
The period number tells us how many shells are occupied. An element in Period 3 has three electron shells.
周期号告诉我们有多少电子壳层被占据。第 3 周期的元素有三个电子壳层。
7. Metals and Non-metals | 金属与非金属
In the periodic table, metals are found on the left and centre, while non-metals are found on the right. The stepped line between groups 13 and 14 separates metals from non-metals.
在元素周期表中，金属位于左侧和中间，非金属位于右侧。第 13 族和第 14 族之间的阶梯线将金属与非金属分开。
Metals tend to lose electrons to form positive ions, while non-metals tend to gain electrons to form negative ions.
金属倾向于失去电子形成正离子，而非金属倾向于获得电子形成负离子。
- Metals: shiny, malleable, good conductors of heat and electricity.
- Non-metals: dull, brittle, poor conductors (except graphite).
- 金属：有光泽、可延展、良好的热和电导体。
- 非金属：暗淡、易碎、不良导体（石墨除外）。
8. Periodic Trends: Atomic Radius | 周期趋势：原子半径
Across a period, the atomic radius generally decreases. This is because the nuclear charge increases, pulling electrons closer to the nucleus.
在同一周期中，原子半径通常减小。这是因为核电荷增加，将电子拉得更靠近原子核。
Down a group, the atomic radius increases because each new period adds an extra electron shell, which increases the distance from the nucleus.
在同一族中，原子半径增大，因为每增加一个周期就增加一个新的电子壳层，从而增加了原子核与最外层电子的距离。
9. Periodic Trends: Ionisation Energy | 周期趋势：电离能
Ionisation energy is the energy required to remove an electron from an atom. Across a period, ionisation energy generally increases due to a stronger nuclear charge.
电离能是从原子中移去一个电子所需的能量。在同一周期中，电离能通常因核电荷增强而增大。
Down a group, ionisation energy decreases because the outer electrons are further from the nucleus and more shielded from the pull of the nucleus.
在同一族中，电离能下降，因为外层电子离原子核更远，并且受到核吸引的屏蔽效应更强。
10. Forming Ions and Ionic Bonds | 离子形成与离子键
Atoms can form ions to achieve a full outer shell, usually following the octet rule. Metals lose electrons, and non-metals gain electrons.
原子可以通过形成离子来获得全满的外壳，通常遵循八隅体规则。金属失去电子，非金属获得电子。
Ionic bonds occur between positively charged cations and negatively charged anions through electrostatic attraction. For example, sodium chloride is formed from Na⁺ and Cl⁻.
离子键存在于带正电的阳离子与带负电的阴离子之间，通过静电吸引形成。例如，氯化钠由 Na⁺ 和 Cl⁻ 形成。
Na → Na⁺ + e⁻ Cl + e⁻ → Cl⁻
Na → Na⁺ + e⁻ Cl + e⁻ → Cl⁻
11. Common Exam Mistakes | 常见考试错误
Many students confuse atomic number with mass number. Remember: the atomic number is the number of protons, not the total number of particles.
许多学生混淆原子序数与质量数。请记住：原子序数是质子数，而不是粒子总数。
Another common mistake is using the group number to determine the number of shells. The period gives the shell count, not the group.
另一个常见错误是用族号来判断壳层数。周期号才给出壳层数量，而不是族号。
When writing electron configurations, always check the maximum electron capacity: 2, 8, 8 for the first three shells.
在书写电子排布时，始终检查最大电子容量：前三个壳层分别为 2、8、8。
12. Summary of Key Equations | 关键公式总结
The following relationships are essential for solving IGCSE questions.
以下关系对于解决 IGCSE 问题至关重要。
| Quantity | Formula | Example |
| Neutrons | Mass number − Atomic number | For ³⁵Cl: 35 − 17 = 18 |
| Electrons in neutral atom | Number of protons | Na has 11 electrons |
| Group number | Number of outer electrons | Oxygen (2,6) is Group 6 |
Neutrons = Mass number − Atomic number
中子数 = 质量数 − 原子序数
Mastering atomic structure and the periodic table gives you a solid base for chemical bonding, equations and rates of reaction. Keep practising with past papers and focus on trend explanations.
掌握原子结构和元素周期表为你学习化学键、方程式和反应速率打下坚实基础。请持续练习历年真题，并重点理解趋势的解释。
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