📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
The atom is the fundamental building block of all matter. Understanding its structure and how it relates to the periodic table is a core topic in IGCSE Science. This article explains key concepts step by step, from subatomic particles to periodic trends, with worked examples to prepare you for exams.
原子是所有物质的基本构成单位。理解原子结构以及它与元素周期表的关系，是IGCSE科学的核心主题。本文逐步讲解关键概念，从亚原子粒子到周期趋势，并提供例题助你备考。
1. Development of the Atomic Model | 原子模型的发展
Early ideas: John Dalton proposed that atoms were solid, indivisible spheres. In 1897, J.J. Thomson discovered electrons and suggested the plum pudding model, where positive charge was spread evenly with electrons stuck inside.
早期观点：约翰·道尔顿提出原子是实心、不可再分的球体。1897年，J.J.汤姆逊发现电子，并提出葡萄干布丁模型，认为正电荷均匀分布，电子镶嵌其中。
In 1911, Ernest Rutherford’s gold foil experiment showed that most of the atom is empty space, with a tiny, dense, positive nucleus. Niels Bohr then described electrons orbiting the nucleus in fixed energy shells, a model still used in IGCSE.
1911年，欧内斯特·卢瑟福的金箔实验表明，原子大部分是空的空间，中央有一个微小、致密、带正电的原子核。随后尼尔斯·玻尔提出电子在固定能量壳层中绕核运动，这个模型至今仍用于IGCSE教学。
2. Subatomic Particles | 亚原子粒子
An atom consists of three types of subatomic particle: protons, neutrons and electrons. Protons and neutrons are found in the nucleus, while electrons move in shells around the nucleus.
原子由三种亚原子粒子组成：质子、中子和电子。质子和中子位于原子核内，电子则在原子核周围的壳层中运动。
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Proton: relative mass 1, relative charge +1. | 质子：相对质量为1，相对电荷为+1。
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Neutron: relative mass 1, relative charge 0. | 中子：相对质量为1，相对电荷为0。
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Electron: relative mass 1/1836, relative charge -1. | 电子：相对质量为1/1836，相对电荷为-1。
The number of protons defines the element, as it determines the atomic number. In a neutral atom, the number of electrons equals the number of protons.
质子数决定了元素种类，因为它决定了原子序数。在电中性的原子中，电子数等于质子数。
3. Atomic Number, Mass Number and Isotopes | 原子序数、质量数与同位素
Atomic number (Z) is the number of protons in the nucleus of an atom. Mass number (A) is the total number of protons and neutrons.
原子序数（Z）是指原子核中的质子数。质量数（A）是质子数和中子数的总和。
A = Z + N
质量数 = 质子数 + 中子数
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. For example, carbon-12 (¹²C) has 6 protons and 6 neutrons, while carbon-14 (¹⁴C) has 6 protons and 8 neutrons.
同位素是同一元素的原子，具有相同的质子数但不同中子数。例如，碳-12（¹²C）有6个质子和6个中子，而碳-14（¹⁴C）有6个质子和8个中子。
Isotopes have identical chemical properties because chemical behaviour depends on the electron configuration, which is the same for isotopes of an element.
同位素具有相同的化学性质，因为化学行为取决于电子排布，同一元素的同位素电子排布相同。
4. Electronic Configuration | 电子排布
Electrons occupy shells (energy levels) around the nucleus. The first shell holds a maximum of 2 electrons, the second and third shells hold up to 8 electrons each.
电子占据原子核外的壳层（能级）。第一壳层最多容纳2个电子，第二和第三壳层最多各容纳8个电子。
To write the electronic configuration, fill the shells in order: first 2, then 8, then 8 (for elements up to calcium in IGCSE). For example, sodium (Na, Z=11) has the configuration 2,8,1.
书写电子排布时，按顺序填充壳层：先2，再8，再8（在IGCSE范围内最多到钙）。例如，钠（Na，Z=11）的排布为2,8,1。
The electrons in the outermost shell are called valence electrons. They determine the chemical reactivity of the element.
最外层电子称为价电子。它们决定元素的化学活泼性。
5. Organisation of the Periodic Table | 元素周期表的组织
The periodic table arranges elements in order of increasing atomic number. Each horizontal row is called a period, and each vertical column is called a group.
元素周期表按原子序数递增的顺序排列元素。每一横行为一个周期，每一纵列为一个族。
Elements in the same group have the same number of outer electrons, so they show similar chemical properties. For example, Group 1 elements all have one outer electron, Group 7 elements all have seven outer electrons.
同一族的元素具有相同的最外层电子数，因此化学性质相似。例如，第1族元素都只有1个最外层电子，第7族元素都有7个最外层电子。
The group number equals the number of valence electrons for main-group elements, except helium which has 2 outer electrons despite being in Group 0 (Group 8).
对于主族元素，族号等于价电子数，但氦例外，它在第0族（第8族）却有2个外层电子。
6. Groups and Periods | 族与周期
Period 1 contains hydrogen and helium. Period 2 and 3 each contain 8 elements. The period number indicates how many electron shells are occupied.
第1周期包含氢和氦。第2和第3周期各含8种元素。周期号表示元素占据了多少个电子壳层。
Group 1 (alkali metals) are soft, reactive metals with low density. Group 7 (halogens) are non-metals that exist as diatomic molecules. Group 0 (noble gases) are unreactive because they have full outer shells.
第1族（碱金属）是柔软、活泼、密度小的金属。第7族（卤素）是非金属，以双原子分子形式存在。第0族（稀有气体）因具有全满的电子壳层而不易反应。
These group trends help predict properties of unfamiliar elements. For example, chlorine (Group 7) reacts with sodium to form sodium chloride; iodine also reacts similarly but less vigorously.
这些族趋势帮助预测未知元素的性质。例如，氯（第7族）与钠反应生成氯化钠；碘也能发生类似反应，但反应更不明显。
7. Metals and Non-Metals | 金属与非金属
Metals are found on the left and in the middle of the periodic table. They typically have high melting points, conduct electricity and are malleable. Non-metals are found on the right, and they are usually poor conductors and brittle.
金属位于周期表的左侧和中间。它们通常熔点高、导电、有延展性。非金属位于右侧，通常不导电且易碎。
Metallic elements tend to lose electrons to form positive ions (cations), while non-metals tend to gain electrons to form negative ions (anions).
金属元素倾向于失去电子形成正离子（阳离子），而非金属倾向于获得电子形成负离子（阴离子）。
Metalloids, such as silicon and germanium, have properties between metals and non-metals and are often used in semiconductors.
准金属，如硅和锗，性质介于金属与非金属之间，常用于半导体。
8. Trends in the Periodic Table | 元素周期表中的趋势
Across a period from left to right, the atomic radius generally decreases because the increasing nuclear charge pulls electrons closer.
在同一周期从左到右，原子半径通常减小，因为核电荷增加将电子拉得更近。
Down a group, atomic radius increases because each new shell is added, making the atom larger. Ionisation energy generally decreases down a group due to increased shielding and distance from nucleus.
在族中从上到下，原子半径增大，因为每增加一个新壳层使原子变大。电离能通常随族往下减小，因为屏蔽效应增加且电子离核更远。
| Property | 性质 | Across a period (left → right) | 同周期（左→右） | Down a group (top → bottom) | 同族（上→下） |
| Atomic radius | 原子半径 | Decreases | 减小 | Increases | 增大 |
| Metallic character | 金属性 | Decreases | 减弱 | Increases | 增强 |
| Reactivity (Group 1) | 活泼性（第1族） | — | Increases | 增强 |
These trends are important for predicting relative reactivities and physical properties in exam questions.
这些趋势在考题中常用于预测反应活性和物理性质的相对变化。
9. Formation of Ions | 离子的形成
Atoms gain or lose electrons to achieve a stable electron configuration, usually a full outer shell.
原子通过获得或失去电子来达到稳定电子排布，通常是填满外壳层。
Sodium (2,8,1) loses its outer electron to form Na⁺ with configuration 2,8. Chlorine (2,8,7) gains one electron to form Cl⁻ with configuration 2,8,8.
钠（2,8,1）失去最外层电子形成Na⁺，排布为2,8。氯（2,8,7）获得一个电子形成Cl⁻，排布为2,8,8。
Positive ions are smaller than their parent atoms because losing electrons removes the entire outer shell, reducing the radius. Negative ions are larger because the gain of electrons increases electron-electron repulsion.
阳离子比其母原子小，因为失去电子相当于移除了整个外壳层，使半径减小。阴离子则比母原子大，因为获得电子增加了电子之间的排斥。
10. Applications of Isotopes | 同位素的应用
Radioactive isotopes have many uses. Carbon-14 is used in radiocarbon dating to determine the age of ancient organic materials.
放射性同位素有多种用途。碳-14用于放射性测年，以确定古代有机材料的年代。
Cobalt-60 emits gamma rays and is used in cancer radiotherapy and to sterilise medical equipment. Iodine-131 is used to treat thyroid disorders by targeting thyroid tissue.
钴-60发射伽马射线，用于癌症放射治疗和医疗器械灭菌。碘-131用于治疗甲状腺疾病，通过靶向甲状腺组织。
Non-radioactive isotopes, such as the carbon-13 in NMR spectroscopy, help scientists study chemical structures without harmful radiation.
非放射性同位素，如核磁共振波谱中的碳-13，帮助科学家在不产生有害辐射的情况下研究化学结构。
11. Worked Example: Calculating Subatomic Particles | 例题：计算亚原子粒子
Question: An atom of potassium has a mass number of 39 and an atomic number of 19. Determine the number of protons, neutrons and electrons in a neutral atom.
题目：钾原子的质量数为39，原子序数为19。求中性原子中的质子数、中子数和电子数。
Protons = atomic number = 19
电子 = 原子序数 = 19
Neutrons = mass number − atomic number = 39 − 19 = 20
中子数 = 质量数 − 原子序数 = 39 − 19 = 20
For an ion such as K⁺, one electron is removed, so the number of electrons becomes 18 while protons and neutrons remain the same.
对于像K⁺这样的离子，会失去一个电子，因此电子数变为18，而质子数和中子数保持不变。
12. Summary | 总结
Key points to remember: protons and neutrons form the nucleus; electrons occupy shells; atomic number equals proton number; mass number equals protons plus neutrons; isotopes differ in neutrons; periodic table groups are based on outer electrons; trends can be explained by nuclear charge and shell number.
需要记住的关键点：质子和中子构成原子核；电子占据壳层；原子序数等于质子数；质量数等于质子数加中子数；同位素中子数不同；周期表族的划分基于最外层电子；趋势可由核电荷和壳层数解释。
Practice writing electron configurations and identifying particles for different elements to build confidence. These basics will support your understanding of ionic and covalent bonding in the next topic.
多练习书写电子排布和确定不同元素的粒子数量，以增强信心。这些基础知识将帮助你理解下一主题的离子键和共价键。
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