📚 Year 9 AQA Chemistry: Formula & Theorem Quick Reference Guide | Year 9 AQA 化学：公式定理速查手册
This quick reference guide summarises the essential formulas, rules and theorems that every Year 9 AQA Chemistry student needs to master. From electronic configurations to moles and reacting masses, this handbook provides clear explanations with worked examples to support your revision and exam preparation.
本速查手册总结了每位 Year 9 AQA 化学学生必须掌握的核心公式、规则和定理。从电子排布到摩尔和反应质量，本手册提供清晰的解释和例题，以辅助你的复习和备考。
1. Electronic Configuration Rules | 电子排布规则
Electrons in an atom are arranged in shells (energy levels) around the nucleus. The first shell can hold a maximum of 2 electrons; the second and third shells can each hold up to 8 electrons. This is known as the 2,8,8 rule for the first 20 elements. For example, oxygen (atomic number 8) has the electronic configuration 2,6. Calcium (atomic number 20) is 2,8,8,2.
原子中的电子分层排布在原子核外的电子层中。第一层最多容纳2个电子，第二和第三层最多各容纳8个电子——这就是前20号元素遵循的2,8,8规则。例如氧（原子序数8）的电子排布为2,6。钙（原子序数20）的电子排布为2,8,8,2。
A full outer shell (usually 8 electrons, or 2 for the first shell) gives the atom a stable, noble gas configuration. This drives the formation of ions and covalent bonds.
最外层填满（通常8个电子，第一层为2个）使原子具有稳定的惰性气体结构，这一趋势驱动了离子和共价键的形成。
2. Relative Atomic Mass & Relative Formula Mass | 相对原子质量与相对分子质量
The relative atomic mass (Ar) of an element is the average mass of its atoms compared to 1/12th the mass of a carbon‑12 atom. Ar values are given on the periodic table and have no units.
元素的相对原子质量（Ar）是其原子的平均质量与碳‑12原子质量的1/12的比值。Ar 值列在元素周期表上，没有单位。
The relative formula mass (Mr) of a compound is the sum of the Ar values of all the atoms in its formula. For example, the Mr of CaCO₃ = Ar(Ca) + Ar(C) + 3 × Ar(O) = 40 + 12 + (3 × 16) = 100.
化合物的相对分子质量（Mr）是其化学式中所有原子 Ar 的总和。例如，CaCO₃ 的 Mr = 40 + 12 + (3 × 16) = 100。
Mr = Sum of Ar values of all atoms present
3. Valency & Writing Chemical Formulae | 化合价与化学式的书写
The valency (or combining power) of an element is related to the number of electrons it needs to gain, lose or share to achieve a full outer shell. For ions, the charge number often equals the valency. To write the formula of an ionic compound, swap the valencies as subscripts and simplify if needed: the valency of the positive ion becomes the subscript for the negative ion, and vice versa.
元素的化合价（结合能力）与其需要得失或共用多少电子以达到满层结构相关。对离子而言，电荷数常等于化合价。书写离子化合物化学式时，将化合价交叉作为对方的下标，必要时约简：阳离子的化合价作为阴离子的下标，反之亦然。
Example: Magnesium chloride. Magnesium forms Mg²⁺ ions (valency 2), chloride forms Cl⁻ ions (valency 1). Swapping gives Mg₁Cl₂, simplified to MgCl₂. Aluminium oxide: Al³⁺ and O²⁻, valencies 3 and 2, formula Al₂O₃.
示例：氯化镁。镁形成 Mg²⁺ 离子（化合价2），氯形成 Cl⁻ 离子（化合价1），交叉得到 Mg₁Cl₂，简写为 MgCl₂。氧化铝：Al³⁺ 和 O²⁻，化合价3和2，化学式为 Al₂O₃。
4. Balancing Chemical Equations & Conservation of Mass | 化学方程式的配平与质量守恒
The law of conservation of mass states that atoms are neither created nor destroyed in a chemical reaction. Therefore, a chemical equation must have the same number of each type of atom on both the reactant and product sides. Balancing is done by placing coefficients (large numbers) in front of the formulas.
质量守恒定律指出，在化学反应中原子的种类和数目均保持不变。因此，化学方程式必须配平，使得反应前后每种原子的总数相等。配平通过在化学式前添加系数（计量数）来完成。
₂ → 2 H₂O. Now right has 4 H, so balance H by placing 2 in front of H₂: 2 H₂ + O₂ → 2 H₂O.
示例：H₂ + O₂ → H₂O。计算原子数：左侧 2 H, 2 O；右侧 2 H, 1 O。在 H₂O 前加2配平 O：H₂ + O₂ → 2 H₂O。此时右侧有4 H，故在左侧 H₂ 前加2：2 H₂ + O₂ → 2 H₂O。
Total mass of reactants always equals total mass of products. If 4 g of hydrogen reacts with 32 g of oxygen, 36 g of water is produced.
反应物的总质量总等于生成物的总质量。若4 g氢气与32 g氧气反应，则生成36 g水。
5. The Mole Concept & Molar Mass | 摩尔概念与摩尔质量
ole (mol) is the amount of substance that contains exactly 6.02 × 10²³ particles (Avogadro' mass of one mole of it, in grams per mole (g/mol). For an element, the molar mass is its Ar in grams; for a compound, it is the Mr in grams.
1摩尔（mol）是含有精确 6.02 × 10²³ 个微粒（阿伏伽德罗常数）的物质量。物质的摩尔质量（M）是1摩尔该物质的质量，单位为克每摩尔（g/mol）。对元素，摩尔质量在数值上等于其 Ar；对化合物，则等于其 Mr。
Amount (mol) = mass (g) / molar mass (g/mol) or n = m / M
Example: What is the amount of 8 g of oxygen gas (O₂)? M of O₂ = 2 × 16 = 32 g/mol, so n = 8 ÷ 32 = 0.25 mol.
示例：8 g 氧气（O₂）的物质的量为多少？M(O₂) = 2 × 16 = 32 g/mol，故 n = 8 ÷ 32 = 0.25 mol。
6. Stoichiometry & Reacting Mass Calculations | 化学计量与反应质量计算
quation. (2) Convert the known mass to moles using n = m/M. (3) Use the mole ratio from the equation to find moles of the unknown. (4) Convert moles back to mass using m = n × M.
利用配平后的化学方程式和摩尔概念，可计算所需反应物或生成物的质量。步骤：(1) 写出配平的方程式。(2) 将已知质量换算成物质的量（n = m/M）。(3) 根据方程式中的计量系数比求未知物的物质的量。(4) 将物质的量换算为质量（m = n × M）。
Example: 2Mg + O₂ → 2MgO. How much MgO is produced from 6 g of Mg? O = 2:2 = 1:1, so n(MgO) = 0.25 mol. M(MgO) = 24 + 16 = 40 g/mol, mass = 0.25 × 40 = 10 g.
示例：2Mg + O₂ → 2MgO。6 g Mg 可生成多少 MgO？n(Mg) = 6/24 = 0.25 mol。物质的量比 Mg:MgO = 1:1，故 n(MgO) = 0.25 mol。M(MgO) = 40 g/mol，质量 = 0.25 × 40 = 10 g。
7. Concentration Calculations | 浓度计算
Concentration is a measure of how much solute is dissolved in a given volume of solvent or solution. The most commonly used unit in Year 9 is grams per cubic decimetre (g/dm³).
浓度是衡量在一定体积的溶剂或溶液中溶解了多少溶质的量。Year 9最常用的单位是克每立方分米（g/dm³）。
Concentration (g/dm³) = mass of solute (g) / volume of solution (dm³)
is dissolved in 250 cm³ of water, volume = 0.25 dm³, concentration = 5 / 0.25 = 20 g/dm³.
注意：1 dm³ = 1000 cm³，将 cm³ 换算为 dm³ 需除以1000。若5 g 氯化钠溶于 250 cm³ 水中，体积为0.25 dm³，浓度 = 5 ÷ 0.25 = 20 g/dm³。
pics, but the principle is similar: concentration (mol/dm³) = amount (mol) / volume (dm³).
以 mol/dm³ 为单位的浓度将在更高年级涉及，原理类似：浓度 (mol/dm³) = 物质的量 (mol) / 体积 (dm³)。
8. Chromatography & Rf Values | 色谱与比移值
Paper chromatography separates mixtures of soluble substances. Each component travels up the paper at a different rate. The retention factor (Rf) is used to identify a substance by comparing the distance it moves to the distance the solvent front moves.
纸色谱法用于分离可溶性混合物。各组分在纸上以不同速率移动。比移值（Rf）通过比较物质移动距离与溶剂前沿移动距离来鉴定物质。
Rf = distance moved by spot (mm) / distance moved by solvent front (mm)
Rf is always between 0 and 1 and has no units. If a spot travels 2.4 cm and the solvent front travels 6.0 cm, Rf = 2.4/6.0 = 0.4. The value can be compared to known standards under identical conditions.
Rf 值总是在 0 到 1 之间，无单位。若斑点移动 2.4 cm，溶剂前沿移动 6.0 cm，则 Rf = 2.4 / 6.0 = 0.4。该值可与相同条件下已知物质的 Rf 值进行比较。
9. Rate of Reaction | 反应速率
The rate of a chemical reaction tells us how quickly reactants are used up or products are formed. The mean (average) rate can be calculated by measuring the change in mass, volume of gas produced, or colour change over time.
化学反应速率表示反应物消耗或生成物产生的快慢。平均速率可通过测量质量变化、产生气体的体积或颜色变化随时间的变化来计算。
Mean rate = quantity of reactant used or product formed / time taken
/20 = 2.4 cm³/s. The rate is fastest at the start and slows down as the reaction proceeds.
常用单位有 g/s（质量变化）或 cm³/s（气体体积）。例如，若在20秒内收集到 48 cm³ 氢气，平均速率 = 48 ÷ 20 = 2.4 cm³/s。反应开始时速率最快，随着反应进行而减慢。
10. Bond Energies & Energy Changes | 键能与能量变化
In a chemical reaction, bonds in the reactants are broken and new bonds are formed in the products. Bond energy (kJ/mol) is the energy needed
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