📚 Core Principles of Chemistry (Edexcel Combined Science Unit 1) | 化学核心原理（Edexcel 综合科学第一单元）
Welcome to your ultimate revision guide for the Edexcel International A Level Combined Science Unit 1: The Core Principles of Chemistry. This unit builds the foundational knowledge you need to understand atomic structure, bonding, energetics, organic chemistry and periodic trends. Whether you are preparing for a classroom test or the final examination, mastering these concepts will give you the confidence to tackle both theoretical and calculation-based questions. We have structured this article into ten key topics, each explained with clear definitions, worked examples and memory aids to help you revise efficiently.
欢迎阅读 Edexcel 国际 A Level 综合科学第一单元“化学核心原理”完整复习指南。这一单元为你搭建了理解原子结构、化学键、能量学、有机化学和周期趋势所必需的知识框架。无论你是在准备课堂测验还是最终大考，吃透这些概念都能让你从容应对理论与计算题目。本文将内容划分为十大关键主题，每个主题都配有清晰定义、例题和记忆技巧，帮助你高效复习。
1. Atomic Structure and the Periodic Table | 原子结构与周期表
Atoms contain a tiny, dense nucleus made of protons and neutrons, surrounded by electrons arranged in shells. The atomic number (Z) is the number of protons in the nucleus and determines the element’s identity. The mass number (A) is the sum of protons and neutrons. In a neutral atom, the number of electrons equals the atomic number. Modern periodic table arranges elements in order of increasing atomic number, with elements in the same group having similar chemical properties because they have the same number of outer-shell electrons.
原子内部有一个微小致密的原子核，由质子和中子构成，核外电子按照壳层排布。原子序数（Z）是原子核中的质子数，它决定了元素的种类。质量数（A）等于质子数与中子数之和。中性原子中，电子数等于原子序数。现代周期表按原子序数递增的顺序排列元素，同一族的元素因最外层电子数相同而表现出相似的化学性质。
2. Isotopes and Relative Atomic Mass | 同位素与相对原子质量
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties but slightly different physical properties. The relative atomic mass (Ar) of an element is the weighted average mass of its isotopes relative to 1/12th the mass of a carbon‑12 atom. You can calculate Ar from isotopic abundances using the formula: Ar = Σ (isotopic mass × % abundance) / 100.
同位素是指质子数相同而中子数不同的同种元素的原子。它们的化学性质完全相同，但物理性质略有差异。元素的相对原子质量（Ar）是其各种同位素质量的加权平均值，以碳‑12 原子质量的 1/12 为基准。你可以利用同位素丰度计算 Ar，公式为：Ar = Σ（同位素质量 × 百分丰度） / 100。
Ar = (mass₁ × %₁ + mass₂ × %₂) / 100
3. Chemical Bonding: Ionic, Covalent and Metallic | 化学键：离子键、共价键与金属键
Ionic bonding occurs when a metal transfers one or more electrons to a non‑metal, forming oppositely charged ions that attract each other in a giant ionic lattice. For example, sodium chloride (NaCl) consists of Na⁺ and Cl⁻ ions. Covalent bonding involves the sharing of electron pairs between two non‑metal atoms, forming discrete molecules or giant covalent structures like diamond and silicon dioxide. Metallic bonding is the attraction between a lattice of positive metal ions and a sea of delocalised electrons – this explains the high electrical conductivity and malleability of metals.
金属原子将一个或多个电子转移给非金属原子时，形成带相反电荷的离子，它们通过静电引力构成巨大的离子晶格，这就是离子键。例如氯化钠（NaCl）由 Na⁺ 和 Cl⁻ 构成。共价键是两个非金属原子通过共用电子对结合在一起，可以形成离散分子或像金刚石、二氧化硅这样的巨型共价结构。金属键则是正金属离子晶格与“离域电子海”之间的吸引力，这解释了金属优异的导电性和延展性。
4. Shapes of Molecules and Intermolecular Forces | 分子形状与分子间作用力
Electron‑pair repulsion theory (VSEPR) states that electron pairs around a central atom arrange themselves to be as far apart as possible, determining the molecule’s shape. Key shapes include linear (e.g. BeCl₂, CO₂), trigonal planar (e.g. BF₃), tetrahedral (e.g. CH₄, NH₄⁺), pyramidal (e.g. NH₃) and bent (e.g. H₂O). Intermolecular forces – London dispersion forces, dipole‑dipole interactions and hydrogen bonding – influence physical properties like boiling point. Hydrogen bonding, which occurs when hydrogen is bonded to highly electronegative N, O or F, is the strongest intermolecular force and explains the unusually high boiling point of water.
电子对互斥理论（VSEPR）指出，中心原子周围的电子对会尽量远离彼此，从而决定了分子的形状。常见的分子形状有直线形（例如 BeCl₂、CO₂）、平面三角形（例如 BF₃）、正四面体形（例如 CH₄、NH₄⁺）、三角锥形（例如 NH₃）和折线形（例如 H₂O）。分子间作用力——伦敦色散力、偶极‑偶极作用力和氢键——影响着熔点、沸点等物理性质。氢键发生在氢与电负性极强的 N、O 或 F 成键的情况下，是最强的分子间作用力，它解释了水的异常高沸点。
| Shape / 形状 | Bond Pairs / 成键电子对 | Lone Pairs / 孤电子对 | Example / 例子 |
| Linear / 直线形 | 2 | 0 | CO₂ |
| Trigonal Planar / 平面三角形 | 3 | 0 | BF₃ |
| Tetrahedral / 正四面体形 | 4 | 0 | CH₄ |
| Pyramidal / 三角锥形 | 3 | 1 | NH₃ |
| Bent / 折线形 | 2 | 2 | H₂O |
5. The Mole Concept and Stoichiometry | 摩尔概念与化学计量
One mole of any substance contains exactly 6.022 × 10²³ particles (Avogadro’s constant). The number of moles (n) is linked to mass (m) and molar mass (Mr) by the equation n = m / Mr. Using balanced chemical equations, you can calculate the masses of reactants consumed and products formed. This quantitative relationship is the core of stoichiometry. Always check that your equation is balanced before performing any mole calculation.
任何物质的一摩尔都精确包含 6.022 × 10²³ 个粒子（阿伏伽德罗常数）。物质的量（n）、质量（m）与摩尔质量（Mr）的关系式为 n = m / Mr。利用配平的化学方程式，可以计算反应物消耗的质量和生成物产生的质量。这种定量关系就是化学计量的核心。在做任何摩尔计算前，一定要先确认方程式已经配平。
n = m / Mr
6. Empirical and Molecular Formulae | 实验式与分子式
The empirical formula gives the simplest whole‑number ratio of atoms in a compound, while the molecular formula shows the actual number of each atom per molecule. To find the empirical formula, divide the mass (or percentage) of each element by its relative atomic mass, then divide by the smallest number of moles you obtain to get a simple ratio. The molecular formula is a multiple of the empirical formula, found by comparing the relative molecular mass with the empirical formula mass.
实验式表示化合物中原子的最简整数比，而分子式则给出每个分子中各种原子的实际数目。计算实验式的步骤是：用各元素的质量或百分含量除以它的相对原子质量，再把得到的摩尔数除以其中的最小值，得到简单整数比。分子式是实验式的整数倍，可以通过比较相对分子质量与实验式质量来求得。
Molecular formula = n × Empirical formula
For example, if the empirical formula is CH₂ and the relative molecular mass is 56, then n = 56 / 14 = 4, giving the molecular formula C₄H₈. | 例如，若实验式为 CH₂，相对分子质量为 56，则 n = 56/14 = 4，分子式为 C₄H₈。
7. Enthalpy Changes and Calorimetry | 焓变与量热法
Enthalpy change (ΔH) is the heat energy transferred in a reaction at constant pressure. Exothermic reactions (ΔH negative) release energy to the surroundings, often raising the temperature, while endothermic reactions (ΔH positive) absorb energy. You can determine ΔH experimentally by measuring the temperature change of a known mass of water using the equation q = mcΔT, where c is the specific heat capacity of water (4.18 J g⁻¹ K⁻¹). Then ΔH = −q / n, where n is the number of moles of fuel or reactant.
焓变（ΔH）是恒压条件下反应中转移的热量。放热反应（ΔH 为负值）向环境释放能量，通常使温度升高；吸热反应（ΔH 为正值）则从环境吸收能量。实验测定 ΔH 的方法是通过测量已知质量水的温度变化，利用公式 q = mcΔT（c 为水的比热容 4.18 J g⁻¹ K⁻¹）。然后 ΔH = −q / n，其中 n 为燃料或反应物的物质的量。
q = mcΔT
ΔH = −q / n
8. Hess’s Law and Enthalpy Cycles | 赫斯定律与焓循环
Hess’s Law states that the total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. This allows you to calculate enthalpy changes that are difficult to measure directly, such as the standard enthalpy of formation or combustion. By constructing an enthalpy cycle using known enthalpy changes, you can easily find the unknown ΔH. Remember to reverse the sign when you reverse a reaction, and to multiply by the appropriate factor when scaling the equation.
赫斯定律指出，一个化学反应的总焓变只取决于反应的始态和终态，与反应途径无关。利用这一定律，我们可以计算那些难以直接测定的焓变，如标准生成焓或标准燃烧焓。通过构建包含已知焓变的焓循环，可以简便地求出未知的 ΔH。注意，反应倒转时要改变 ΔH 的符号，方程式放大时要相应乘以倍数。
ΔH₁ = ΔH₂ + ΔH₃
9. Introduction to Organic Chemistry: Alkanes and Alkenes | 有机化学导论：烷烃与烯烃
Organic chemistry is the study of carbon‑containing compounds. Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They mainly undergo combustion and substitution reactions with halogens in the presence of UV light. Alkenes are unsaturated hydrocarbons containing a carbon‑carbon double bond (C=C), with the general formula CₙH₂ₙ. The double bond makes alkenes much more reactive; they undergo addition reactions, for example with bromine, hydrogen, water and hydrogen halides. The test for unsaturation is the decolourisation of orange bromine water.
有机化学是研究含碳化合物的学科。烷烃是通式为 CₙH₂ₙ₊₂ 的饱和烃，主要发生燃烧反应，以及在紫外光下与卤素发生的取代反应。烯烃是含有碳碳双键（C=C）的不饱和烃，通式为 CₙH₂ₙ。双键使烯烃的化学反应活性远高于烷烃，它们能发生加成反应，例如与溴、氢气、水和卤化氢加成。检验不饱和烃的方法是将橙色的溴水加入样品中，溴水褪色即表明存在不饱和键。
10. Trends in Groups 1, 2 and 7 | 第1、2和7族元素趋势
Group 1 (alkali metals) become more reactive down the group: the outer electron is further from the nucleus and more easily lost. They react vigorously with water to produce metal hydroxide and hydrogen gas. Group 2 (alkaline earth metals) also show increasing reactivity down the group, though they react less vigorously than Group 1. They react with oxygen to form oxides, and with water (or steam) to form hydroxides. Group 7 (halogens) become less reactive down the group: the atoms get larger, so the attraction for an extra electron decreases. Displacement reactions can be used to confirm the reactivity trend: a more reactive halogen will displace a less reactive halide ion from its salt.
第1族（碱金属）越往下金属性越强：最外层电子离核越来越远，更容易失去。它们与水剧烈反应生成金属氢氧化物和氢气。第2族（碱土金属）也表现出越往下越活泼的趋势，但反应不如碱金属剧烈。它们与氧反应生成氧化物，与水（或水蒸气）反应生成氢氧化物。第7族（卤素）的反应活性则是越往下越弱：原子半径增大，原子核对一个外来电子的吸引力减弱。可以利用置换反应验证这一趋势：较活泼的卤素单质能从盐溶液中把较不活泼的卤素离子置换出来。
| Halogen / 卤素 | Colour in solution / 溶液中颜色 | Reactivity / 活泼性 |
| Chlorine / 氯 | Pale green / 浅绿色 | Most reactive / 最活泼 |
| Bromine / 溴 | Orange / 橙色 | Moderate / 中等 |
| Iodine / 碘 | Brown / 棕色 | Least reactive / 最不活泼 |
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