Writing Chemical Formulas & Chemical Equations | 化学式与化学方程式书写规则

📚 Writing Chemical Formulas & Chemical Equations | 化学式与化学方程式书写规则

In A-Level Chemistry (CIE), writing correct chemical formulas and balanced chemical equations is a fundamental skill that underpins nearly every exam question. A single incorrect symbol, subscript, or state symbol can cost you marks even if the rest of your reasoning is sound. This guide covers the essential rules for writing chemical formulas and chemical equations, tailored specifically for the CIE syllabus.

在 A-Level 化学（CIE）考试中，正确书写化学式与配平化学方程式是一项贯穿几乎所有考题的基础技能。哪怕只是一个符号、下标或状态符号写错，也可能导致失分，即使你的其余推理完全正确。本指南将系统讲解化学式和化学方程式的书写规则，专为 CIE 考纲量身定制。

1. Chemical Formula Basics | 化学式基础

A chemical formula represents the types and numbers of atoms in a substance using element symbols and subscripts. The subscript indicates the number of atoms of the element immediately to its left. For example, H₂O means each molecule contains two hydrogen atoms and one oxygen atom. The formula of an ionic compound is always written with the cation first, followed by the anion.

化学式用元素符号和下标表示物质中原子的种类与数量。下标表示其左侧紧邻元素的原子个数。例如，H₂O 表示每个分子含有两个氢原子和一个氧原子。离子化合物的化学式总是先写阳离子，再写阴离子。

For ionic compounds, the total positive charge must equal the total negative charge. This is called the “cross-over” rule: the charge number of one ion becomes the subscript of the other ion, then simplify the ratio if possible. For example, Al³⁺ and O²⁻ combine as Al₂O₃ (2 × 3 = 6+ and 3 × 2 = 6−).

对于离子化合物，总正电荷必须等于总负电荷。这就是“交叉法”规则：一种离子的电荷数成为另一种离子的下标，然后尽可能化简比例。例如，Al³⁺ 与 O²⁻ 结合为 Al₂O₃（2 × 3 = 6+，3 × 2 = 6−）。

2. Ionic Charges and Polyatomic Ions | 离子电荷与多原子离子

You must memorise the common charges of ions. Groups in the periodic table give the charge for simple monatomic ions: Group 1 metals form +1 ions, Group 2 form +2 ions, Group 3 form +3 ions, Group 5 non-metals form −3 ions, Group 6 form −2 ions, and Group 7 form −1 ions. Transition metals have variable charges such as Fe²⁺/Fe³⁺ and Cu⁺/Cu²⁺.

你必须牢记常见离子的电荷。周期表主族元素可给出简单单原子离子的电荷：第1族金属形成 +1 离子，第2族形成 +2 离子，第3族形成 +3 离子，第5族非金属形成 −3 离子，第6族形成 −2 离子，第7族形成 −1 离子。过渡金属具有可变电荷，如 Fe²⁺/Fe³⁺ 和 Cu⁺/Cu²⁺。

Polyatomic ions behave as single units. The most common ones include: ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, hydrogencarbonate HCO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, and phosphate PO₄³⁻. When more than one polyatomic ion is needed, enclose it in brackets before adding the subscript, e.g., Ca(OH)₂, (NH₄)₂SO₄, Fe₂(SO₄)₃.

多原子离子作为一个整体单元参与结合。最常见的多原子离子包括：铵离子 NH₄⁺、氢氧根 OH⁻、硝酸根 NO₃⁻、碳酸氢根 HCO₃⁻、碳酸根 CO₃²⁻、硫酸根 SO₄²⁻ 和磷酸根 PO₄³⁻。当一个化学式需要多个多原子离子时，先加括号再写下标，例如 Ca(OH)₂、(NH₄)₂SO₄、Fe₂(SO₄)₃。

Roman numerals in names indicate the oxidation state: Fe²⁺ = iron(II), Fe³⁺ = iron(III).

名称中的罗马数字表示氧化态：Fe²⁺ = iron(II)（铁(II)），Fe³⁺ = iron(III)（铁(III)）。
The suffix “-ide” is used for simple anions (Cl⁻ = chloride, O²⁻ = oxide), while “-ate” and “-ite” indicate oxyanions (NO₃⁻ = nitrate, NO₂⁻ = nitrite).

后缀“-ide”用于简单阴离子（Cl⁻ = chloride，O²⁻ = oxide），而“-ate”和“-ite”表示含氧酸根离子（NO₃⁻ = nitrate，NO₂⁻ = nitrite）。

3. Empirical and Molecular Formulas | 实验式与分子式

The empirical formula shows the simplest whole-number ratio of atoms in a compound, while the molecular formula shows the actual number of atoms of each element in one molecule. For example, glucose has the molecular formula C₆H₁₂O₆ but its empirical formula is CH₂O. The molecular formula is always a whole-number multiple of the empirical formula.

实验式表示化合物中各原子最简整数比，而分子式表示一个分子中每种元素的实际原子数目。例如，葡萄糖的分子式为 C₆H₁₂O₆，但其实验式为 CH₂O。分子式总是实验式的整数倍。

To find the empirical formula from percentage composition: divide each percentage by the relative atomic mass, then divide all values by the smallest result to obtain the simplest ratio. If the ratio is not close to whole numbers, multiply all values by a whole number (e.g., 2 or 3) to obtain whole-number ratios.

由质量百分组成求实验式的方法：将各元素的百分质量除以相对原子质量，再将所有结果除以最小值，得到最简整数比。若比值不是接近整数，则将所有数值乘以一个整数（如2或3），得到整数比。

Example question	A compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen. Find its empirical formula.
Solution	C: 40.0/12 = 3.33; H: 6.7/1 = 6.7; O: 53.3/16 = 3.33. Ratio C:H:O = 3.33:6.7:3.33 = 1:2:1. Empirical formula = CH₂O.

Molecular formula = (empirical formula)ₙ, where n = molar mass / empirical formula mass

分子式 = (实验式)ₙ，其中 n = 摩尔质量 / 实验式摩尔质量

4. Writing Ionic Formulae | 离子化学式的书写

To write the formula of an ionic compound from its name:

由名称书写离子化合物的化学式，步骤如下：

Identify the cation and anion with their charges.

确定阳离子和阴离子及其电荷。
Write the cation first, then the anion.

先写阳离子，再写阴离子。
Combine the ions in a ratio that makes the total charge zero.

按总电荷为零的比例组合离子。
Reduce the ratio to its simplest form unless the compound is a peroxide or a special case.

将比例化为最简形式，除非是过氧化物等特殊情况。

For example, calcium (Ca²⁺) and phosphate (PO₄³⁻): the LCM of 2 and 3 is 6, so we need 3 Ca²⁺ and 2 PO₄³⁻. The formula is Ca₃(PO₄)₂. Always place brackets around a polyatomic ion when its subscript is greater than 1.

例如，钙离子 Ca²⁺ 与磷酸根 PO₄³⁻：2 和 3 的最小公倍数为 6，因此需要 3 个 Ca²⁺ 和 2 个 PO₄³⁻。化学式为 Ca₃(PO₄)₂。当多原子离子的下标大于 1 时，务必加上括号。

5. Writing Chemical Equations | 化学方程式的书写

A chemical equation summarises a reaction using chemical formulas and symbols. The reactants are written on the left side, the products on the right side, with an arrow (→) separating them. The law of conservation of mass requires that the same number of each type of atom appears on both sides; therefore the equation must be balanced.

化学方程式用化学式和符号概括一个化学反应。反应物写在左侧，生成物写在右侧，中间用箭头（→）隔开。质量守恒定律要求每种原子在两边数量相同，因此方程式必须配平。

Steps for writing a chemical equation:

书写化学方程式的步骤：

Write the correct formula for each reactant and product — never change the formula during balancing.

写出每个反应物和生成物的正确化学式——配平过程中绝不能更改化学式。
Count the number of atoms of each element on both sides.

统计两边每种元素的原子数目。
Balance the atoms by placing coefficients in front of the formulas. Start with the most complex compound, then balance one element at a time.

通过在化学式前添加系数来配平原子。从最复杂的化合物开始，逐种元素依次配平。
Finally balance hydrogen and oxygen atoms.

最后配平氢和氧原子。
Check that all coefficients are in the smallest whole-number ratio and add state symbols.

检查所有系数是否为最简整数比，并添加状态符号。

6. Balancing Equations: The Inspection Method | 配平方程式：视察法

The inspection (or trial-and-error) method is the most common approach. For example, balance the combustion of propane:

视察法（即试错法）是最常用的配平方法。例如，配平丙烷的燃烧反应：

C₃H₈ + O₂ → CO₂ + H₂O

Step 1: Balance carbon: 3 CO₂ on the right.

第一步：配平碳原子——右侧写 3 CO₂。

C₃H₈ + O₂ → 3 CO₂ + 4 H₂O

Step 2: Balance hydrogen: 4 H₂O on the right gives 8 H atoms.

第二步：配平氢原子——右侧写 4 H₂O 提供 8 个氢原子。

Step 3: Balance oxygen: 3 × 2 + 4 × 1 = 10 O atoms on the right, so 5 O₂ on the left.

第三步：配平氧原子——右侧共有 3 × 2 + 4 × 1 = 10 个氧原子，因此左侧写 5 O₂。

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

All coefficients are whole numbers and cannot be simplified further. Always multiply through if the ratio has fractions, e.g., C₂H₆ + 3½O₂ → 2CO₂ + 3H₂O becomes 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.

所有系数均为整数且不能进一步化简。若配平出现分数系数，则需整体乘以分母消去分数，例如 C₂H₆ + 3½O₂ → 2CO₂ + 3H₂O 应改为 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O。

7. Balancing Redox Equations: Half-Equation Method | 氧化还原方程式配平：半方程式法

In A-Level chemistry, redox equations are best balanced using the half-equation method. First, write the two half-equations (oxidation and reduction), balance atoms other than O and H, balance O by adding H₂O, balance H by adding H⁺ (in acidic solution) or OH⁻ (in alkaline solution), then balance charge by adding electrons.

在 A-Level 化学中，氧化还原方程式最好采用半方程式法配平。首先写出两个半反应式（氧化与还原），配平除 O 和 H 以外的原子，用 H₂O 配平氧原子，用 H⁺（酸性条件）或 OH⁻（碱性条件）配平氢原子，最后用电子的得失配平电荷。

Example: balance the reaction between MnO₄⁻ and Fe²⁺ in acid.

示例：配平酸性条件下 MnO₄⁻ 与 Fe²⁺ 的反应。

Reduction half-equation:

还原半反应式：

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Oxidation half-equation:

氧化半反应式：

Fe²⁺ → Fe³⁺ + e⁻

Multiply the oxidation half-equation by 5 to equalise electrons, then add:

将氧化半反应式乘以 5 使电子数相等，然后相加合并：

MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺

In alkaline medium, use OH⁻ and H₂O to balance the atoms instead of H⁺.

在碱性介质中，用 OH⁻ 和 H₂O 来配平原子，而不是用 H⁺。

8. State Symbols and Reaction Conditions | 状态符号与反应条件

State symbols are written after each chemical formula in parentheses: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution. They are essential in equations that involve solubility, precipitation reactions, or ionic equations.

状态符号写在每个化学式后面的括号中：(s) 表示固态，(l) 表示液态，(g) 表示气态，(aq) 表示水溶液。在涉及溶解度、沉淀反应或离子方程式中，状态符号必不可少。

Symbol	Meaning	Example
(s)	Solid	NaCl(s)
(l)	Pure liquid	H₂O(l)
(g)	Gas	CO₂(g)
(aq)	Aqueous solution	NaOH(aq)

Reaction conditions are usually written above the arrow. Common conditions include “heat” (Δ), “catalyst”, “high pressure”, “UV light”, and the temperature value. For example, the Haber process is written as:

反应条件通常写在箭头上方。常见条件包括“加热”（Δ）、“催化剂”、“高压”、“紫外线”以及具体温度值。例如，哈伯法合成氨写为：

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) Fe catalyst, 450°C, 200 atm

The symbol ⇌ is used for reversible reactions, while → indicates a reaction that proceeds to completion.

符号 ⇌ 用于可逆反应，而 → 表示反应进行完全。

9. Ionic Equations | 离子方程式

Ionic equations show only the species that actually take part in a reaction, omitting spectator ions. For example, the reaction between AgNO₃(aq) and NaCl(aq) can be written as:

离子方程式只显示实际参加反应的粒子，省略旁观离子。例如，AgNO₃(aq) 与 NaCl(aq) 的反应可写为：

Full equation:

完整方程式：

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

Ionic equation (cancel Na⁺ and NO₃⁻):

离子方程式（消去 Na⁺ 和 NO₃⁻）：

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

In ionic equations, the charges must balance. If a precipitation reaction involves a solid product, the product is written as (s), not (aq). Always check that the total charge on the left equals the total charge on the right.

在离子方程式中，电荷必须守恒。若沉淀反应生成固体产物，产物应写为 (s)，而不是 (aq)。务必检查左右两侧总电荷是否相等。

10. Organic Chemistry Structures | 有机化学结构式

In organic chemistry, the molecular formula alone is often not enough; you may need the structural formula or displayed formula. For example, C₂H₆O could be ethanol (CH₃CH₂OH) or methoxymethane (CH₃OCH₃). The structural formula shows how atoms are connected, and the displayed formula shows every bond explicitly.

在有机化学中，仅用分子式往往不够，你可能需要结构式或示性式。例如，C₂H₆O 既可能是乙醇（CH₃CH₂OH），也可能是甲氧基甲烷（CH₃OCH₃）。结构式展示原子之间的连接方式，示性式则明确画出每一条化学键。

When writing organic equations, use structural formulas to be unambiguous. For example, the esterification of ethanoic acid with ethanol is:

书写有机反应方程式时，使用结构式以避免歧义。例如，乙酸与乙醇的酯化反应为：

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Remember that in organic equations, heat and catalysts are commonly required, such as concentrated H₂SO₄ in esterification. State symbols are still required.

请记住，有机反应通常需要加热和催化剂，例如酯化反应中使用浓硫酸。状态符号仍然必须标注。

11. Common Mistakes and Exam Tips | 常见错误与考试技巧

One frequent error is changing a formula while attempting to balance an equation. You must only add coefficients, never subscripts. For example, writing AlCl₂ instead of AlCl₃ is a formula error, not a balancing error. Another mistake is omitting brackets for polyatomic ions, such as writing Ca(OH)₂ incorrectly as CaOH₂.

一个常见错误是在配平过程中修改化学式。你只能添加系数，绝不能修改下标。例如，把 AlCl₃ 写成 AlCl₂ 属于化学式错误，而不是配平错误。另一个常见错误是忘记给多原子离子加括号，例如把 Ca(OH)₂ 误写成 CaOH₂。

Always reduce coefficients to the simplest whole-number ratio.

始终将系数化为最简整数比。
Use the smallest possible whole numbers for all coefficients.

所有系数应使用尽可能小的整数。
Do not forget state symbols when the question explicitly asks for them.

当题目明确要求时，不要遗漏状态符号。
In multiple-choice questions, check that both the formula and the balancing are correct before selecting.

在选择题中，先确认化学式正确且已配平，再作答。

12. Quick Reference: Writing Ionic Formulas | 快速参考：离子化学式书写

Below is a quick summary of the charge relationships that are frequently tested:

以下为常见考试高频的电荷关系速查表：

Cation	Anion	Compound formula
Na⁺	Cl⁻	NaCl
Mg²⁺	Cl⁻	MgCl₂
Mg²⁺	NO₃⁻	Mg(NO₃)₂
Al³⁺	SO₄²⁻	Al₂(SO₄)₃
NH₄⁺	CO₃²⁻	(NH₄)₂CO₃

When writing the formula, if the charges have a common factor (e.g., Ca²⁺ and O²⁻), simplify the ratio to CaO rather than Ca₂O₂. This is a common trap in exam questions.

书写化学式时，如果电荷数有公因数（如 Ca²⁺ 与 O²⁻），应化简比例写为 CaO，而不是 Ca₂O₂。这是考试中常见的陷阱。

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Writing Chemical Formulas & Chemical Equations | 化学式与化学方程式书写规则