📚 Atoms & Elements: OCR A-Level Science Key Points | 原子与元素：OCR A-Level 科学考点精讲
Atoms are the fundamental building blocks of matter. In OCR A-Level Science, a deep understanding of atomic structure and the properties of elements is the foundation for topics like bonding, periodicity, and chemical reactivity. This revision guide distils the essential concepts, definitions, and patterns you need to master for the exam.
原子是构成物质的基本单元。在 OCR A-level 科学考试中，透彻理解原子结构与元素性质是掌握化学键、周期性和化学反应等知识的基础。这份复习指南提炼了考试必须掌握的核心概念、定义和规律。
1. Atomic Structure – Protons, Neutrons, and Electrons | 原子结构——质子、中子和电子
According to the nuclear model, an atom consists of a small, dense nucleus containing positively charged protons and uncharged neutrons. Negatively charged electrons move around the nucleus in regions called shells or energy levels. Almost all the mass of the atom is concentrated in the nucleus.
根据核模型，原子由一个致密的原子核和核外运动的电子构成。原子核内含带正电荷的质子和不带电的中子，核外电子在称为壳层或能级的区域内运动。原子的几乎全部质量都集中在原子核上。
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1/1836 |
质子带一个单位正电荷，相对质量约为 1；中子不带电，相对质量约为 1；电子带一个单位负电荷，相对质量极小，仅为质子的 1/1836 左右。原子整体呈电中性，因此质子数等于电子数。
2. Atomic Number (Z) and Mass Number (A) | 原子序数(Z)与质量数(A)
The atomic number (Z) is defined as the number of protons in the nucleus of an atom. It determines the identity of the element – every atom of the same element has the same Z. The mass number (A) is the total number of protons and neutrons in the nucleus. The number of neutrons can be found using A − Z.
原子序数(Z)定义为原子核内的质子数，它决定了元素的种类——同种元素的所有原子具有相同的 Z 值。质量数(A)是原子核内质子数与中子数的总和，因此中子数等于 A − Z。
For example, a carbon atom with 6 protons and 6 neutrons has Z = 6 and A = 12, written as ¹²C. An oxygen atom with 8 protons and 8 neutrons has Z = 8, A = 16, represented as ¹⁶O.
例如，含有 6 个质子和 6 个中子的碳原子，Z = 6，A = 12，记作 ¹²C。拥有 8 个质子和 8 个中子的氧原子，Z = 8，A = 16，记作 ¹⁶O。
3. Isotopes: Definition and Notation | 同位素：定义与表示法
Isotopes are atoms of the same element that have the same atomic number (Z) but different mass numbers (A). They possess the same number of protons but different numbers of neutrons. Isotopes exhibit identical chemical properties because chemical behaviour is governed by the electron arrangement, which remains unchanged. However, their physical properties such as mass and density differ.
同位素是指质子数相同而质量数不同的同种元素的原子，即质子数相同、中子数不同。由于化学性质取决于核外电子排布，而电子结构并未改变，因此同位素的化学性质几乎完全相同；但它们的物理性质（如质量和密度）存在差异。
Carbon has three naturally occurring isotopes: ¹²C (6 neutrons), ¹³C (7 neutrons) and ¹⁴C (8 neutrons). Chlorine consists of two stable isotopes, ³⁵Cl and ³⁷Cl, in a roughly 3:1 ratio.
碳有三种天然同位素：¹²C（6 个中子）、¹³C（7 个中子）和 ¹⁴C（8 个中子）。氯有两种稳定同位素 ³⁵Cl 和 ³⁷Cl，丰度比约为 3:1。
4. Relative Atomic Mass and Mass Spectrometry | 相对原子质量与质谱法
Relative atomic mass (Ar) is the weighted average mass of an atom of an element compared to 1/12th of the mass of an atom of carbon‑12. It is calculated using the mass numbers and relative abundances of all naturally occurring isotopes. Mass spectrometry is the experimental technique used to measure isotopic masses and their abundances.
相对原子质量(Ar)是元素的原子质量与一个碳‑12 原子质量的 1/12 相比较所得的加权平均值。它由各同位素的质量数和相对丰度计算得出。质谱法就是用来测定同位素质量及其丰度的实验技术。
A simple mass spectrometer operates in four stages: ionisation (gaseous atoms are bombarded with high‑energy electrons to form positive ions), acceleration, deflection (by a magnetic field), and detection. Lighter ions are deflected more, and more highly charged ions are deflected more strongly. The resulting mass spectrum displays relative abundance against mass‑to‑charge ratio (m/z).
质谱仪的基本工作流程分为四步：电离（气态原子被高能电子轰击形成正离子）、加速、偏转（通过磁场）和检测。质量越小的离子偏转越大，带电荷越多的离子也偏转越大。得到的质谱图显示相对丰度对质荷比(m/z)的分布。
For chlorine, if the abundance of ³⁵Cl is 75% and that of ³⁷Cl is 25%, then Ar(Cl) = (35 × 75 + 37 × 25) / 100 = 35.5. This non‑integer value explains why most elements have fractional relative atomic masses.
以氯为例，若 ³⁵Cl 丰度为 75%，³⁷Cl 为 25%，则 Ar(Cl) = (35×75 + 37×25)/100 = 35.5。这个非整数值解释了为何多数元素的相对原子质量不是整数。
5. Electron Shells and Energy Levels | 电子壳层与能级
Electrons are not randomly distributed; they occupy discrete energy levels, often called shells, around the nucleus. Each shell is assigned a principal quantum number n, where n = 1 is the lowest energy level closest to the nucleus. The maximum number of electrons a shell can hold is given by the formula 2n². Thus, the first shell (n=1) holds up to 2 electrons, the second shell (n=2) up to 8, the third up to 18, and the fourth up to 32.
电子并非随机分布，而是占据原子核外分立的能级，通常称为电子壳层。每一壳层用主量子数 n 表示，n=1 的能量最低、离核最近。壳层可容纳的最多电子数为 2n²：第一壳层最多 2 个电子，第二壳层最多 8 个，第三壳层最多 18 个，第四壳层最多 32 个。
Electrons fill the lowest available energy levels first – this is the Aufbau principle. When a shell is full, electrons occupy the next shell outward. The outer‑shell electrons, called valence electrons, are responsible for the chemical bonding and reactions of the element.
电子优先填充能量最低的能级，这就是构造原理。一个壳层填满后，电子才会占据更外层的壳层。最外层电子（称为价电子）决定了元素的化学键合与化学反应。
6. Sub-shells and Atomic Orbitals (s, p, d) | 亚层与原子轨道(s, p, d)
Each electron shell is further divided into sub‑shells labelled s, p, d and f. A sub‑shell contains one or more atomic orbitals. An orbital is a region of space around the nucleus where there is a high probability of finding an electron. Each orbital can hold a maximum of two electrons with opposite spins.
每个电子壳层可以细分为亚层，符号为 s、p、d 和 f。亚层中包含一定数量的原子轨道。轨道是原子核外电子出现概率最高的空间区域。每个轨道最多容纳两个自旋相反的电子。
Sub‑shell capacities: the s sub‑shell has 1 orbital (max 2 electrons); the p sub‑shell has 3 orbitals (max 6 electrons); the d sub‑shell has 5 orbitals (max 10 electrons); the f sub‑shell has 7 orbitals (max 14 electrons). The shape of an s orbital is spherical, while p orbitals are dumbbell‑shaped and oriented along the x, y and z axes.
各亚层的轨道数与电子容量：s 亚层含 1 个轨道（最多 2 个电子）；p 亚层含 3 个轨道（最多 6 个电子）；d 亚层含 5 个轨道（最多 10 个电子）；f 亚层含 7 个轨道（最多 14 个电子）。s 轨道呈球形，p 轨道呈哑铃形且沿 x、y、z 轴方向伸展。
In a shell with principal quantum number n, the possible sub‑shells are up to n−1. For n=1: only 1s. For n=2: 2s and 2p. For n=3: 3s, 3p and 3d. The energy order of filling for the first four levels is 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p. Note that the 4s sub‑shell fills before 3d because of its lower energy in neutral atoms.
对于主量子数为 n 的壳层，亚层可到 n−1。n=1：仅 1s；n=2：2s 和 2p；n=3：3s、3p 和 3d。电子填充的能级顺序为：1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p。注意 4s 亚层的能量低于 3d，因此电子先填充 4s 后再进入 3d。
7. Writing Electron Configurations | 书写电子排布
The electron configuration of an atom shows the distribution of electrons among the sub‑shells. It is written by listing the sub‑shells in order of increasing energy and adding a superscript to indicate the number of electrons in each. For example, oxygen (Z=8) has the configuration 1s² 2s² 2p⁴. Sodium (Z=11) is 1s² 2s² 2p⁶ 3s¹.
电子排布式表示电子在各个亚层中的分布情况。书写时按能量由低到高的顺序列出亚层，并用上标数字注明该亚层中的电子数。例如，氧(Z=8)的排布为 1s² 2s² 2p⁴；钠(Z=11)为 1s² 2s² 2p⁶ 3s¹。
For transition metals, remember that 4s is filled before 3d. Iron (Z=26): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. However, chromium (Z=24) and copper (Z=29) are exceptions. Chromium adopts [Ar] 4s¹ 3d⁵ and copper [Ar] 4s¹ 3d¹⁰, because half‑filled and fully‑filled d sub‑shells confer extra stability.
对于过渡金属，牢记 4s 先于 3d 填充。铁(Z=26)的排布为 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶。但铬(Z=24)和铜(Z=29)是例外：铬的排布为 [Ar] 4s¹ 3d⁵，铜为 [Ar] 4s¹ 3d¹⁰，因为半充满和全充满的 d 亚层能带来额外的稳定性。
When writing configurations for ions, electrons are removed first from the outermost shell. For Fe²⁺: remove the two 4s electrons, giving 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶. For Fe³⁺: remove one more electron from 3d, yielding 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵.
书写离子电子排布时，电子总是从最外层开始失去。Fe²⁺ 的排布：先失去两个 4s 电子，得到 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶；Fe³⁺ 再失去一个 3d 电子，得到 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵。
8. The Periodic Table: Periods, Groups, and Blocks | 元素周期表：周期、族与区块
The modern periodic table arranges elements in order of increasing atomic number. Horizontal rows are called periods; the period number tells you the highest principal quantum number n that is occupied. Vertical columns are groups; elements in the same group have the same number of valence electrons in similar configurations, resulting in similar chemical properties.
现代元素周期表按原子序数递增的顺序排列。横行称为周期，周期数代表该行元素电子占据的最高主量子数 n。纵列称为族，同族元素具有相同的价电子数和相似的电子排布，因而化学性质相似。
The table is also divided into blocks – s, p, d and f – based on the sub‑shell being filled. Groups 1 and 2 form the s‑block; groups 13 to 18 are the p‑block; the transition metals in groups 3–12 belong to the d‑block; and the inner transition metals (lanthanides and actinides) are the f‑block. Knowledge of the block an element belongs to helps predict its electron configuration and properties.
周期表可根据填充的亚层划分为 s、p、d、f 四个区块。第 1、2 族属于 s 区；第 13 至 18 族属于 p 区；第 3 至 12 族过渡金属构成 d 区；镧系和锕系元素属于 f 区。知道某元素所属的区块，有助于推断其电子排布与性质。
9. First Ionisation Energy Trends | 第一电离能趋势
The first ionisation energy (IE) of an element is the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. It is an endothermic process, e.g. Na(g) → Na⁺(g) + e⁻.
第一电离能(IE)是指从 1 摩尔气态原子中移走 1 摩尔最外层电子，生成 1 摩尔气态+1 价离子所需的能量。这是个吸热过程，如 Na(g) → Na⁺(g) + e⁻。
Across a period, the first ionisation energy generally increases. This is because nuclear charge increases while electrons are added to the same principal energy level, so the atomic radius decreases and the attraction between nucleus and outer electrons becomes stronger. More energy is needed to remove an electron.
沿着周期从左到右，第一电离能总体呈增大趋势。这是因为核电荷递增，而增加的电子都进入同一主能层，导致原子半径减小，原子核对外层电子的吸引力增强，移走电子所需的能量增大。
There are two key dips in this trend across Period 3, and similar patterns appear in Period 2. The first dip occurs between Group 2 and Group 13 (e.g. Mg → Al). Aluminium’s outermost electron occupies the 3p sub‑shell, which is of slightly higher energy than the 3s sub‑shell in magnesium. This lower‑energy barrier means less energy is required. The second dip is between Group 15 and Group 16 (e.g. P → S). Sulphur’s electron configuration is 3p⁴, which contains a pair of electrons in one of
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