📚 AS Chemistry: Atomic Structure Key Points | AS 化学：原子结构 考点精讲
Atomic structure is the foundation of AS Chemistry. Understanding the composition of atoms, how subatomic particles behave, and how electrons are arranged explains both chemical properties and reactivity patterns. This article walks you through every essential concept for the exam, from subatomic particles to successive ionisation energies.
原子结构是 AS 化学的基础。理解原子的组成、亚原子粒子的行为以及电子的排布方式，能够解释化学性质和反应规律。本文带你深入每个核心考点，从亚原子粒子到逐级电离能，逐一剖析。
1. Subatomic Particles | 亚原子粒子
Atoms are composed of three fundamental particles: protons, neutrons, and electrons. Their properties determine the identity and behaviour of an element.
原子由三种基本粒子组成：质子、中子和电子。它们的性质决定元素的身份和化学行为。
Protons carry a relative charge of +1 and a relative mass of 1. They are located in the nucleus.
质子相对电荷为 +1，相对质量为 1，位于原子核内。
Neutrons are neutral (charge 0) and also have a relative mass of 1. They reside in the nucleus alongside protons.
中子不带电（电荷为 0），相对质量也为 1，与质子共同构成原子核。
Electrons possess a charge of -1 and a negligible mass (1/1836 of a proton). They move around the nucleus in specific energy levels.
电子电荷为 -1，质量极小（约为质子的 1/1836），在原子核外的特定能级上运动。
2. Atomic Number and Mass Number | 原子序数与质量数
The atomic number (Z) is the number of protons in the nucleus. It defines the element and is unique for each element.
原子序数（Z）是原子核内的质子数。它定义了元素，每个元素的原子序数唯一。
The mass number (A) is the total number of protons and neutrons in the nucleus.
质量数（A）是原子核中质子数与中子数之和。
Therefore, the number of neutrons can be calculated as A – Z.
因此，中子数可由 A – Z 计算得到。
In a neutral atom, the number of electrons equals the number of protons.
在中性原子中，电子数等于质子数。
3. Isotopes | 同位素
Isotopes are atoms of the same element with the same atomic number but different mass numbers. They contain the same number of protons but different numbers of neutrons.
同位素是同一元素的原子，它们具有相同的原子序数但质量数不同。它们的质子数相同，中子数不同。
For example, carbon has three naturally occurring isotopes: ¹²C, ¹³C, and ¹⁴C. All have 6 protons, but 6, 7, and 8 neutrons respectively.
例如，碳有三种天然同位素：¹²C、¹³C 和 ¹⁴C，都含有 6 个质子，但中子数分别为 6、7、8。
Isotopes exhibit identical chemical properties because chemical behaviour is determined by electrons, which are the same in number for all isotopes.
同位素的化学性质相同，因为化学行为取决于电子，而所有同位素的电子数一致。
Physical properties such as density and rate of diffusion may differ slightly due to different masses.
物理性质（如密度和扩散速率）可能因质量不同而略有差异。
4. Relative Atomic Mass | 相对原子质量
The relative atomic mass (Aᵣ) is the weighted average mass of an atom of an element, compared to 1/12 the mass of a carbon‑12 atom, taking into account the relative abundances of its isotopes.
相对原子质量（Aᵣ）是元素一个原子的加权平均质量与一个碳‑12 原子质量的 1/12 之比，计算时考虑了同位素的相对丰度。
It has no units because it is a ratio of masses.
相对原子质量没有单位，因为它是质量的比值。
Aᵣ = (mass₁ × %₁ + mass₂ × %₂ + …) / 100
Aᵣ = （质量₁ × 百分比₁ + 质量₂ × 百分比₂ + …）/ 100
Where mass₁, mass₂ are the isotopic masses and %₁, %₂ are the percentage abundances.
其中 mass₁、mass₂ 为同位素质量，%₁、%₂ 为丰度百分比。
5. Mass Spectrometry | 质谱分析
Mass spectrometry is used to determine the relative isotopic masses and their abundances, which enables calculation of relative atomic mass.
质谱法用于测定相对同位素质量及其丰度，从而计算相对原子质量。
The main stages of a time‑of‑flight mass spectrometer are: vaporisation (or atomisation), ionisation (electron impact or electrospray), acceleration, deflection (in older instruments) or time‑of‑flight separation, and detection.
飞行时间质谱仪的主要步骤有：气化（或原子化）、电离（电子轰击或电喷雾）、加速、偏转（旧式仪器）或飞行时间分离，以及检测。
In the ionisation stage, a gaseous sample is bombarded with high‑energy electrons, forming positive ions: M(g) → M⁺(g) + e⁻.
在电离阶段，气态样品被高能电子轰击，形成正离子：M(g) → M⁺(g) + e⁻。
The ions are accelerated by an electric field, and their time of flight depends on mass‑to‑charge ratio. Lighter ions and those with higher charges reach the detector faster.
离子被电场加速，其飞行时间取决于质荷比。质量越轻、电荷越高的离子越快到达检测器。
A mass spectrum plots relative abundance against mass‑to‑charge ratio (m/z), producing peaks for each isotope. Relative atomic mass can then be calculated.
质谱图以质荷比 (m/z) 为横坐标，相对丰度为纵坐标，每种同位素产生一个峰，据此可计算相对原子质量。
6. Electronic Structure: Energy Levels and Subshells | 电子结构：能级与亚层
Electrons exist in principal energy levels (shells), labelled n = 1, 2, 3, 4 … The higher the value of n, the higher the energy and the further the shell from the nucleus.
电子存在于主能级（电子层）中，标记为 n = 1, 2, 3, 4 …… n 值越大，能量越高，层离核越远。
Each principal level is split into subshells: s, p, d, f. The type of subshells available depends on n.
每个主能级分为亚层：s、p、d、f。可用的亚层种类取决于 n。
n = 1 has only an s subshell; n = 2 has s and p; n = 3 has s, p, and d; n = 4 has s, p, d, and f.
n = 1 只有 s 亚层；n = 2 有 s 和 p；n = 3 有 s、p、d；n = 4 有 s、p、d、f。
7. Orbitals and Their Shapes | 轨道及其形状
Each subshell contains a fixed number of orbitals. An orbital is a region where there is a high probability of finding an electron. Each orbital can hold a maximum of two electrons with opposite spins.
每个亚层包含一定数量的轨道。轨道是电子出现概率较高的区域，每个轨道最多容纳两个自旋相反的电子。
s subshell: 1 orbital, spherical shape. p subshell: 3 orbitals, dumbbell shaped, oriented along x, y, z axes.
s 亚层：1 个轨道，球形。p 亚层：3 个轨道，哑铃形，分别沿 x、y、z 轴取向。
d subshell contains 5 orbitals, f subshell contains 7 orbitals. Their shapes are more complex.
d 亚层含 5 个轨道，f 亚层含 7 个轨道，形状更复杂。
8. Rules for Filling Orbitals | 轨道填充规则
Electrons occupy the lowest available energy levels first – this is the Aufbau principle.
电子优先占据能量最低的轨道——这是构造原理。
Each orbital can hold a maximum of two electrons, and their spins must be opposite (Pauli exclusion principle).
每个轨道最多容纳两个电子，且它们自旋必须相反（泡利不相容原理）。
When filling degenerate orbitals (e.g., the three p orbitals), electrons occupy separate orbitals singly before pairing up – this is Hund’s rule, minimising repulsion.
在填充简并轨道（如三个 p 轨道）时，电子先以自旋相同的方向单独占据不同轨道，再配对——这是洪特规则，可减少斥力。
The order of filling for the first 20 elements is: 1s, 2s, 2p, 3s, 3p, 4s. Note that 4s fills before 3d because it is lower in energy for K and Ca. For transition metals, 3d fills after 4s but exceptions (Cr, Cu) exist.
前 20 号元素的填充顺序为：1s, 2s, 2p, 3s, 3p, 4s。注意 4s 先于 3d 填充，因为对 K 和 Ca 而言 4s 能量更低。过渡金属先填 4s 后填 3d，但存在铬、铜等例外。
9. Electron Configurations of Atoms and Ions | 原子与离子的电子排布
An electron configuration shows the distribution of electrons among the subshells. For example, sodium (Z = 11): 1s² 2s² 2p⁶ 3s¹.
电子排布表示电子在亚层上的分布。例如钠（Z = 11）：1s² 2s² 2p⁶ 3s¹。
A shorthand notation uses the previous noble gas in square brackets: [Ne] 3s¹ for sodium.
简写式用前一周期稀有气体符号加方括号表示内层电子：钠写作 [Ne] 3s¹。
For transition metals, there are two key anomalies: chromium (Cr, Z = 24) is [Ar] 4s¹ 3d⁵ instead of 4s² 3d⁴; copper (Cu, Z = 29) is [Ar] 4s¹ 3d¹⁰. These arise because half‑filled and fully filled d subshells confer extra stability.
过渡元素有两个重要特例：铬 (Cr, Z = 24) 电子排布为 [Ar] 4s¹ 3d⁵ 而非 4s² 3d⁴；铜 (Cu, Z = 29) 为 [Ar] 4s¹ 3d¹⁰。半满和全满 d 亚层提供额外稳定性。
When ions form, electrons are removed from the outermost shell first. For transition metals, 4s electrons are lost before 3d electrons. For example, Fe²⁺: [Ar] 3d⁶, not [Ar] 4s² 3d⁴.
形成离子时，电子从最外层开始失去。过渡金属先失去 4s 电子，再失 3d 电子。例如 Fe²⁺ 为 [Ar] 3d⁶，而不是 [Ar] 4s² 3d⁴。
10. First Ionisation Energy | 第一电离能
The first ionisation energy (IE₁) is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
第一电离能 (IE₁) 是指从 1 mol 气态原子中移除 1 mol 电子形成 1 mol 气态 1+ 离子所需的能量。
Equation: X(g) → X⁺(g) + e⁻ ΔH = IE₁ (kJ mol⁻¹)
方程式：X(g) → X⁺(g) + e⁻ 焓变 = IE₁ (kJ mol⁻¹)
It is an endothermic process. The magnitude of IE₁ reflects how strongly an electron is attracted to the nucleus.
此过程吸热。第一电离能的大小反映原子核对电子的吸引强弱。
11. Factors Affecting Ionisation Energy | 影响电离能的因素
Three main factors influence ionisation energy: nuclear charge, distance of the outermost electron from the nucleus, and shielding by inner electrons.
影响电离能的三个主要因素：核电荷、最外层电子离核的距离，以及内层电子的屏蔽作用。
A higher nuclear charge (more protons) means a greater attractive force, increasing IE.
核电荷越高（质子数越多），核对电子的吸引力越强，电离能越大。
Greater distance of the outer electron from the nucleus reduces attraction, lowering IE.
最外层电子离核越远，吸引力越弱，电离能降低。
Inner shells of electrons shield the outer electrons from the full nuclear charge. More shielding reduces the effective nuclear charge experienced by the outer electron, so IE decreases.
内层电子对外层电子起屏蔽作用，减少有效核电荷，导致电离能下降。
12. Successive Ionisation Energies | 逐级电离能
Successive ionisation energies are the energies required to remove each subsequent electron. IE₂ corresponds to: X⁺(g) → X²⁺(g) + e⁻.
逐级电离能是依次移除每个电子所需的能量。第二电离能 IE₂ 对应：X⁺(g) → X²⁺(g) + e⁻。
Successive ionisation energies always increase because the ion becomes more positively charged and electrons are held tighter.
逐级电离能总是增大，因为离子正电性增强，对电子的束缚更紧。
A large jump in ionisation energy occurs when an electron is removed from a lower principal energy level, which is much closer to the nucleus and less shielded. This provides evidence for electron shells.
当从更低的主能级移除电子时，电离能会出现大幅跳跃，因为该电子离核更近、屏蔽更少。这为电子层的存在提供了证据。
For example, sodium (1s²2s²2p⁶3s¹) shows a huge jump between IE₁ and IE₂, confirming a single outer electron. The next large jump occurs after removing the eight 2p and 2s electrons, revealing the inner 1s² shell.
例如钠 (1s²2s²2p⁶3s¹) 的 IE₁ 和 IE₂ 间有一次大幅跃升，证明只有一个外层电子。移走八个 2p 和 2s 电子后再次大幅跃升，揭示内层 1s² 电子。
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