Atoms, Elements and Compounds | 原子、元素与化合物

📚 Atoms, Elements and Compounds | 原子、元素与化合物

Everything around us is made of matter, and matter is built from atoms. In the Edexcel IGCSE Science course, understanding atoms, elements and compounds is the foundation for all other topics in chemistry and physics. This article breaks down the essential facts and skills you need for your exam, with clear explanations and worked examples.

我们周围的一切都是由物质构成的，而物质由原子组成。在 Edexcel IGCSE 科学课程中，理解原子、元素和化合物是所有化学和物理主题的基础。本文分解了考试所需的关键知识和技能，并配有清晰的解释和例题。

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1. The Structure of an Atom | 原子的结构

An atom is the smallest particle of an element that can take part in a chemical reaction. It consists of three subatomic particles: protons, neutrons and electrons. Protons and neutrons are found in the central nucleus, while electrons move around the nucleus in shells (energy levels).

原子是元素的最小粒子，可以参与化学反应。它由三种亚原子粒子组成：质子、中子和电子。质子和中子位于中心原子核内，而电子在核外壳层（能级）中运动。

• Protons: positively charged, relative mass 1, located in the nucleus.

  质子：带正电荷，相对质量为 1，位于原子核中。

• Neutrons: neutral (no charge), relative mass 1, located in the nucleus.

  中子：电中性（无电荷），相对质量为 1，位于原子核中。

• Electrons: negatively charged, relative mass 1/1840 (approximately 0), located in shells.

  电子：带负电荷，相对质量约为 1/1840（近似为 0），位于电子壳层中。

Atom = protons + neutrons + electrons

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2. 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 which element the atom belongs to. The mass number (A) is the total number of protons and neutrons in the nucleus.

原子序数（Z）是原子核中的质子数。它决定了该原子属于哪种元素。质量数（A）是原子核中质子数和中子数的总和。

Quantity	Definition	Units
Atomic number (Z)	Number of protons	none
Mass number (A)	Protons + neutrons	atomic mass units

To find the number of neutrons, simply subtract the atomic number from the mass number: neutrons = mass number – atomic number.

要计算中子数，只需用质量数减去原子序数：中子数 = 质量数 – 原子序数。

Neutrons = A − Z

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3. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. For example, carbon-12 and carbon-14 are both carbon atoms, but carbon-14 has two extra neutrons.

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。这意味着它们的原子序数相同，但质量数不同。例如，碳-12 和碳-14 都是碳原子，但碳-14 多了两个中子。

• Same chemical properties because they have the same electron arrangement.

  化学性质相同，因为它们的电子排列相同。

• Different physical properties, such as density and rate of diffusion.

  物理性质不同，如密度和扩散速率。

A common exam question asks you to identify the number of each subatomic particle in an isotope. For example: how many neutrons are in ²³₁₁Na? Mass number = 23, atomic number = 11, so neutrons = 23 − 11 = 12.

一个常见考试问题是让你找出同位素中各亚原子粒子的数量。例如：²³₁₁Na 中有多少个中子？质量数 = 23，原子序数 = 11，所以中子数 = 23 − 11 = 12。

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4. Relative Atomic Mass | 相对原子质量

The relative atomic mass (Aᵣ) of an element is the average mass of an atom of that element compared with one-twelfth of the mass of an atom of carbon-12. Because most elements exist as a mixture of isotopes, the Aᵣ is a weighted average.

元素的相对原子质量（Aᵣ）是该元素一个原子的平均质量与一个碳-12 原子质量的 1/12 之比。由于大多数元素以同位素混合物形式存在，Aᵣ 是一个加权平均值。

The equation used to calculate Aᵣ from isotope abundances is:

Aᵣ = (mass₁ × abundance₁ + mass₂ × abundance₂ + …) / total abundance

For chlorine, 75% is ³⁵Cl and 25% is ³⁷Cl, so Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5.

对于氯，75% 是 ³⁵Cl，25% 是 ³⁷Cl，所以 Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5。

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5. Electron Arrangement | 电子排布

Electrons occupy energy levels (shells) around the nucleus. The first shell can hold up to 2 electrons, and the second and third shells can hold up to 8 electrons each. Electrons fill the inner shells first before moving to outer shells.

电子占据原子核周围的能级（壳层）。第一层最多可容纳 2 个电子，第二层和第三层各最多可容纳 8 个电子。电子先填满内层，再向外层填充。

For example, sodium has 11 electrons: the arrangement is 2, 8, 1. Magnesium (12 electrons) has arrangement 2, 8, 2. The electron arrangement of an atom determines its chemical reactivity and its position in the periodic table.

例如，钠有 11 个电子：排列为 2, 8, 1。镁（12 个电子）排列为 2, 8, 2。原子的电子排布决定了其化学反应活性及其在元素周期表中的位置。

Element	Electrons	Arrangement (shell 1, 2, 3)
Helium	2	2
Neon	10	2, 8
Aluminium	13	2, 8, 3

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6. The Periodic Table: Groups and Periods | 元素周期表：族和周期

The periodic table organises elements in order of increasing atomic number. A row is called a period, and a column is called a group. Elements in the same group have the same number of outer-shell electrons, so they have similar chemical properties.

元素周期表按原子序数递增的顺序排列元素。横行称为周期，纵列称为族。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。

• Group 1 (alkali metals): 1 outer electron, very reactive metals.

  第 1 族（碱金属）：1 个最外层电子，非常活泼的金属。

• Group 7 (halogens): 7 outer electrons, reactive non-metals.

  第 7 族（卤素）：7 个最外层电子，活泼的非金属。

• Group 0 (noble gases): full outer shell (2 or 8), extremely unreactive.

  第 0 族（稀有气体）：最外层电子满（2 或 8），极不活泼。

The group number (for groups 1, 2, 7 and 0) often tells you the number of electrons in the outer shell for elements in that group.

对于第 1、2、7 和 0 族，族号通常告诉你该族元素最外层的电子数。

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7. Metals and Non-metals | 金属和非金属

Elements are classified as metals or non-metals based on their physical and chemical properties. In the periodic table, metals appear on the left and centre, while non-metals appear on the right (including hydrogen at the top left).

元素根据其物理和化学性质被分类为金属或非金属。在周期表中，金属位于左侧和中间，非金属位于右侧（包括左上角的氢）。

Property	Metals	Non-metals
Appearance	Shiny (lustrous)	Dull
Conduction of heat/electricity	Good conductors	Poor conductors (except graphite)
State at room temperature	Usually solid (except mercury)	Solid, liquid or gas
Malleable / ductile	Yes	Brittle when solid

Metal oxides tend to form basic solutions in water, while non-metal oxides tend to form acidic solutions.

金属氧化物在水中通常形成碱性溶液，而非金属氧化物通常形成酸性溶液。

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8. Compounds and Molecules | 化合物和分子

A compound is a substance formed when two or more different elements are chemically combined in fixed proportions. A molecule is a group of atoms held together by covalent bonds (usually between non-metals). Compounds can be molecular or ionic.

化合物是由两种或两种以上不同元素按固定比例化学结合而成的物质。分子是由共价键（通常在非金属之间）连接的一组原子。化合物可以是分子型或离子型。

• Ionic compounds: formed between metals and non-metals when electrons are transferred (e.g., sodium chloride, NaCl).

  离子化合物：金属和非金属之间通过电子转移形成（如氯化钠，NaCl）。

• Covalent compounds: formed between non-metals when electrons are shared (e.g., water, H₂O).

  共价化合物：非金属之间通过共用电子对形成（如水，H₂O）。

Remember: an element is a pure substance that cannot be broken down into simpler substances by chemical methods. A compound is different from a mixture because a mixture has no chemical bonds between its components.

记住：元素是无法通过化学方法分解为更简单物质的纯净物。化合物与混合物不同，因为混合物中各成分之间没有化学键。

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9. Chemical Formulae and Equations | 化学式和化学方程式

Chemical formulae show the number and type of atoms in a formula unit. For example, CaCO₃ contains one calcium atom, one carbon atom and three oxygen atoms. Subscripts are used to show the number of atoms of each element.

化学式表示一个式量单位中原子的种类和数量。例如，CaCO₃ 含有一个钙原子、一个碳原子和三个氧原子。下标用于表示每种元素的原子的数量。

When writing equations, the total mass of reactants must equal the total mass of products. You may be asked to balance equations by adding coefficients.

书写方程式时，反应物的总质量必须等于生成物的总质量。你可能会被要求通过添加系数来配平方程式。

2H₂ + O₂ → 2H₂O

This equation shows four hydrogen atoms and two oxygen atoms on both sides, so it is balanced.

这个方程式表明两边都有四个氢原子和两个氧原子，因此它是配平的。

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10. Common Exam Pitfalls | 常见考试陷阱

Many students lose marks on this topic because of simple errors. Here are the most frequent mistakes to avoid.

很多学生在这个主题上因为简单的错误而失分。以下是最常见的要避免的错误。

• Confusing atomic number and mass number – always read the question carefully.

  混淆原子序数和质量数——仔细审题。

• Forgetting that electrons have practically no mass.

  忘记电子的质量几乎为零。

• Writing electron arrangements in the wrong order (e.g., 8,2,1 instead of 2,8,1 for sodium).

  错误地书写电子排布（例如钠写成 8,2,1 而不是 2,8,1）。

• Using the word ‘molecule’ when describing ionic compounds like NaCl.

  在描述 NaCl 等离子化合物时误用“分子”一词。

Learning the definitions exactly as they appear in the specification is crucial for 1-mark questions.

按照考纲中的原话记住定义对 1 分题至关重要。

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11. Quick Revision Summary | 快速复习总结

Here is a condensed checklist of the most important points from this article.

以下是本文最重要考点的简明清单。

Term	Key fact
Proton	+1 charge, mass 1, in nucleus
Neutron	0 charge, mass 1, in nucleus
Electron	−1 charge, negligible mass, in shells
Isotopes	Same protons, different neutrons
Group	Column in periodic table, same outer electrons
Period	Row in periodic table

Practice drawing the electron arrangements for the first 20 elements. Use the atomic number to add electrons shell by shell (2, 8, 8, 2 for potassium and calcium).

练习绘制前 20 号元素的电子排布。根据原子序数按壳层逐层添加电子（钾和钙为 2, 8, 8, 2）。

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