📚 Atoms and Elements: Key Exam Points for IGCSE Edexcel Science | IGCSE Edexcel 科学：原子与元素 考点精讲
Atoms are the fundamental building blocks of all matter. Understanding atoms and elements is crucial for mastering the chemistry component of IGCSE Edexcel Science, whether you are taking Double Award or Separate Sciences. This guide breaks down the key concepts, definitions, and exam tips to help you ace this topic.
原子是所有物质的基本构建单元。无论你参加的是双奖科学还是单科科学，理解原子和元素对于掌握 IGCSE Edexcel 科学中的化学部分至关重要。本指南分解了关键概念、定义和考试技巧，助你轻松应对这一主题。
1. What is an Atom? | 什么是原子？
An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are incredibly small – their diameter is about 10⁻¹⁰ m. Each atom consists of a central nucleus containing protons and neutrons, surrounded by electrons arranged in shells. The nucleus is positively charged, holds almost all the mass, while the electron cloud takes up most of the space.
原子是能参与化学反应的元素的最小粒子。原子极其微小，直径约为 10⁻¹⁰ 米。每个原子都有一个包含质子和中子的中心核，核外是分层排布的电子。原子核带正电且几乎集中于全部质量，而电子云则占据了绝大部分体积。
The nucleus is much smaller than the whole atom. Its diameter is around 10⁻¹⁴ m. This means most of an atom is empty space. Even though electrons are tiny, they are arranged in energy levels called shells, and their arrangement controls how the atom reacts.
原子核比整个原子小得多，其直径约为 10⁻¹⁴ 米。这意味着原子的大部分是空的。尽管电子很小，它们排列在称为电子层的能级中，而电子的排布决定了原子的反应方式。
2. Subatomic Particles | 亚原子粒子
There are three main subatomic particles: protons, neutrons, and electrons. Their relative masses and charges are summarised below. Protons and neutrons are found in the nucleus, while electrons move around the nucleus in shells.
共有三种主要的亚原子粒子：质子、中子和电子。它们的相对质量和电荷总结如下。质子和中子位于原子核内，电子则在核外的电子层中运动。
Their properties are given in the table:
其性质如下表所示：
| Particle | Relative Mass | Relative Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 (negligible) | -1 | Shells |
Remember: in a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero.
记住：在中性原子中，质子数等于电子数，因此总电荷为零。
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 of an atom. It defines the element – every atom of the same element has the same atomic number. For example, all carbon atoms have 6 protons. The mass number (A) is the total number of protons plus neutrons in the nucleus.
原子序数 (Z) 是原子核中的质子数。它决定了元素种类——同种元素的每个原子都有相同的原子序数。例如，所有碳原子都有 6 个质子。质量数 (A) 是原子核中质子数和中子数的总和。
In a neutral atom, the number of electrons equals the number of protons. Therefore, you can find the number of neutrons by subtracting the atomic number from the mass number.
在中性原子中，电子数等于质子数。因此，用质量数减去原子序数即可得到中子数。
A = Z + number of neutrons
number of neutrons = A − Z
electrons in a neutral atom, and 23 − 11 = 12 neutrons. The symbol can be written as ²³Na.
例如，钠的原子序数为 11，质量数为 23。因此钠有 11 个质子，中性原子中有 11 个电子，以及 23 − 11 = 12 个中子。符号可写作 ²³Na。
4. Elements, Compounds and Mixtures | 元素、化合物和混合物
An element is a substance made of only one type of atom. It cannot be split into simpler substances by chemical means. A compound contains two or more elements chemically combined in fixed proportions. A mixture contains two or more substances that are not chemically combined and can be separated by physical methods.
元素是仅由一种原子组成的物质，不能通过化学方法分解为更简单的物质。化合物由两种或更多元素以固定比例化学结合而成。混合物包含两种或更多未化学结合的物质，可通过物理方法分离。
- Element examples: Cu, O₂, Fe – 元素示例：铜、氧气、铁
- Compound examples: H₂O, CO₂, NaCl – 化合物示例：水、二氧化碳、氯化钠
- Mixture examples: air, seawater, soil – 混合物示例：空气、海水、土壤
Atoms are the building blocks of elements. When different elements react, their atoms bond together to form compounds. In a compound, the elements lose their individual properties and the compound has new properties. In a mixture, each substance keeps its own properties.
原子是元素的构建单元。当不同元素反应时，它们的原子结合形成化合物。在化合物中，元素失去各自的特性，化合物具有新的性质。在混合物中，每种物质保持自身性质。
5. Isotopes | 同位素
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers. For example, carbon-12, carbon-13 and carbon-14 are isotopes of carbon. All have 6 protons, but they have 6, 7 and 8 neutrons respectively.
同位素是同一元素的原子，质子数相同但中子数不同。它们具有相同的原子序数但质量数不同。例如，碳-12、碳-13 和碳-14 是碳的同位素。它们都有 6 个质子，但分别有 6、7 和 8 个中子。
Isotopes have the same chemical properties because chemical reactions depend mainly on electrons. However, their physical properties, such as density or rate of diffusion, may differ slightly because of the different masses. Some isotopes are radioactive, such as carbon-14, which is used in carbon dating.
同位素具有相同的化学性质，因为化学反应主要取决于电子。然而，由于质量不同，它们的物理性质（如密度或扩散速率）可能略有差异。有些同位素具有放射性，例如碳-14 可用于碳定年。
6. Electron Shells and Electronic Configuration | 电子层与电子排布
Electrons occupy shells around the nucleus. The first shell can hold a maximum of 2 electrons. The second shell can hold a maximum of 8 electrons. For the first 20 elements, the third shell can also hold up to 8 electrons. The electronic configuration is written as numbers separated by commas.
电子占据原子核周围的电子层。第一层最多容纳 2 个电子。第二层最多容纳 8 个电子。对于前 20 号元素，第三层最多也可容纳 8 个电子。电子排布用逗号分隔的数字表示。
Na: 2,8,1
Cl: 2,8,7
The number of electrons in the outermost shell determines how an atom reacts. Atoms with full outer shells, such as noble gases, are unreactive. Atoms with one or two outer electrons tend to lose them, while atoms with six or seven outer electrons tend to gain electrons. This links directly to the formation of ions and the periodic table.
最外层电子数决定原子的反应方式。最外层已满的原子，如稀有气体，不易反应。有 1 或 2 个外层电子的原子倾向于失去电子，而有 6 或 7 个外层电子的原子倾向于获得电子。这与离子形成和元素周期表直接相关。
7. The Periodic Table and Atomic Structure | 元素周期表与原子结构
The periodic table arranges elements in order of increasing atomic number. The group number tells you the number of electrons in the outer shell for main group elements. The period number tells you the number of electron shells. For example, sodium is in Group 1 and Period 3, so it has 1 outer electron and 3 shells.
元素周期表按原子序数递增排列元素。对于主族元素，族数表示最外层电子数。周期数表示电子层数。例如，钠位于第 1 族第 3 周期，因此它有 1 个外层电子和 3 个电子层。
- Group 1: alkali metals, 1 outer electron, lose 1 electron to form 1+ ions – 第 1 族：碱金属，1 个外层电子，失去 1 个电子形成 1+ 离子
- Group 7: halogens, 7 outer electrons, gain 1 electron to form 1− ions – 第 7 族：卤素，7 个外层电子，获得 1 个电子形成 1− 离子
- Group 0: noble gases, full outer shell, unreactive – 第 0 族：稀有气体，最外层已满，不易反应
Atomic structure explains the patterns in the periodic table. As you go across a period, the number of protons increases, and the outer shell gradually fills. As you go down a group, the number of shells increases, and the outer electrons are further from the nucleus.
原子结构解释了元素周期表中的规律。沿周期从左到右，质子数增加，最外层逐渐填满。沿族从上到下，电子层数增加，外层电子离原子核更远。
8. Ions and Ionic Charge | 离子与电荷
Ions are charged particles formed when atoms lose or gain electrons. Metals lose electrons to form positive ions, called cations. Non-metals gain electrons to form negative ions, called anions. The charge on an ion can be calculated from the number of protons and electrons.
离子是原子失去或获得电子时形成的带电粒子。金属失去电子形成正离子，称为阳离子。非金属获得电子形成负离子，称为阴离子。离子所带电荷可通过质子数和电子数计算。
charge = number of protons − number of electrons
For example, a sodium atom has 11 protons and 11 electrons. When it loses 1 electron, it becomes Na⁺ with 11 protons and 10 electrons, so the charge is +1. A chlorine atom has 17 protons and 17 electrons. When it gains 1 electron, it becomes Cl⁻ with 17 protons and 18 electrons, so the charge is −1. Other examples include Mg²⁺, O²⁻ and Ca²⁺.
例如，钠原子有 11 个质子和 11 个电子。失去 1 个电子后变成 Na⁺，有 11 个质子和 10 个电子，因此电荷为 +1。氯原子有 17 个质子和 17 个电子。获得 1 个电子后变成 Cl⁻，有 17 个质子和 18 个电子，因此电荷为 −1。其他例子包括 Mg²⁺、O²⁻ 和 Ca²⁺。
9. Chemical Symbols and Formulae | 化学符号与化学式
Each element has a chemical symbol, such as H, C, O, Na and Cl. A chemical formula shows the number and type of atoms in a compound. Subscripts tell you how many atoms of each element are present. For example, H₂O contains 2 hydrogen atoms and 1 oxygen atom. CO₂ contains 1 carbon atom and 2 oxygen atoms.
每种元素都有化学符号，例如 H、C、O、Na 和 Cl。化学式表示化合物中原子的种类和数量。下标告诉你每种元素的原子数。例如，H₂O 含有 2 个氢原子和 1 个氧原子。CO₂ 含有 1 个碳原子和 2 个氧原子。
| Formula | Atoms Present | 中文说明 |
|---|---|---|
| H₂O | 2 H, 1 O | 2 个氢原子，1 个氧原子 |
| CO₂ | 1 C, 2 O | 1 个碳原子，2 个氧原子 |
| NaCl | 1 Na, 1 Cl | 1 个钠原子，1 个氯原子 |
| MgCl₂ | 1 Mg, 2 Cl | 1 个镁原子，2 个氯原子 |
When writing formulae, you must balance the charges of ions. For example, magnesium forms Mg²⁺ and chlorine forms Cl⁻. Two chloride ions are needed to balance one magnesium ion, giving MgCl₂. This is a key skill in IGCSE Edexcel Science.
书写化学式时，必须平衡离子电荷。例如，镁形成 Mg²⁺，氯形成 Cl⁻。需要一个镁离子对应两个氯离子，因此得到 MgCl₂。这是 IGCSE Edexcel 科学中的关键技能。
10. Relative Atomic Mass and Calculations | 相对原子质量与计算
Relative atomic mass (Ar) is the weighted average mass of the atoms of an element compared with 1/12 of the mass of a carbon-12 atom. It is not the same as mass number. Mass number is a whole number for a specific isotope, while relative atomic mass can be a decimal because it takes into account the abundance of different isotopes.
相对原子质量 (Ar) 是元素原子的加权平均质量，与碳-12 原子质量的 1/12 相比较。它与质量数不同。质量数是特定同位素的整数，而相对原子质量考虑不同同位素的丰度，因此可以是小数。
Ar = (mass₁ × abundance₁ + mass₂ × abundance₂ + ...) ÷ 100
has two main isotopes: chlorine-35 and chlorine-37. If chlorine-35 has an abundance of 75% and chlorine-37 has an abundance of 25% This explains why the relative atomic mass of chlorine is 35.5, even though no single chlorine atom has a mass of 35.5.
例如，氯有两种主要同位素：氯-35 和氯-37。如果氯-35
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