📚 What Are Compounds? | 什么是化合物？
In KS3 chemistry, a compound is one of the most important ideas you will meet. It helps explain why water, salt, carbon dioxide and countless other substances have their own special properties, and why chemists can predict what happens when substances react. This article introduces the definition, formation and everyday examples of compounds, and then builds up the exam skills that go with them, including formulae, naming and word equations.
在 KS3 化学中，化合物是你将遇到的最重要的概念之一。它帮助我们理解为什么水、食盐、二氧化碳和无数其他物质具有各自独特的性质，也帮助化学家预测物质反应时会发生什么。本文介绍化合物的定义、形成过程以及日常生活中的实例，并在此基础上讲解化学式、命名与文字方程式等考试技能。
1. Elements and Atoms | 元素与原子
All substances are made from tiny particles called atoms. An atom is the smallest particle of an element that can exist. An element is a pure substance made of only one type of atom, such as oxygen (O), hydrogen (H) or iron (Fe). There are about 100 different elements shown in the periodic table, and each one has its own chemical symbol.
所有物质都由微小的粒子——原子组成。原子是元素能够存在的最小微粒。元素是只由一种原子组成的纯净物，例如氧 (O)、氢 (H) 或铁 (Fe)。元素周期表中列出了大约 100 种不同的元素，每一种都有自己的化学符号。
Elements can exist in different forms: as single atoms (helium, He), as molecules made of two atoms (oxygen, O₂), or as giant structures (iron, Fe). Whichever form they take, they still contain only one type of atom, so they are still elements.
元素可以以不同形式存在：单原子形式（氦 He）、由两个原子组成的分子形式（氧气 O₂），或者巨型结构形式（铁 Fe）。无论采取哪种形式，它们都只含有一种原子，因此仍然属于元素。
Atoms of different elements can join together chemically. When this happens, they form a new substance with completely different properties. That new substance is called a compound.
不同元素的原子可以通过化学方式结合在一起。当这种情况发生时，它们会形成一种性质完全不同的新物质。这种新物质就叫做化合物。
2. What Exactly Is a Compound? | 究竟什么是化合物？
A compound is a pure substance made of two or more different elements that are chemically combined. Four key words in that sentence matter for every exam question you will ever see on this topic.
化合物是由两种或两种以上不同元素通过化学方式结合而成的纯净物。这句话里有四个关键词，对你今后遇到的每一道相关考题都很重要。
- Two or more elements - one element on its own can never be a compound.
两种或两种以上的元素——只有一种元素的物质永远不可能是化合物。 - Different elements - they must be different from each other; oxygen and oxygen do not make a compound.
不同的元素——它们必须互不相同，氧与氧之间并不能构成化合物。 - Chemically combined - the atoms are joined by chemical bonds, not simply stirred together.
化学结合——原子之间通过化学键连接，而不仅仅是搅拌混合在一起。 - Fixed composition - the elements are always present in the same fixed ratio, whatever the source.
固定组成——无论来源如何，各元素总是以相同的固定比例存在。
Water is the classic example. It contains hydrogen and oxygen in a fixed ratio of 2 hydrogen atoms to every 1 oxygen atom, and its formula is H₂O. Carbon dioxide (CO₂), sodium chloride (NaCl) and methane (CH₄) are other familiar compounds.
水是最经典的例子。它含有氢和氧，比例固定为每 1 个氧原子对应 2 个氢原子，化学式为 H₂O。二氧化碳 (CO₂)、氯化钠 (NaCl) 和甲烷 (CH₄) 是其他常见的化合物。
3. How Atoms Join: Chemical Bonding | 原子如何结合：化学键
Atoms join because they become more stable when they gain, lose or share electrons. At KS3 you do not need the full quantum picture, but you do need the three simple bonding patterns.
原子之所以结合，是因为它们在得失电子或共用电子后变得更加稳定。在 KS3 阶段你不需要完整的量子理论，但需要掌握三种简单的成键方式。
- Ionic bonding - a metal atom transfers electrons to a non-metal atom, making charged ions that attract each other.
离子键——金属原子把电子转移给非金属原子，形成带电荷的离子，离子之间相互吸引。 - Covalent bonding - two non-metal atoms share pairs of electrons.
共价键——两个非金属原子共用电子对。 - Metallic bonding - metal atoms only; this gives us metals and alloys, which are not compounds.
金属键——只涉及金属原子，形成金属与合金，它们都不是化合物。
Na → Na⁺ + e⁻ Cl + e⁻ → Cl⁻ Na⁺ + Cl⁻ → NaCl
The first two patterns give compounds. Sodium loses one electron to become Na⁺, chlorine gains one electron to become Cl⁻, and the two ions pack together to form sodium chloride.
前两种成键方式会产生化合物。钠失去一个电子变成 Na⁺，氯得到一个电子变成 Cl⁻，两种离子堆积在一起就形成了氯化钠。
4. Compounds and Mixtures: What Is the Difference? | 化合物与混合物：区别在哪里？
A mixture contains two or more substances that are not chemically combined. Air, sea water and a bowl of cereal are all mixtures. Compounds and mixtures differ in almost every practical way chemists care about.
混合物含有两种或多种没有化学结合的物质。空气、海水和一碗麦片粥都是混合物。化合物与混合物在化学家关心的几乎所有实际方面都不相同。
| Feature | 特征 | Compound | 化合物 | Mixture | 混合物 |
| Bonding | 结合方式 | Chemically combined | 化学结合 | Not chemically combined | 未化学结合 |
| Ratio | 比例 | Fixed | 固定 | Variable | 可变 |
| Properties | 性质 | New, own properties | 全新的、自身性质 | Similar to components | 与成分相似 |
| Separation | 分离方法 | Chemical reaction needed | 需要化学反应 | Physical methods | 物理方法 |
| Example | 例子 | Water, H₂O | 水 H₂O | Salt water, air | 盐水、空气 |
This is why sea water can be separated by distillation, while the hydrogen and oxygen inside water cannot be separated without an electric current or another chemical reaction.
这就是为什么海水可以用蒸馏法分离，而水中的氢和氧如果没有电流或其他化学反应就无法分开。
5. Compounds and Elements: Spotting the Difference | 化合物与元素：如何区分
You can usually tell compounds and elements apart by asking one question: how many different types of atom are present?
通常只需要问一个问题就能区分化合物与元素：其中含有多少种不同的原子？
| Substance | 物质 | Contains | 含有 | Type | 类型 |
| O₂ | 氧气 | Oxygen atoms only | 仅氧原子 | Element | 元素 |
| Fe | 铁 | Iron atoms only | 仅铁原子 | Element | 元素 |
| H₂O | 水 | Hydrogen and oxygen | 氢和氧 | Compound | 化合物 |
| CO₂ | 二氧化碳 | Carbon and oxygen | 碳和氧 | Compound | 化合物 |
| NaCl | 氯化钠 | Sodium and chlorine | 钠和氯 | Compound | 化合物 |
| Air | 空气 | N₂, O
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