From Name to Formula: A-Level Cambridge Chemistry | 从名称到化学式：A-Level 剑桥化学

📚 From Name to Formula: A-Level Cambridge Chemistry | 从名称到化学式：A-Level 剑桥化学

Converting a chemical name into a correct formula is one of the most important practical skills in Cambridge AS/A Level Chemistry. It tests your understanding of oxidation states, ionic charges, polyatomic ions, Greek prefixes, acids, salts and organic homologous series.

将化学名称转换为正确的化学式是剑桥 AS/A Level 化学中最重要的实用技能之一。它考查你对氧化态、离子电荷、多原子离子、希腊数字前缀、酸、盐以及有机同系列的理解。

This article breaks the process into systematic sections so that you can move confidently from a given name to its formula, whether the substance is ionic, covalent, acidic, hydrated or organic.

本文将把这一过程拆分为系统的若干部分，使你能自信地从给定名称写出化学式，无论该物质是离子型、共价型、酸、水合物还是有机物。

1. Why Naming Rules Matter | 为什么命名规则重要

Systematic chemical names are designed so that any chemist can convert a name into an unambiguous formula and vice versa. In Cambridge AS/A Level papers, you may be given a name such as iron(III) sulfate or dinitrogen tetroxide and asked to write the formula.

系统化学名称的设计目的是让任何化学工作者都能从名称写出唯一确定的化学式，反之亦然。在剑桥 AS/A Level 考试中，你可能会看到如 iron(III) sulfate（硫酸铁(III)）或 dinitrogen tetroxide（四氧化二氮）这样的名称，并要求写出化学式。

The type of bonding controls which naming system applies. Ionic compounds use charges and oxidation states, while simple covalent molecules use Greek prefixes.

化学键类型决定了使用哪一套命名规则。离子化合物使用电荷与氧化态，而简单共价分子则使用希腊数字前缀。

If you learn the rules for each compound type, you can avoid common errors such as writing CaNO₃₂ instead of Ca(NO₃)₂, or FeCl₂ when iron(III) chloride is required.

如果掌握了每种化合物类型的规则，就能避免常见错误，例如把 Ca(NO₃)₂ 写成 CaNO₃₂，或在需要氯化铁(III) 时错误写成 FeCl₂。

2. Element Symbols and Oxidation States | 元素符号与氧化态

Before writing any formula, you must know the correct symbols and common oxidation states of elements. Group 1 metals are always +1 in compounds, Group 2 metals are +2, and aluminium is +3.

在书写任何化学式之前，必须掌握正确的元素符号和常见氧化态。第 1 族金属在化合物中始终为 +1，第 2 族为 +2，铝为 +3。

Common non-metals such as oxygen often show −2, fluorine is −1, and hydrogen is +1 when bonded to non-metals but −1 in metal hydrides such as NaH.

常见非金属如氧通常为 −2，氟为 −1；氢与非金属结合时为 +1，但在金属氢化物如 NaH 中为 −1。

You should also memorise the symbols of common elements: Na, K, Mg, Ca, Al, C, N, P, O, S, Cl, Br, I, Fe, Cu, Zn, Ag and Pb. These appear repeatedly in formula-writing questions.

还需熟记常见元素符号：Na、K、Mg、Ca、Al、C、N、P、O、S、Cl、Br、I、Fe、Cu、Zn、Ag 和 Pb。它们在化学式书写题中反复出现。

3. Binary Ionic Compounds: Metal + Non-metal | 二元离子化合物：金属 + 非金属

A binary ionic compound is formed from a metal cation and a non-metal anion. The name gives the metal first, then the non-metal with an -ide ending: sodium chloride contains Na⁺ and Cl⁻.

二元离子化合物由金属阳离子和非金属阴离子组成。命名时先写金属，再将非金属词尾改为 -ide：sodium chloride（氯化钠）含 Na⁺ 和 Cl⁻。

To obtain the formula, balance the total positive and negative charges. For magnesium chloride, Mg²⁺ needs two Cl⁻ ions, so the formula is MgCl₂.

要得到化学式，需要平衡正负电荷总数。以氯化镁为例，Mg²⁺ 需要两个 Cl⁻，因此化学式为 MgCl₂。

For aluminium oxide, Al³⁺ and O²⁻ combine in a 2:3 ratio to give Al₂O₃. The metal is always written first, and the charges are not shown in the final formula.

对于氧化铝，Al³⁺ 与 O²⁻ 按 2:3 比例结合，得到 Al₂O₃。化学式中总是先写金属，且最终式不标电荷。

4. Polyatomic Ions and Brackets | 多原子离子与括号

Polyatomic ions are groups of atoms with an overall charge, such as nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻, hydroxide OH⁻, ammonium NH₄⁺ and phosphate PO₄³⁻.

多原子离子是带有整体电荷的原子团，例如硝酸根 NO₃⁻、硫酸根 SO₄²⁻、碳酸根 CO₃²⁻、氢氧根 OH⁻、铵根 NH₄⁺ 和磷酸根 PO₄³⁻。

When more than one polyatomic ion is needed, place it inside brackets and write the number outside as a subscript. For example, calcium nitrate is Ca(NO₃)₂, not CaNO₃₂.

当需要多个多原子离子时，应将其放入括号中，并把个数写在括号外作为下标

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