📚 Year 9 AQA Chemistry: Formula and Theorem Quick Reference Handbook | Year 9 AQA 化学：公式定理速查手册
This quick reference handbook summarises the essential formulas, rules and theorems you will encounter in Year 9 AQA Chemistry. Mastering these concepts will help you solve quantitative problems, write correct chemical formulae, and understand reactions at a deeper level. Use it for revision and homework support.
本速查手册总结了 Year 9 AQA 化学中涉及的重要公式、规则和定理。掌握这些概念将帮助你解决定量问题，正确书写化学式，并更深入地理解化学反应。可用于复习和完成作业。
1. Conservation of Mass | 质量守恒定律
The law of conservation of mass states that during a chemical reaction, atoms are neither created nor destroyed. The total mass of reactants always equals the total mass of products.
质量守恒定律指出，在化学反应中，原子既不会创生也不会消失。反应物的总质量始终等于生成物的总质量。
This is because reactions simply rearrange atoms into new combinations; the number and type of each atom remain unchanged. For example, when 12 g of carbon reacts with 32 g of oxygen, exactly 44 g of carbon dioxide is produced.
这是因为化学反应只是将原子重新组合成新的物质；每种原子的数目和种类保持不变。例如，12 克碳与 32 克氧气反应，恰好生成 44 克二氧化碳。
ows that one C atom and one O₂ molecule recombine without any loss of atoms. If a reaction appears to gain or lose mass, it is usually because a gas has entered or escaped from the reaction vessel.
配平后的方程式 C + O₂ → CO₂ 表明一个碳原子和一个氧分子重新组合，没有任何原子损失。如果反应看似质量增加或减少，通常是因为有气体进入或逸出反应容器。
2. Atomic Structure | 原子结构
Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons orbiting in shells. The atomic number (Z) is the number of protons in an atom, and it defines the element.
原子由包含质子和中子的原子核以及核外分层排布的电子组成。原子序数（Z）是原子中质子的数目，决定了元素的种类。
s. You can calculate the number of neutrons using: neutrons = mass number − atomic number.
对于中性原子，电子数等于质子数。质量数（A）是原子核中质子与中子数目之和。可用公式计算中子数：中子数 = 质量数 − 原子序数。
Protons = Z | Electrons = Z (neutral) | Neutrons = A − Z
质子数 = Z | 电子数 = Z（中性） | 中子数 = A − Z
Most elements exist as a mixture of isotopes – atoms with the same atomic number but different mass numbers due to different numbers of neutrons. Isotopes have the same chemical properties because they have the same electron arrangement.
大多数元素以同位素的混合物形式存在——同位素具有相同的原子序数，但因中子数不同而质量数不同。同位素的化学性质相同，因为它们具有相同的电子排布。
3. Ions and Valency | 离子与化合价
An ion is a charged particle formed when an atom gains or loses electrons. Metals tend to lose electrons to form positive ions (cations); non‑metals tend to gain electrons to form negative ions (anions).
离子是原子得到或失去电子后形成的带电粒子。金属倾向于失去电子形成阳离子；非金属倾向于得到电子形成阴离子。
s is equal to the charge number. For example, sodium (Na) loses 1 electron → Na⁺, valency 1; oxygen (O) gains 2 electrons → O²⁻, valency 2.
化合价（或称结合力）表示一个原子需要得到、失去或共享多少个电子以达到稳定外层。简单离子的化合价等于其电荷数。例如，钠（Na）失去 1 个电子 → Na⁺，化合价为 1；氧（O）得到 2 个电子 → O²⁻，化合价为 2。
| Common Cations / 常见阳离子 | Formula / 符号 | Common Anions / 常见阴离子 | Formula / 符号 |
|---|---|---|---|
| Sodium / 钠 | Na⁺ | Chloride / 氯 | Cl⁻ |
| Potassium / 钾 | K⁺ | Oxide / 氧 | O²⁻ |
| Calcium / 钙 | Ca²⁺ | Sulfate / 硫酸根 | SO₄²⁻ |
| Magnesium / 镁 | Mg²⁺ | Nitrate / 硝酸根 | NO₃⁻ |
| Aluminium / 铝 | Al³⁺ | Carbonate / 碳酸根 | CO₃²⁻ |
4. Writing Chemical Formulae | 化学式的书写
The 'cross‑over' method uses valencies to write correct chemical formulae. Write the ions side by side, then swap the charge numbers (ignoring + and − signs) to become the subscript of the opposite ion. Simplify the ratio if possible.
“交叉法”利用化合价书写正确的化学式。将离子并排书写，然后将电荷数（忽略正负号）交换成为另一离子右下方的下标。如可以，简化比例。
neutral because (2 × +3) + (3 × −2) = 0. For magnesium chloride, Mg²⁺ and Cl⁻ give MgCl₂.
示例：氧化铝由 Al³⁺ 和 O²⁻ 形成。交换电荷数得到 Al₂O₃。该化合物整体不带电，因 (2 × +3) + (3 × −2) = 0。对于氯化镁，Mg²⁺ 与 Cl⁻ 得到 MgCl₂。
Naming conventions: the metal is written first, followed by the non‑metal with the ending changed to '-ide' for two elements (e.g. sodium chloride, NaCl). For compounds containing oxygen, '-ate' is used if oxygen is part of a group (e.g. copper sulfate, CuSO₄).
命名规则：金属在前，非金属在后，双元素化合物非金属词尾改为“-ide”（如氯化钠，NaCl）。含氧酸根时，通常用“-ate”（如硫酸铜，CuSO₄）。
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