📚 Year 9 SQA Chemistry: Quick-Reference Formula & Principle Handbook | Year 9 SQA 化学：公式定理速查手册
This quick-reference handbook gathers the essential formulae, principles and relationships you need for Year 9 SQA Chemistry. It covers atomic structure, the mole, equations, concentrations, yields and reaction rates, with worked examples to cement your understanding. Use it alongside your class notes for revision and exam practice.
这本速查手册汇集了 Year 9 SQA 化学所需的核心公式、原理与关系。内容涵盖原子结构、摩尔、方程式、浓度、产率和反应速率，并配以例题帮助你巩固理解。可配合课堂笔记用于复习和备考训练。
1. Atomic Structure & Key Numbers | 原子结构与关键数字
Every atom consists of a dense nucleus made of protons and neutrons, orbited by electrons. The atomic number (symbol Z) tells you how many protons are in the nucleus; in a neutral atom the number of electrons is also Z.
每个原子由致密的原子核（内含质子和中子）与绕核运动的电子组成。原子序数（符号 Z）表示核内质子数；在中性原子中，电子数也等于 Z。
The mass number (symbol A) is the sum of protons and neutrons in the nucleus. You can find the number of neutrons by subtracting the atomic number from the mass number.
质量数（符号 A）是核内质子数与中子数之和。用质量数减去原子序数即可求出中子数。
Number of neutrons = A – Z
中子数 = A – Z
For example, a sodium atom has Z = 11 and A = 23, so it contains 23 – a⁺ has lost one electron (10 electrons), while Cl⁻ has gained one electron (18 electrons).
例如，钠原子的 Z = 11、A = 23，因此含有 23 – 11 = 12 个中子。原子形成离子时仅电子数变化：Na⁺ 失去一个电子（10 个电子），Cl⁻ 则得到一个电子（18 个电子）。
Isotopes are variants of the same element with identical atomic number but different mass numbers because they have different numbers of neutrons. The relative atomic mass (Aᵣ) shown on the Periodic Table is a weighted average of the masses of an element's isotopes, compared to 1/12 of a carbon-12 atom.
同位素是同一元素的变体，具有相同的原子序数但质量数不同，因为中子数不同。周期表上显示的相对原子质量（Aᵣ）是元素各同位素质量的加权平均值，相对于碳‑12 原子质量的 1/12。
Aᵣ = Σ (isotope mass × % abundance) ÷ 100
相对原子质量 = Σ（同位素质量 × 丰度%）÷ 100
2. Writing Chemical Formulae Using Valency | 用化合价书写化学式
The valency of an atom or ion is its combining power. For ionic compounds, valency is equal to the magnitude of the charge on the ion. To write the correct formula, swap the numerical charges and write them as subscripts, omitting the '+' or '−' sign.
原子或离子的化合价表示其结合能力。对于离子化合物，化合价等于离子所带电荷的数值。正确书写化学式时，交换电荷数值并写为下标，省去正负号。
If a polyatomic ion (e.g., SO₄²⁻, NO₃⁻) needs a subscript greater than 1, enclose the ion in brackets before adding the subscript.
若多原子离子（如 SO₄²⁻、NO₃⁻）需要大于 1 的下标，须先用括号将离子括起，再加下标。
Example 1: magnesium chloride – Mg²⁺ + Cl⁻ → Mg₁Cl₂ → MgCl₂.
例 1：氯化镁——Mg²⁺ + Cl⁻ → Mg₁Cl₂ → MgCl₂。
Example 2: aluminium oxide – Al³⁺ + O²⁻ → swap charges → Al₂O₃.
例 2：氧化铝——Al³⁺ + O²⁻ → 交换电荷 → Al₂O₃。
Example 3: calcium nitrate – Ca²⁺ + NO₃⁻ → Ca(NO₃)₂.
例 3：硝酸钙——Ca²⁺ + NO₃⁻ → Ca(NO₃)₂。
Always reduce the subscripts to the simplest whole-number ratio if possible (e.g
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