📚 The Nuclear Atom | 核型原子
The nuclear atom is the fundamental model describing the structure of an atom: a dense, positively charged nucleus surrounded by a cloud of electrons. This concept emerged from experiments in the late 19th and early 20th centuries, overturning the ancient idea of indivisible particles and Dalton's solid-sphere model. Understanding the nuclear atom is essential for explaining chemical bonding, periodicity, and the behaviour of elements. This article covers the historical development, subatomic particles, isotopes, mass spectrometry, and electron configurations that form the core of IB and A-Level chemistry.
核型原子是描述原子结构的基本模型：一个致密、带正电的原子核被电子云所包围。这一概念源于19世纪末至20世纪初的一系列实验，推翻了古老的不可分微粒思想和道尔顿的实心球模型。理解核型原子对解释化学键、周期性以及元素行为至关重要。本文将涵盖历史发展、亚原子粒子、同位素、质谱分析以及电子排布等IB和A-Level化学的核心内容。
1. Historical Development of the Atomic Model | 原子模型的历史演变
John Dalton proposed in 1808 that atoms were tiny, indivisible spheres. This model explained the law of conservation of mass and constant composition but could not account for electrical phenomena. J.J. Thomson's discovery of the electron in 1897 led to the 'plum pudding' model, where negative electrons were embedded in a positive sphere. In 1911, Ernest Rutherford's gold foil experiment showed that most alpha particles passed straight through, but a few were deflected at large angles. He concluded that the atom must have a small, dense, positively charged nucleus, with electrons orbiting around it. Niels Bohr later refined the model by suggesting electrons move in fixed energy levels, explaining the stability of atoms and line spectra.
约翰·道尔顿于1808年提出原子是微小的、不可分割的实心球体。该模型解释了质量守恒与定比定律，但无法说明电现象。J.J.汤姆逊在1897年发现电子后，提出了“葡萄干布丁”模型，即带负电的电子嵌在正电球体中。1911年，欧内斯特·卢瑟福的金箔实验表明，大多数α粒子径直穿过，但少数以大角度反弹。他由此得出结论：原子必定具有一个极小、致密且带正电的核，电子绕其运行。尼尔斯·玻尔后来进一步修正，提出电子在固定能级上运动，解释了原子的稳定性和线状光谱。
2. Subatomic Particles: Proton, Neutron, Electron | 亚原子粒子：质子、中子、电子
Atoms consist of three fundamental particles. Their properties are summarised below:
原子由三种基本粒子构成，其性质总结如下：
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton (p) | 1 | +1 | Nucleus |
| Neutron (n) | 1 | 0 | Nucleus |
| Electron (e⁻) | 1/1836 | -1 | Orbitals |
The mass of an electron is almost 2000 times smaller than that of a proton or neutron; therefore, almost all the mass of an atom resides in the nucleus. The number of protons defines the element, while neutrons contribute to the mass and stability. The number of electrons equals the number of protons in a neutral atom.
电子的质量几乎是质子或中子的1/2000，因此原子的近乎全部质量都集中在原子核。质子的数目决定了元素种类，中子则贡献质量与稳定性。中性原子中，电子数等于质子数。
3. Nuclear Symbol Notation | 核符号表示法
Chemists use a standard notation to represent the composition of a nucleus: AZX, where X is the element symbol, Z is the atomic number (number of protons), and A is the mass number (total number of protons and neutrons). For example, 126 n a neutral atom) 6 electrons. The number of neutrons is calculated as A - Z = 12 - 6 = 6.
化学家使用标准符号来表示原子核的组成：AZX，其中X为元素符号，Z为原子序数（质子数），A为质量数（质子与中子总数）。例如，126C具有6个质子、6个中子，中性原子中还有6个电子。中子数计算为A - Z = 12 - 6 = 6。
When writing nuclear symbols, the mass number is placed as a superscript to the left, and the atomic number as a subscript. For ions, the charge is written as a superscript to the right: e.g., 2311Na+. This notation is universally used in nuclear equations and isotopic labelling.
书写核符号时，质量数放在左上角，原子序数放在左下角。对于离子，电荷则写在右上角，如2311Na+。该表示法普遍用于核反应方程式和同位素标记中。
4. Isotopes | 同位素
Isotopes are atoms of the same element (same Z) that have different numbers of neutrons (different A). Since chemical properties are determined by the electron configuration, which depends on Z, isotopes exhibit nearly identical chemical behaviour. However, their physical properties, such as mass, density, and radioactivity, differ. For example, carbon-12 (126C) and carbon-14 (146C) both have 6 protons and 6 electrons, but 6 and 8 neutrons respectively.
同位素是质子数（Z）相同而中子数（A不同）的同一元素原子。由于化学性质由电子排布决定，而电子排布依赖于Z，因此同位素表现出几乎相同的化学行为。然而，它们的物理性质如质量、密度和放射性则不同。例如，碳-12 (126C) 和碳-14 (146C) 都有6个质子和6个电子，但中子数分别为6和8。
Radioisotopes have unstable nuclei and decay by emitting radiation. They are widely used in medicine (e.g., 13153I for thyroid treatment), carbon dating (146C), and industrial tracers. The presence of isotopes explains why most atomic masses on the periodic table are not whole numbers; they are weighted averages of isotopic masses.
放射性同位素原子核不稳定，会通过辐射衰变。它们在医学（如13153I用于甲状腺治疗）、碳定年法（146C）以及工业示踪剂中有广泛应用。同位素的存在解释了为什么周期表中大多数原子量不是整数——它们是同位素质量的加权平均值。
5. Introduction to Mass Spectrometry | 质谱法简介
A mass spectrometer separates gaseous ions according to their mass-to-charge ratio (m/z) to determine isotopic abundances and relative atomic masses. The process involves five main stages: vaporisation, ionisation, acceleration, deflection, and detection. The sample is first vaporised, then bombarded with high-energy electrons to form positive ions. These ions are accelerated by an electric field and passed through a magnetic field, which deflects them; lighter ions and those with higher charges are deflected more. Finally, ions hit a detector, generating a current proportional to their abundance.
质谱仪根据离子的质荷比（m/z）分离气态离子，从而测定同位素丰度和相对原子质量。该过程包含五个主要阶段：蒸发、电离、加速、偏转和检测。样品首先被蒸发，随后受到高能电子轰击形成正离子。这些离子在电场中加速后通过磁场，磁场使其偏转，质量较小或电荷较高的离子偏转更大。最后，离子撞击检测器，产生与其丰度成正比的电流。
The key relationship governing deflection is that the radius of curvature increases with mass and decreases with charge. In many instruments, charge is usually +1, so m/z effectively equals the mass of the ion. The output is a mass spectrum with m/z on the x-axis and relative abundance on the y-axis.
控制偏转的关键关系是曲率半径随质量增大而增大，随电荷增加而减小。在许多仪器中，离子电荷通常为+1，因此m/z实质上等于离子的质量。输出结果为质谱图，横坐标为m/z，纵坐标为相对丰度。
6. Interpreting Mass Spectra | 质谱图解析
To calculate the relative atomic mass (Ar) of an element from its mass spectrum, you take the weighted mean of all isotopic masses. For each peak, multiply the isotopic mass by its relative abundance (as a fraction), sum these values, and divide by the total abundance if using percentages, or simply use the abundance directly if normalised. For example, chlorine consists of two main isotopes: 35Cl (75%) and 37Cl (25%). The relative atomic mass is (35 × 0.75) + (37 × 0.25) = 35.5.
要从质谱图计算元素的相对原子质量（Ar），需取所有同位素质量的加权平均值。对每个峰，将同位素质量乘以其相对丰度（作为分数），求和，若使用百分比则除以总丰度，若丰度已归一化则直接相加。例如，氯主要有两种同位素：35Cl (75%) 和 37Cl (25%)。相对原子质量为 (35 × 0.75) + (37 × 0.25) = 35.5。
For molecules, mass spectra also show peaks resulting from fragmentation. The peak with the highest m/z (molecular ion peak, M+) gives the relative molecular mass. Additional peaks at lower m/z represent fragments that help deduce the structure of the molecule. In diatomic elements like Cl2, peaks appear at m/z 70, 72 and 74 due to combinations of the isotopes 35Cl and 37Cl.
对于分子，质谱还会显示碎片离子峰。具有最大m/z的峰（分子离子峰，M+）给出相对分子质量。较低m/z的额外峰代表碎片，有助于推断分子结构。在双原子分子如Cl2中，由于35Cl和37Cl的组合，m/z 70、72和74处会出现峰。
7. Electron Energy Levels and Sublevels | 电子能级和亚层
as s and p; n=3 has s, p, and d; and n=4 has s, p, d, and f. Each sublevel consists of a specific number of orbitals: s has 1 orbital, p has 3, d has 5, and f has 7. Each orbital can hold a maximum of 2 electrons.
电子占据不连续的能级（壳层），用主量子数n = 1, 2, 3, ...标记。随着n增加，能量及与核的平均距离增大。每个壳层包含亚层（子壳层），分别标为s、p、d、f。n=1壳层仅有s亚层；n=2有s和p；n=3有s、p、d；n=4有s、p、d、f。每个亚层由一定数目的轨道构成：s有1个轨道，p有3个，d有5个，f有7个。每个轨道最多容纳2个电子。
The maximum number of electrons in a shell is given by 2n². Thus, shell 1 holds 2 electrons, shell 2 holds 8, shell 3 holds 18, and shell 4 holds 32. However, these numbers are fully realised only for elements further down the periodic table. The order of filling orbitals follows the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.
一个壳层最多可容纳的电子数为2n²。因此，第1壳层容纳2个电子，第2壳层8个，第3壳层18个，第4壳层32个。但这些数目只在周期表较后的元素中充分体现。轨道的填充顺序遵循构造原理：1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p等。
8. Writing Electron Configurations | 书写电子构型
Electron configurations describe the arrangement of electrons in an atom' rbitals. Using the Aufbau principle, electrons fill the lowest energy orbitals first. Hund' ation of oxygen (Z=8) is 1s² 2s² 2p⁴. The 2p sublevel has three orbitals; with four electrons, two orbitals have one electron each and one orbital is doubly occupied.
电子构型描述原子轨道中电子的排布。按构造原理，电子优先填充能量最低的轨道。洪特规则指出，电子在简并轨道上先以单电子占据，再配对。泡利不相容原理规定，同一原子中没有两个电子具有完全相同的四个量子数，因此一个轨道最多容纳两个自旋相反的电子。例如，氧（Z=8）的电子构型为1s² 2s² 2p⁴。2p亚层有三个轨道，四个电子填充时，两个轨道各有一个电子，一个轨道容纳两个电子。
s² 3d⁶. Exceptions occur for chromium and copper due to the extra stability of half-filled and fully filled d sublevels.
对于前20种元素，电子构型遵循简单的模式。钙之后，3d亚层在4s之后开始填充，形成过渡金属。简写式用方括号内前一个稀有气体表示：如Fe (Z=26) 为 [Ar] 4s² 3d⁶。铬和铜由于半满和全满d亚层的额外稳定性而出现例外。
9. Ionisation Energy and Electron Configuration | 电离能与电子排布
s across periods and down groups provide evidence for the shell model. Across a period, ionisation energy generally increases because nuclear charge increases and electrons are added to the same main shell, so shielding is similar but the attraction to the nucleus is stronger. Down a group, ionisation energy decreases as outer electrons are in higher shells, farther from the nucleus, and experience more shielding.
第一电离能是指从1摩尔气态原子中移去1摩尔电子，生成1摩尔气态1+离子所需的能量：X(g) → X⁺(g) + e⁻。周期和族间的电离能变化趋势为壳层模型提供了证据。在同一周期中，电离能通常递增，因为核电荷增加，电子加入同一主壳层，屏蔽效应相近，核对电子的吸引力更强。在族中自上而下，电离能递减，因为最外层电子处于更高能级，离核更远，且受到更多屏蔽。
Successive ionisation energies show large jumps when an electron is removed from an inner shell. For example, aluminium (1s² 2s² 2p⁶ 3s² 3p¹) shows a steady increase for the first three ionisations (removing the 3p and 3s electrons), but a huge jump for the fourth, confirming that the fourth electron comes from the 2p sublevel, which is closer to the nucleus and experiences a much higher effective nuclear charge. This pattern provides direct experimental evidence for the electron sublevels.
连续电离能在电子取自内层壳层时会出现巨大跃升。例如，铝（1s² 2s² 2p⁶ 3s² 3p¹）的前三个电离能（移去3p和3s电子）逐渐增加，但第四电离能急剧跃升，证明第四个电子来自2p亚层，它离核更近，受到的有效核电荷大得多。这一模式为电子亚层的存在提供了直接的实验证据。
10. Summary | 总结
The nuclear atom model integrates the discoveries of subatomic particles, the nucleus, isotopes, and the quantum behaviour of electrons. It allows chemists to explain atomic mass, chemical reactivity, and periodicity. Mass spectrometry gives precise isotopic abundances, and electron configurations govern how atoms bond. This foundation underpins all later topics in chemistry, from bonding theory to reaction mechanisms.
核型原子模型整合了对亚原子粒子、原子核、同位素和电子量子行为的发现。它使化学家能够解释原子质量、化学反应性与周期性。质谱法提供了精确的同位素丰度，而电子排布决定了原子的成键方式。这一基础支撑着化学中所有后续主题，从键合理论到反应机理。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图