📚 IGCSE Chemistry Conventions: Signs, Symbols, Terminology and Nomenclature | IGCSE化学规范：符号、术语与命名法
In IGCSE Chemistry, conventions are the agreed rules for writing signs, symbols, names, formulae and units. They allow chemists worldwide to read the same equation, formula or term without ambiguity. This article summarises the key conventions that examiners expect you to use accurately.
在IGCSE化学中，规范是书写符号、化学式、名称、公式和单位的公认规则。它们让世界各地的化学工作者能够无歧义地阅读同一化学方程式、化学式或术语。本文总结考试中要求准确掌握的关键规范。
1. Chemical Symbols and Element Names | 化学符号与元素名称
Every element has a unique chemical symbol. A one-letter symbol is written as a capital letter, while a two-letter symbol has a capital first letter and a lowercase second letter. Examples include H, C, N, O, He, Li, Mg, Ca and Cl.
每种元素都有唯一的化学符号。单字母符号用大写字母书写，双字母符号首字母大写、第二个字母小写。例如 H、C、N、O、He、Li、Mg、Ca 和 Cl。
Some symbols come from Latin names rather than the modern English name. Common examples are Na for sodium (natrium), K for potassium (kalium), Fe for iron (ferrum), Cu for copper (cuprum), Ag for silver (argentum) and Pb for lead (plumbum).
有些符号来自拉丁语名称，而不是现代英语名称。常见的例子有 Na 表示钠（natrium）、K 表示钾（kalium）、Fe 表示铁（ferrum）、Cu 表示铜（cuprum）、Ag 表示银（argentum）和 Pb 表示铅（plumbum）。
2H₂ + O₂ → 2H₂O
2. Writing Chemical Formulae | 化学式的书写
A chemical formula shows the symbols of elements and the ratio of atoms in a substance. A molecular formula gives the actual number of atoms, for example H₂O, CO₂, NH₃ and CH₄. An empirical formula gives the simplest whole-number ratio of atoms.
化学式表示物质中元素的符号和原子个数比。分子式给出实际原子数，例如 H₂O、CO₂、NH₃ 和 CH₄。经验式给出最简单的整数原子个数比。
For ionic compounds, the formula is written so that the total positive charge balances the total negative charge. Examples are NaCl, MgCl₂, Al₂O₃, Ca(OH)₂ and (NH₄)₂SO₄. Brackets are used when more than one polyatomic ion is needed.
对于离子化合物，书写化学式时要使正电荷总数与负电荷总数平衡。例如 NaCl、MgCl₂、Al₂O₃、Ca(OH)₂ 和 (NH₄)₂SO₄。当需要多个多原子离子时使用括号。
Hydrogen, nitrogen, oxygen and the halogens exist as diatomic molecules, so their formulae are written as H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
氢、氮、氧和卤素以双原子分子存在，因此它们的化学式写作 H₂、N₂、O₂、F₂、Cl₂、Br₂ 和 I₂。
3. Naming Binary Ionic Compounds | 二元离子化合物的命名
Binary ionic compounds are named by writing the metal first and then the non-metal with its ending changed to -ide. Examples include sodium chloride, magnesium oxide, calcium sulfide and potassium bromide.
二元离子化合物的命名方法是先写金属名称，再将非金属名称的词尾改为 -ide。例如氯化钠、氧化镁、硫化钙和溴化钾。
When a transition metal can form ions with different charges, Roman numerals are used to show the charge. For example, FeCl₂ is iron(II) chloride, FeCl₃ is iron(III) chloride, Cu₂O is copper(I) oxide and CuO is copper(II) oxide.
当过渡金属可以形成不同电荷的离子时，使用罗马数字表示电荷。例如 FeCl₂ 称为氯化铁(II)，FeCl₃ 称为氯化铁(III)，Cu₂O 称为氧化铜(I)，CuO 称为氧化铜(II)。
4. Naming Covalent Compounds | 共价化合物的命名
Covalent compounds use Greek numerical prefixes to show the number of each atom. The prefixes are mono-, di-, tri-, tetra-, penta- and hexa-. The second element still ends in -ide. Examples are CO carbon monoxide, CO₂ carbon dioxide, SO₂ sulfur dioxide and PCl₅ phosphorus pentachloride.
共价化合物使用希腊数字前缀表示每种原子的个数。前缀有 mono-、di-、tri-、tetra-、penta- 和 hexa-。第二种元素仍以 -ide 结尾。例如 CO 一氧化碳、CO₂ 二氧化碳、SO₂ 二氧化硫和 PCl₅ 五氯化磷。
The prefix mono- is usually omitted for the first element when only one atom of that element is present. Some covalent compounds keep common names, such as water H₂O, ammonia NH₃ and methane CH₄.
当第一种元素只有一个原子时，通常省略前缀 mono-。一些共价化合物保留常用名称，例如水 H₂O、氨 NH₃ 和甲烷 CH₄。
5. Ionic Charges and Oxidation States | 离子电荷与氧化态
Ions are shown with charge superscripts. A positive ion is called a cation and a negative ion is called an anion. The charge is written as a number followed by + or -, for example Na⁺, Mg²⁺, Al³⁺, Cl⁻ and O²⁻.
离子用上标电荷表示。带正电的离子称为阳离子，带负电的离子称为阴离子。电荷写作数字后加 + 或 -，例如 Na⁺、Mg²⁺、Al³⁺、Cl⁻ 和 O²⁻。
The table below lists common ions that you should learn for IGCSE Chemistry.
下表列出了IGCSE化学中需要掌握的常见离子。
| Charge | Positive ions | Negative ions |
| 1+ / 1- | Li⁺, Na⁺, K⁺, Ag⁺, NH₄⁺ | F⁻, Cl⁻, Br⁻, I⁻, OH⁻, NO₃⁻, HCO₃⁻ |
| 2+ / 2- | Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺, Cu²⁺, Fe²⁺ | O²⁻, S²⁻, SO₄²⁻, CO₃²⁻ |
| 3+ / 3- | Al³⁺, Fe³⁺ | N³⁻, PO₄³⁻ |
Use these charges to write correct ionic formulae. For example, aluminium sulfate contains Al³⁺ and SO₄²⁻, so the formula is Al₂(SO₄)₃.
利用这些电荷书写正确的离子化学式。例如硫酸铝含有 Al³⁺ 和 SO₄²⁻，因此化学式为 Al₂(SO₄)₃。
6. State Symbols in Equations | 方程式中的状态符号
State symbols are written after each formula in a chemical equation to show the physical state. The symbols are (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution, meaning dissolved in water.
状态符号写在化学方程式中每个化学式之后，表示物理状态。符号 (s) 表示固体，(l) 表示液体，(g) 表示气体，(aq) 表示水溶液，即溶于水。
State symbols are important because
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