📚 Atomic Structure Core: Models and Composition | 原子结构核心：模型与组成剖析
The atom is the fundamental unit of matter, yet its internal architecture has been refined through more than a century of experimental discovery. This article explores the evolution of atomic models and the precise composition of subatomic particles, providing a clear framework for IB and CIE chemistry examinations.
原子是物质的基本单位，然而其内部构造经过了一个多世纪的实验发现才逐步清晰。本文将梳理原子模型的演变历程，并剖析亚原子粒子的精确组成，为 IB 与 CIE 化学考试提供清晰的知识框架。
1. From Dalton to Rutherford | 从道尔顿到卢瑟福
John Dalton proposed that atoms were indivisible solid spheres. This model explained the law of conservation of mass and the law of constant composition, but it offered no insight into the internal structure of the atom.
约翰·道尔顿提出原子是不可分割的实心球体。该模型解释了质量守恒定律与定组成定律，但没有揭示原子的内部结构。
J.J. Thomson discovered the electron in 1897 and proposed the “plum pudding” model, in which negatively charged electrons were embedded in a uniform positive charge.
J.J. 汤姆逊于1897年发现电子，并提出“葡萄干布丁”模型，认为带负电的电子镶嵌在均匀的正电荷之中。
Ernest Rutherford’s gold-foil experiment (1911) showed that most alpha particles passed straight through, but a few were deflected at large angles. This led to a nuclear model with a dense, positive nucleus and mostly empty surrounding space.
欧内斯特·卢瑟福的金箔实验（1911年）表明，大多数α粒子直穿而过，少数发生大角度偏转，于是提出原子核模型：一个致密的正电原子核，周围绝大部分是空旷的空间。
2. Bohr’s Planetary Model and Its Limits | 玻尔行星模型及其局限
Niels Bohr refined the nuclear model by placing electrons in discrete circular orbits, or energy levels, around the nucleus. Electrons could absorb or emit energy when jumping between these quantised levels, successfully explaining the hydrogen emission spectrum.
尼尔斯·玻尔改进核模型，将电子置于原子核周围分立的圆形轨道即能级上。电子在这些量子化的能级间跃迁时吸收或发射能量，成功解释了氢原子发射光谱。
However, Bohr’s model failed for atoms with more than one electron, could not explain the fine structure of spectral lines, and ignored the wave nature of electrons.
然而，玻尔模型无法解释多电子原子，也不能解释谱线的精细结构，更忽略了电子的波动性。
3. Quantum Mechanical Model | 量子力学模型
Modern atomic theory describes electrons not as particles in fixed orbits, but as probability clouds described by wave functions. The Schrödinger equation gives the energies and distributions of these orbitals, known as atomic orbitals.
现代原子理论不将电子视为固定轨道上的粒子，而是由波函数描述的几率云。薛定谔方程给出这些轨道（即原子轨道）的能量和分布。
Each orbital is characterised by three quantum numbers: principal (n), azimuthal (l), and magnetic (mₗ). The spin quantum number (mₛ) further distinguishes the two possible spin states of an electron.
每个轨道由三个量子数表征：主量子数 n、角量子数 l、磁量子数 mₗ。自旋量子数 mₛ 进一步区分电子的两种可能自旋状态。
4. Subatomic Particles: Protons, Neutrons, Electrons | 亚原子粒子：质子、中子、电子
An atom contains three key subatomic particles. Protons are positively charged, located in the nucleus, and each carries a relative charge of +1. Neutrons have no charge and also reside in the nucleus. Electrons have a relative charge of −1 and move in the space around the nucleus.
原子包含三种关键的亚原子粒子。质子带正电，位于原子核内，每个质子相对电荷为 +1。中子不带电荷，同样位于原子核内。电子相对电荷为 −1，在核外空间运动。
| Particle 粒子 | Relative charge 相对电荷 | Relative mass 相对质量 |
| Proton 质子 | +1 | 1 |
| Neutron 中子 | 0 | 1 |
| Electron 电子 | −1 | 1/1840 |
The mass of an electron is negligible relative to protons and neutrons. Therefore, the mass of an atom is essentially concentrated in the nucleus.
电子的质量相对于质子与中子可以忽略不计。因此，原子的质量几乎全部集中在原子核中。
5. 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 of an atom. It defines the element and determines its chemical identity. In a neutral atom, the number of electrons equals the number of protons.
原子序数 Z 是原子核中的质子数。它决定了元素的种类及其化学性质。在中性原子中，电子数等于质子数。
The mass number (A) is the total number of protons and neutrons in the nucleus. Nucleons is the collective term for protons and neutrons.
质量数 A 是原子核中质子数与中子数之和。核子是指质子和中子的统称。
A = Z + N
where N is the number of neutrons. This relationship is essential for calculating neutron numbers from isotopic notation.
其中 N 为中子数。这一关系对于从同位素符号计算中子数至关重要。
6. Isotopes and Relative Atomic Mass | 同位素与相对原子质量
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They share the same chemical properties but differ in physical properties such as density and reaction rate.
同位素是同一元素中具有相同质子数但不同中子数的原子。它们具有相同的化学性质，但在密度、反应速率等物理性质上有所不同。
For example, carbon-12 (¹²C) has 6 protons and 6 neutrons, while carbon-14 (¹⁴C) has 6 protons and 8 neutrons. Both belong to the element carbon.
例如，碳-12（¹²C）有6个质子和6个中子，而碳-14（¹⁴C）有6个质子和8个中子，二者均属于碳元素。
The relative atomic mass (Aᵣ) of an element is the weighted mean mass of its atoms compared to 1/12 of the mass of one carbon-12 atom. This weighting accounts for the natural abundance of each isotope.
元素的相对原子质量 Aᵣ 是该元素原子的加权平均质量与一个碳-12原子质量的1/12之比。该加权考虑了每个同位素的天然丰度。
7. Mass Spectrometry and Isotopic Abundance | 质谱分析法与同位素丰度
Mass spectrometry is used to determine the relative atomic mass and isotopic abundance of elements. The sample is vaporised and ionised, then accelerated and deflected by electric and magnetic fields before being detected.
质谱分析法用于测定元素的相对原子质量和同位素丰度。样品先被汽化并电离，然后在电场和磁场中加速和偏转，最后被检测器接收。
The resulting mass spectrum shows a series of peaks. Each peak corresponds to a specific isotope, with the peak height representing its relative abundance.
所得质谱图显示一系列峰，每个峰对应一种特定的同位素，峰高代表其相对丰度。
For example, if chlorine has isotopes ³⁵Cl (75%) and ³⁷Cl (25%), the relative atomic mass is calculated as:
例如，若氯元素含同位素 ³⁵Cl（75%）和 ³⁷Cl（25%），则相对原子质量的计算如下：
Aᵣ(Cl) = (35 × 75 + 37 × 25) / 100 = 35.5
8. Electronic Structure: Energy Levels and Sublevels | 电子结构：能级与亚层
Electrons occupy principal energy levels, denoted by n = 1, 2, 3, etc. Higher n values correspond to higher energy and larger average distance from the nucleus.
电子占据主能级，用 n = 1, 2, 3……表示。n 值越大，能量越高，平均离核越远。
Each principal level contains one or more sublevels: s, p, d, and f. The number of sublevels in principal level n is equal to n.
每个主能级含有一个或多个亚层：s、p、d、f。主能级 n 中亚层数目等于 n。
Each orbital can hold a maximum of two electrons with opposite spins. An s sublevel has 1 orbital, a p sublevel has 3 orbitals, a d sublevel has 5 orbitals, and an f sublevel has 7 orbitals.
每个轨道最多容纳自旋方向相反的两个电子。s 亚层有1个轨道，p 亚层有3个轨道，d 亚层有5个轨道，f 亚层有7个轨道。
9. Filling Rules: Aufbau, Hund and Pauli | 填充规则：构造原理、洪德规则与泡利原理
The Aufbau principle states that electrons fill the lowest energy orbitals first. The order of filling is commonly represented as: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s, and so on.
构造原理指出，电子优先填入能量最低的轨道。填充顺序通常表示为：1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s 等。
Hund’s rule says that when electrons occupy degenerate orbitals (same energy), they must first occupy each orbital singly with parallel spins before pairing occurs.
洪德规则指出，当电子占据简并轨道（能量相同）时，必须先以相同自旋方向单独占据每个轨道，然后才能配对。
The Pauli exclusion principle states that no two electrons in the same atom can have the same set of all four quantum numbers. Consequently, an orbital can hold at most two electrons, and they must have opposite spins.
泡利不相容原理指出，同一原子中不存在四个量子数完全相同的两个电子。因此，一个轨道最多容纳两个电子，且二者自旋必须相反。
10. Electron Configurations and Orbital Diagrams | 电子构型与轨道图
Electron configurations describe how electrons are distributed among the atomic orbitals. For example, the configuration of oxygen (Z = 8) is 1s² 2s² 2p⁴.
电子构型描述电子在原子轨道中的分布。例如，氧（Z = 8）的电子构型为 1s² 2s² 2p⁴。
Orbital diagrams are drawn using boxes or lines to represent orbitals and arrows to represent electrons. Up and down arrows indicate opposite spins.
轨道图用方框或短横线表示轨道，用箭头表示电子，向上和向下的箭头代表相反的自旋。
Some transition metals show exceptions to the expected filling order. For example, chromium (Z = 24) has configuration [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s², because the half-filled d⁵ subshell is extra stable.
某些过渡金属表现出与预期填充顺序不同的例外。例如，铬（Z = 24）的构型为 [Ar] 3d⁵ 4s¹ 而非 [Ar] 3d⁴ 4s²，因为半充满的 d⁵ 亚层具有额外稳定性。
11. Core Concepts in Chemical Bonding Applications | 化学键应用中的核心概念
The electronic structure of the atom determines its chemical reactivity. Elements with incomplete outer shells tend to lose, gain, or share electrons to achieve a stable noble-gas electron configuration.
原子的电子结构决定其化学反应活性。外层电子未充满的元素倾向于失去、获得或共享电子，以达到稳定的稀有气体电子构型。
The period number corresponds to the highest occupied principal energy level, while the group number (for s and p block elements) equals the number of outer-shell electrons.
周期数对应最高占据主能级，而族数（s区与p区元素）等于外层电子数。
Transition elements exhibit variable oxidation states because electrons from both the 4s and 3d sublevels can participate in bonding. This is a common examination topic in IB and CIE chemistry.
过渡元素呈现可变氧化态，因为 4s 和 3d 亚层的电子均可参与成键。这是 IB 与 CIE 化学中的常见考点。
12. Summary and Exam Tips | 总结与备考建议
Mastery of atomic structure requires understanding the evidence behind each model, the exact definitions of subatomic particles, and the ability to write electron configurations for any element.
掌握原子结构需要理解每个模型背后的实验证据、亚原子粒子的精确定义，以及为任何元素写出电子构型的能力。
For exam success, remember: the atomic number uniquely identifies an element; mass number equals protons plus neutrons; isotopes differ in neutron number; and electron filling follows Aufbau, Hund, and Pauli rules.
为了考试成功，请牢记：原子序数唯一确定元素；质量数等于质子数加中子数；同位素中子数不同；电子填充遵循构造原理、洪德规则和泡利原理。
Practise interpreting mass spectra and writing configurations from quantum number assignments, as these frequently appear in free-response questions.
练习解读质谱图以及从量子数分配写出电子构型，因为这些内容在简答题中频繁出现。
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