📚 KS3 Science: Atoms, Elements and Compounds | KS3 科学：原子、元素与化合物
All substances around us, from the water we drink to the metal in a bicycle, are built from tiny particles called atoms. In KS3 Science you learn how atoms join, how the periodic table organises elements, and why compounds have different properties from their parent elements.
我们周围的所有物质——从饮用水到自行车上的金属——都由被称为原子的微小粒子构成。在 KS3 科学中，你将学习原子如何结合、元素周期表如何组织元素，以及为什么化合物的性质与其组成元素不同。
1. What Are Atoms? | 什么是原子？
An atom is the smallest building block of matter. Each atom has a tiny central nucleus made of protons and neutrons, surrounded by electrons moving around the nucleus.
原子是物质的最小构建单元。每个原子都有一个由质子和中子组成的微小中心核，核外有电子围绕原子核运动。
Protons carry a positive charge, electrons carry a negative charge, and neutrons have no charge. In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges balance.
质子带正电，电子带负电，中子不带电。在电中性原子中，质子数等于电子数，因此正负电荷相互平衡。
Everything from oxygen gas to a diamond is made of atoms. Even a single grain of sugar contains an enormous number of atoms.
从氧气到钻石，一切物质都由原子组成。即使是一粒糖也含有数量极为庞大的原子。
2. Elements and Chemical Symbols | 元素与化学符号
An element is a substance made of only one type of atom. There are about 100 known elements, and each element has its own chemical symbol, usually one or two letters.
元素是只由一种原子组成的物质。已知元素约有 100 种，每种元素都有自己的化学符号，通常由一或两个字母组成。
The first letter of a chemical symbol is always capitalised, and the second letter, if there is one, is always lower case. For example, H stands for hydrogen, He for helium, O for oxygen, Fe for iron, and Na for sodium.
化学符号的第一个字母必须大写，第二个字母（如果有）必须小写。例如，H 代表氢，He 代表氦，O 代表氧，Fe 代表铁，Na 代表钠。
- Hydrogen (H) - 氢 (H)
- Carbon (C) - 碳 (C)
- Nitrogen (N) - 氮 (N)
- Oxygen (O) - 氧 (O)
- Iron (Fe) - 铁 (Fe)
- Copper (Cu) - 铜 (Cu)
- Sodium (Na) - 钠 (Na)
- Chlorine (Cl) - 氯 (Cl)
Memorising these common symbols will help you write chemical formulae and equations correctly throughout KS3.
记住这些常见符号将帮助你在 KS3 阶段正确书写化学式和化学方程式。
3. The Periodic Table | 元素周期表
The periodic table arranges all known elements in order of increasing atomic number. The atomic number is the number of protons in the nucleus of an atom.
元素周期表按照原子序数递增的顺序排列所有已知元素。原子序数就是原子核中的质子数。
Rows in the periodic table are called periods, and columns are called groups. Elements in the same group usually have similar chemical properties because they have the same number of electrons in their outer shell.
周期表中的横行称为周期，纵列称为族。同一族元素通常具有相似的化学性质，因为它们最外层电子数相同。
Metals are found on the left and centre of the periodic table, while non-metals are found on the right. Group 1 contains the alkali metals, Group 7 contains the halogens, and Group 0 contains the noble gases.
金属位于周期表的左侧和中部，非金属位于右侧。第 1 族是碱金属，第 7 族是卤素，第 0 族是稀有气体。
4. Compounds and Chemical Formulae | 化合物与化学式
A compound is a substance formed when atoms of two or more different elements chemically join together. A compound has a fixed composition and always contains the same elements in the same proportions.
化合物是由两种或多种不同元素的原子化学结合而成的物质。化合物具有固定组成，总是以相同比例含有相同元素。
A chemical formula shows the types and numbers of atoms in a compound. The formula H₂O tells us that a water molecule contains 2 hydrogen atoms and 1 oxygen atom.
化学式表示化合物中原子的种类和数量。化学式 H₂O 表明一个水分子含有 2 个氢原子和 1 个氧原子。
H₂O CO₂ NaCl CaCO₃
For example, CO₂ is carbon dioxide, NaCl is sodium chloride, and CaCO₃ is calcium carbonate.
例如，CO₂ 是二氧化碳，NaCl 是氯化钠，CaCO₃ 是碳酸钙。
5. Mixtures vs Compounds | 混合物与化合物的区别
A mixture contains two or more substances that are not chemically joined together. The substances in a mixture can usually be separated by physical methods such as filtration, evaporation, or distillation.
混合物含有两种或多种没有化学结合的物质。混合物中的物质通常可以通过过滤、蒸发或蒸馏等物理方法分离。
A compound has chemical bonds holding atoms together, so it cannot be separated into its elements by physical methods. For example, iron filings and sulfur can be separated with a magnet when they form a mixture, but heating them produces iron sulfide, a compound that cannot be separated by a magnet.
化合物中的原子由化学键连接在一起，因此不能用物理方法将其分解为元素。例如，铁屑和硫形成混合物时可以用磁铁分离，但加热后生成硫化亚铁，这是一种化合物，无法用磁铁分离。
A compound often has very different properties from the elements that form it. Sodium is a reactive metal and chlorine is a toxic gas, but sodium chloride is the safe salt we eat.
化合物的性质通常与组成它的元素截然不同。钠是一种活泼金属，氯是一种有毒气体，但氯化钠却是我们食用的安全食盐。
6. States of Matter | 物质的三种状态
Atoms and molecules can be arranged differently in solids, liquids, and gases. In a solid, particles are tightly packed in a regular pattern and only vibrate in fixed positions. In a liquid, particles are close but can slide past each other. In a gas, particles are far apart and move rapidly in all directions.
原子和分子在固体、液体和气体中的排列方式不同。在固体中，粒子紧密排列成规则图案，只能在固定位置振动；在液体中，粒子相距较近但可以相互滑动；在气体中，粒子相距很远，向各个方向快速运动。
Changes of state include melting, freezing, boiling, evaporating, and condensing. During these changes, the atoms themselves do not change; only their arrangement and movement change.
物态变化包括熔化、凝固、沸腾、蒸发和冷凝。在这些变化过程中，原子本身不发生改变，改变的只是它们的排列方式和运动状态。
For example, ice, liquid water, and steam all consist of H₂O molecules, but the spacing and motion of the molecules are different in each state.
例如，冰、液态水和水蒸气都由 H₂O 分子组成，但分子之间的间距和运动方式各不相同。
7. Physical and Chemical Changes | 物理变化与化学变化
A physical change does not produce a new substance. Melting ice, dissolving sugar in water, and breaking a glass rod are all physical changes because the chemical identity of the substance stays the same.
物理变化不会产生新物质。冰融化、糖溶于水、折断玻璃棒都是物理变化，因为物质的化学身份保持不变。
A chemical change produces at least one new substance. Burning magnesium, rusting iron, and adding acid to limestone are chemical changes.
化学变化会生成至少一种新物质。镁条燃烧、铁生锈、向石灰石中加酸都是化学变化。
Signs that a chemical reaction has taken place include a colour change, a gas being given off, a temperature change, or a precipitate forming.
判断是否发生化学反应的依据包括：颜色变化、放出气体、温度变化或生成沉淀。
2Mg + O₂ → 2MgO
In this reaction, magnesium metal reacts with oxygen gas to form magnesium oxide, a new white compound.
在这个反应中，金属镁与氧气反应生成氧化镁，一种新的白色化合物。
8. Conservation of Mass | 质量守恒
During a chemical reaction, atoms are rearranged, but they are not created or destroyed. The total mass of the reactants is therefore equal to the total mass of the products.
在化学反应过程中，原子会被重新排列，但不会被创造或消灭。因此，反应物的总质量等于生成物的总质量。
When magnesium is burned in air, the ash may seem heavier than the original metal. This is because oxygen from the air has combined with the magnesium, so the product contains both magnesium and oxygen atoms.
镁在空气中燃烧后，生成的灰烬看起来比原来的金属更重。这是因为空气中的氧与镁结合，产物中同时含有镁原子和氧原子。
If the reaction is carried out in a closed container, the total mass before and after the reaction stays exactly the same.
如果反应在密闭容器中进行，反应前后的总质量完全保持不变。
2H₂ + O₂ → 2H₂O
9. Naming Simple Compounds | 简单化合物的命名
For a compound made from a metal and a non-metal, the metal is named first and the non-metal changes its ending to -ide. For example, sodium and chlorine form sodium chloride, and magnesium and oxygen form magnesium oxide.
对于由金属和非金属组成的化合物，金属名称在前，非金属名称改变词尾为 -ide。例如，钠和氯生成氯化钠，镁和氧生成氧化镁。
When two non-metals combine, prefixes such as mono-, di-, and tri- are often used. Carbon monoxide has one oxygen atom, while carbon dioxide has two oxygen atoms.
两种非金属结合时，常使用 mono-、di-、tri- 等前缀。一氧化碳含有一个氧原子，二氧化碳含有两个氧原子。
- NaCl = sodium chloride - 氯化钠
- MgO = magnesium oxide - 氧化镁
- H₂O = water - 水
- CO₂ = carbon dioxide - 二氧化碳
- CH₄ = methane - 甲烷
Some compounds have common names that do not follow the simple naming rules, such as water and methane, so these need to be learned separately.
有些化合物有不符合简单命名规则的常用名称，例如水和甲烷，这些名称需要单独记忆。
10. Key Practical: Heating and Burning | 关键实验：加热与燃烧
A common KS3 practical is heating a strip of magnesium ribbon in air. The magnesium burns with a bright white flame and leaves a white ash, which is magnesium oxide.
一个常见的 KS3 实验是加热一段镁条。镁在空气中燃烧时发出明亮的白色火焰，并留下白色灰烬，这就是氧化镁。
Safety is important in this practical: wear safety goggles, use tongs to hold the ribbon, and do not look directly at the bright light because it can damage your eyes.
这个实验的安全注意事项很重要：佩戴护目镜，用坩埚钳夹住镁条，不要直视强光，因为强光会伤害眼睛。
Measuring the mass before and after heating shows that the product is heavier than the original metal. The extra mass comes from oxygen in the air that joins the magnesium.
测量加热前后的质量会发现产物比原来的金属更重。额外的质量来自空气中与镁结合的氧。
2Mg + O₂ → 2MgO
This experiment links the ideas of elements, compounds, chemical change, and conservation of mass in one practical activity.
这个实验将元素、化合物、化学变化和质量守恒等概念融合在一个实践活动中。
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