A-Level Chemistry: Writing Chemical Formulae from Compound Names | A-Level 化学：由化合物名称书写化学式

📚 A-Level Chemistry: Writing Chemical Formulae from Compound Names | A-Level 化学：由化合物名称书写化学式

Writing chemical formulae from compound names is a fundamental skill in A-Level Chemistry. It requires a clear understanding of valency, ionic charge, and the systematic naming conventions used by IUPAC. This article provides a step-by-step approach to converting names into correct formulae, with special attention to common pitfalls.

由化合物名称书写化学式是 A-Level 化学中的一项基本技能。它要求清晰理解化合价、离子电荷以及 IUPAC 的系统命名规则。本文将提供从名称到正确化学式的分步方法，并特别关注常见错误。

1. Types of Chemical Formulae | 化学式的类型

Before writing formulae, you must recognise the difference between empirical, molecular, and structural formulae. The empirical formula shows the simplest whole-number ratio of atoms. The molecular formula shows the actual number of each atom in a molecule. The structural formula shows how atoms are connected.

在书写化学式之前，你必须认识实验式、分子式和结构式之间的区别。实验式表示原子的最简整数比；分子式表示一个分子中每种原子的实际数目；结构式则表示原子的连接方式。

Empirical formula: e.g. CH₂O for glucose | 实验式：例如葡萄糖的 CH₂O
Molecular formula: e.g. C₆H₁₂O₆ for glucose | 分子式：例如葡萄糖的 C₆H₁₂O₆
Structural formula: e.g. CH₃CH₂OH for ethanol | 结构式：例如乙醇的 CH₃CH₂OH

For ionic compounds, the empirical formula is almost always used, because ionic compounds exist as giant lattices rather than discrete molecules.

对于离子化合物，通常使用实验式，因为离子化合物以巨型晶格而非独立分子形式存在。

2. Valency and Oxidation State | 化合价与氧化态

Valency is the combining power of an element. In ionic compounds, it equals the charge on the ion. In covalent compounds, it is the number of bonds formed. Oxidation state is a related concept but accounts for electron distribution in bonds.

化合价是元素的化合能力。在离子化合物中，它等于离子的电荷；在共价化合物中，它是形成的共价键数目。氧化态是相关概念，但考虑了成键电子云的分布。

Common valencies you must memorise:

必须记住的常见化合价：

Group 1	+1
Group 2	+2
Group 3	+3 (e.g. Al³⁺)
Group 5	-3 (e.g. N³⁻)
Group 6	-2 (e.g. O²⁻)
Group 7	-1 (e.g. Cl⁻)

3. The Ionic Compound: Metal + Non-metal | 离子化合物：金属 + 非金属

When a metal reacts with a non-metal, the metal loses electrons to form a positive ion (cation), and the non-metal gains electrons to form a negative ion (anion). The formula is determined by balancing the total positive and negative charges.

当金属与非金属反应时，金属失去电子形成阳离子，非金属得到电子形成阴离子。化学式通过平衡总正电荷与总负电荷来确定。

Example: Sodium oxide. Sodium forms Na⁺, oxygen forms O²⁻. To balance, we need two Na⁺ for every O²⁻. The formula is Na₂O.

例如：氧化钠。钠形成 Na⁺，氧形成 O²⁻。为平衡电荷，每需要一个 O²⁻ 就对应两个 Na⁺。化学式为 Na₂O。

Na⁺ + O²⁻ → Na₂O

This principle applies to all binary ionic compounds. Write the cation first, then the anion, and use subscripts to balance charges.

这一原则适用于所有二元离子化合物。先写阳离子，再写阴离子，用下标平衡电荷。

4. The “Swap and Drop” Method | 交换与下移法

A quick way to write formulae is to write the ion symbols with their charges, then swap the numbers (ignoring the sign) and drop them as subscripts. This works well for simple ions.

快速书写化学式的方法是先写出带电荷的离子符号，然后交换数字（忽略正负号）并将它们作为下标写下。这对简单离子非常有效。

Example: Aluminium oxide. Write Al³⁺ and O²⁻. Swap the 3 and 2: Al₂O₃. Check: total positive charge = 2 × (+3) = +6; total negative charge = 3 × (-2) = -6. Balanced.

例如：氧化铝。写出 Al³⁺ 和 O²⁻。交换 3 和 2：Al₂O₃。验证：总正电荷 = 2 × (+3) = +6；总负电荷 = 3 × (-2) = -6。平衡。

Al³⁺ + O²⁻ → Al₂O₃

Remember to simplify the ratio if possible. For example, Mg²⁺ and O²⁻ give Mg₂O₂, which simplifies to MgO.

记住如果可能要化简比例。例如，Mg²⁺ 和 O²⁻ 得到 Mg₂O₂，化简为 MgO。

5. Common Polyatomic Ions | 常见多原子离子

Many compounds contain polyatomic ions. You must memorise their names, formulae, and charges. The prefix “hydrogen” in a name usually means the ion contains a hydrogen atom, e.g. hydrogencarbonate HCO₃⁻, hydrogensulfate HSO₄⁻.

许多化合物含有多原子离子。你必须记住它们的名称、化学式和电荷。名称中的前缀 “hydrogen”（氢）通常表示该离子含有一个氢原子，例如碳酸氢根 HCO₃⁻、硫酸氢根 HSO₄⁻。

Name	Formula	Charge
Ammonium	NH₄⁺	+1
Hydroxide	OH⁻	-1
Nitrate	NO₃⁻	-1
Carbonate	CO₃²⁻	-2
Sulfate	SO₄²⁻	-2
Phosphate	PO₄³⁻	-3

Note the ending “-ate” usually indicates a polyatomic ion containing oxygen. “-ite” means one fewer oxygen atom. For example, sulfate SO₄²⁻ and sulfite SO₃²⁻.

注意后缀 “-ate”（酸根）通常表示含氧的多原子离子。”-ite”（亚酸根）表示少一个氧原子。例如，硫酸根 SO₄²⁻ 和亚硫酸根 SO₃²⁻。

6. Compounds with Polyatomic Ions | 含多原子离子的化合物

When writing formulae for compounds with polyatomic ions, use brackets around the polyatomic ion if a subscript is needed. This prevents ambiguity.

书写含多原子离子的化合物化学式时，如果多原子离子需要下标，请在其周围使用括号，以避免歧义。

Example: Calcium hydroxide. Ca²⁺ and OH⁻. To balance, we need two OH⁻ ions. The correct formula is Ca(OH)₂, not CaOH₂.

例如：氢氧化钙。Ca²⁺ 和 OH⁻。为平衡电荷，我们需要两个 OH⁻ 离子。正确化学式是 Ca(OH)₂，而不是 CaOH₂。

Example: Aluminium sulfate. Al³⁺ and SO₄²⁻. Swap the 3 and 2: Al₂(SO₄)₃. This means 2 Al³⁺ ions and 3 SO₄²⁻ ions.

例如：硫酸铝。Al³⁺ 和 SO₄²⁻。交换 3 和 2：Al₂(SO₄)₃。这表示 2 个 Al³⁺ 离子和 3 个 SO₄²⁻ 离子。

Al³⁺ + SO₄²⁻ → Al₂(SO₄)₃

7. Variable Oxidation States | 可变氧化态

Transition metals often have variable oxidation states. The oxidation state is indicated in the compound name using Roman numerals in parentheses. This is crucial for writing the correct formula.

过渡金属通常具有可变氧化态。化合物名称中使用括号内的罗马数字表示氧化态。这对于书写正确化学式至关重要。

Example: Iron(III) oxide. Iron(III) means Fe³⁺. Oxide is O²⁻. The formula is Fe₂O₃. Iron(II) oxide contains Fe²⁺, giving FeO.

例如：氧化铁(III)。Iron(III) 表示 Fe³⁺。氧化物是 O²⁻。化学式为 Fe₂O₃。氧化铁(II) 含有 Fe²⁺，得到 FeO。

Example: Copper(I) chloride is CuCl, while Copper(II) chloride is CuCl₂. The Roman numeral tells you the charge on the metal ion directly.

例如：氯化铜(I) 是 CuCl，而氯化铜(II) 是 CuCl₂。罗马数字直接告诉你金属离子的电荷。

Be careful with ions such as mercury(I), which exists as Hg₂²⁺, a dimer. Mercury(I) chloride is Hg₂Cl₂, not HgCl.

注意某些离子如汞(I)，它以二聚体 Hg₂²⁺ 形式存在。氯化汞(I) 是 Hg₂Cl₂，而不是 HgCl。

8. Acids and Acid Salts | 酸与酸式盐

Acid names often hint at their formulae. Hydrochloric acid is HCl, sulfuric acid is H₂SO₄, nitric acid is HNO₃, phosphoric acid is H₃PO₄, and ethanoic acid is CH₃COOH.

酸的名称通常暗示其化学式。盐酸是 HCl，硫酸是 H₂SO₄，硝酸是 HNO₃，磷酸是 H₃PO₄，乙酸是 CH₃COOH。

Acid salts are formed when only some hydrogen atoms of an acid are replaced by a metal. The name includes “hydrogen” or “dihydrogen”. For example, sodium hydrogencarbonate NaHCO₃ and sodium dihydrogenphosphate NaH₂PO₄.

酸式盐是酸中只有部分氢原子被金属取代时形成的。名称中包含 “hydrogen”（氢）或 “dihydrogen”（二氢）。例如，碳酸氢钠 NaHCO₃ 和磷酸二氢钠 NaH₂PO₄。

Example: Calcium hydrogensulfate. Ca²⁺ and HSO₄⁻. The formula is Ca(HSO₄)₂. Note the bracket needed because there are two hydrogensulfate ions.

例如：硫酸氢钙。Ca²⁺ 和 HSO₄⁻。化学式为 Ca(HSO₄)₂。注意因为有两个硫酸氢根离子，所以需要括号。

9. Hydrated Compounds | 水合物

Hydrated compounds contain water molecules as part of their crystal structure. The name includes a prefix indicating the number of water molecules, followed by “hydrate”. The formula uses a dot between the salt and water.

水合物在晶体结构中含有水分子。名称包含表示水分子数目的前缀，后接 “hydrate”（水合物）。化学式在盐和水之间使用圆点连接。

Prefixes: mono- = 1, di- = 2, tri- = 3, tetra- = 4, penta- = 5, hexa- = 6.

前缀：mono- = 1，di- = 2，tri- = 3，tetra- = 4，penta- = 5，hexa- = 6。

Example: Copper(II) sulfate pentahydrate is CuSO₄·5H₂O. This is a blue crystalline solid commonly used in schools.

例如：五水合硫酸铜(II) 是 CuSO₄·5H₂O。这是一种常见的蓝色晶体，常用于学校实验。

Example: Sodium carbonate decahydrate is Na₂CO₃·10H₂O. The dot does not mean “multiply”. It separates the anhydrous salt from the water of crystallisation.

例如：十水合碳酸钠是 Na₂CO₃·10H₂O。圆点不表示”乘法”。它分隔无水盐与结晶水。

10. Organic Compounds: Alkanes and Alkenes | 有机化合物：烷烃与烯烃

Organic nomenclature follows different rules. The root name indicates the number of carbon atoms, and the suffix indicates the functional group. For alkanes, the suffix is “-ane”; for alkenes, “-ene”; for alcohols, “-ol”.

有机命名遵循不同规则。词根表示碳原子数目，后缀表示官能团。烷烃后缀为 “-ane”（烷），烯烃为 “-ene”（烯），醇为 “-ol”（醇）。

Number of C	Root	Alkane	Alkene
1	meth-	CH₄	–
2	eth-	C₂H₆	C₂H₄
3	prop-	C₃H₈	C₃H₆
4	but-	C₄H₁₀	C₄H₈

For general formula: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ. Alcohols with one OH group follow the general formula CₙH₂ₙ₊₁OH, e.g. ethanol C₂H₅OH.

通式：烷烃 CₙH₂ₙ₊₂，烯烃 CₙH₂ₙ。含一个 OH 的醇通式为 CₙH₂ₙ₊₁OH，例如乙醇 C₂H₅OH。

11. Functional Group Priority in Names | 名称中官能团的优先顺序

When a compound has multiple functional groups, the highest priority group determines the suffix. Lower priority groups become prefixes. For example, a compound with both an alcohol and a carboxylic acid is named as a carboxylic acid: 2-hydroxypropanoic acid has formula CH₃CH(OH)COOH, not “propanoic acid with an alcohol”.

当化合物含有多个官能团时，最高优先级的官能团决定后缀，较低优先级的官能团作为前缀。例如，同时含有醇羟基和羧基的化合物按羧酸命名：2-羟基丙酸的化学式为 CH₃CH(OH)COOH，而不是”带醇的丙酸”。

Priority order (high to low): carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene > alkyne > alkane.

优先级顺序（从高到低）：羧酸 > 酯 > 酰胺 > 醛 > 酮 > 醇 > 胺 > 烯烃 > 炔烃 > 烷烃。

Example: The name “butan-2-ol” tells you the OH group is on carbon 2. Its formula is CH₃CH(OH)CH₂CH₃. The position number must be included when the functional group can be located at different positions.

例如：”butan-2-ol”（丁-2-醇）表示 OH 基团在 2 号碳上。其化学式为 CH₃CH(OH)CH₂CH₃。当官能团可能位于不同位置时，必须包含位置编号。

12. Common Mistakes and Tips | 常见错误与提示

Students often forget brackets, use incorrect ion charges, or ignore Roman numerals. Here are the top tips to avoid these errors.

学生经常忘记括号、使用错误的离子电荷或忽略罗马数字。以下是最佳提示，帮助你避免这些错误。

Always balance the total charge to zero before writing the formula. | 在书写化学式前，始终确保总电荷为零。
Use brackets when a polyatomic ion is multiplied by a subscript. | 当多原子离子被下标倍增时，使用括号。
Simplify ratios where possible, but never change the charges. | 尽可能化简比例，但绝不要改变电荷。
Memorise the common polyatomic ions and their charges. | 牢记常见多原子离子及其电荷。
Interpret Roman numerals as the oxidation state of the metal. | 将罗马数字理解为金属的氧化态。
Check the hydride vs hydrogen naming: “hydrogencarbonate” is HCO₃⁻, not CO₃H⁻. | 注意 “hydrogencarbonate” 是 HCO₃⁻，而不是 CO₃H⁻。

Practice tip: Write out the ions with charges, cross them over, then check your work by multiplying each charge by its new subscript. The sum must be zero.

练习技巧：写出带电荷的离子，交叉下标，然后通过将每个电荷乘以其新下标来检查。总和必须为零。

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A-Level Chemistry: Writing Chemical Formulae from Compound Names | A-Level 化学：由化合物名称书写化学式