📚 IB Chemistry: Atomic Nucleus Structure and Atomic Models | IB化学：原子核结构与原子模型
Understanding the structure of the atom is the foundation of all chemistry. From early philosophical ideas to the modern quantum mechanical model, the journey to describe the atomic nucleus and the arrangement of electrons has shaped how we interpret chemical behaviour. This article explores the key discoveries, models and data that IB Chemistry students need to master.
理解原子结构是所有化学的基础。从早期哲学思辨到现代量子力学模型，人类对原子核结构和电子排布的探索塑造了我们对化学行为的诠释。本文系统地梳理IB化学课程中必须掌握的原子模型、关键实验证据与相关计算。
1. From Philosophical Atoms to Experimental Science | 从哲学原子走向实验科学
The concept of the atom dates back to ancient Greek thinkers such as Democritus, who suggested that matter could be divided only until a final indivisible particle was reached. However, these ideas were not based on experimental evidence. Modern atomic theory began with John Dalton in the early 19th century, who proposed that each element is made of identical atoms and that atoms combine in fixed ratios to form compounds.
原子的概念可追溯至古希腊思想家德谟克利特，他认为物质最终可被分割到不可再分的粒子。然而，这些想法缺乏实验依据。现代原子理论始于19世纪初的道尔顿，他提出每种元素由相同原子构成，原子以固定比例结合形成化合物。
Dalton’s model treated atoms as solid, indivisible spheres. Later experiments involving electricity and radioactivity revealed that atoms have internal structure, leading to the discovery of electrons, protons and neutrons. These subatomic particles are not only central to atomic models but also determine the identity and stability of every element.
道尔顿模型将原子视为不可再分的实心球体。随后关于电和放射性的实验揭示原子具有内部结构，最终导致电子、质子与中子的发现。这些亚原子粒子不仅是原子模型的核心，也决定每种元素的身份与稳定性。
2. Properties of Subatomic Particles | 亚原子粒子的性质
An atom is composed of three fundamental subatomic particles: protons, neutrons and electrons. Protons and neutrons are located in the nucleus and are collectively called nucleons. Electrons occupy the space around the nucleus. The relative charges and masses of these particles are essential data for solving many IB problems.
原子由三类基本亚原子粒子组成：质子、中子和电子。质子和中子位于原子核中，合称核子；电子占据原子核周围的空间。这些粒子的相对电荷和相对质量是解决许多IB问题的关键数据。
| Particle | 粒子 | Relative Charge | 相对电荷 | Relative Mass | 相对质量 | Location | 位置 |
|---|---|---|---|
| Proton | 质子 | 1+ | 1 | Nucleus | 原子核 |
| Neutron | 中子 | 0 | 1 | Nucleus | 原子核 |
| Electron | 电子 | 1- | 1/1840 | Electron cloud | 电子云 |
The exact charge of an electron is -1.602 × 10⁻¹⁹ C, and the proton has an equal but opposite charge. Such small magnitudes explain why in chemistry we usually use relative charges. The mass of an electron is negligible compared to that of nucleons, which is why atomic mass is almost entirely contributed by protons and neutrons.
电子的精确电荷为-1.602 × 10⁻¹⁹ C，质子具有等量异号电荷。由于电荷数值极小，化学中通常使用相对电荷。电子的质量与核子相比可忽略不计，因此原子质量几乎完全由质子与中子贡献。
3. Atomic Number, Mass Number and Isotopes | 原子序数、质量数与同位素
The atomic number (Z) is the number of protons in the nucleus of an atom. In a neutral atom, the number of electrons also equals Z. The mass number (A) is the total number of protons and neutrons in the nucleus. The notation for a nuclide uses a superscript for the mass number and a subscript for the atomic number, for example ²⁴₁₂Mg.
原子序数(Z)是原子核内的质子数。在电中性原子中，电子数也等于Z。质量数(A)是核内质子数与中子数之和。核素符号左侧上标为质量数，左下角为原子序数，例如²⁴₁₂Mg。
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Therefore, they have the same atomic number but different mass numbers. For instance, carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) are both carbon atoms, but they contain six and eight neutrons respectively.
同位素是同一元素中质子数相同而中子数不同的原子。因此，它们具有相同的原子序数但不同的质量数。例如碳-12(¹²₆C)和碳-14(¹⁴₆C)都是碳原子，但分别含6个和8个中子。
Isotopes of an element display almost identical chemical behaviour because chemical properties are governed by electron configuration. However, their physical properties, such as density and rate of diffusion, can differ due to mass differences.
元素的不同同位素表现出几乎相同的化学行为，因为化学性质由电子排布决定。但物理性质如密度和扩散速率可能因质量差异而不同。
4. The Plum Pudding Model: J.J. Thomson | 葡萄干布丁模型：汤姆逊
In 1897, J.J. Thomson discovered the electron through experiments with cathode rays. He observed that these rays were deflected by electric and magnetic fields in a way that indicated they consisted of negatively charged particles. Since atoms are electrically neutral, Thomson proposed that they must also contain positive charge.
1897年，汤姆逊通过阴极射线实验发现了电子。他观察到这些射线在电场和磁场中偏转，说明它们由带负电的粒子组成。由于原子整体电中性，汤姆逊提出原子中必定还存在正电荷。
Thomson’s “plum pudding” model described the atom as a sphere of positive charge with negatively charged electrons embedded within it, like raisins in a pudding. This model could explain the neutrality of atoms and the existence of electrons, but it gave no insight into how positive charge was distributed or how electrons moved.
汤姆逊的“葡萄干布丁”模型将原子描述为带正电荷的球体，其中嵌有带负电的电子，如同布丁中的葡萄干。该模型解释了原子的电中性和电子的存在，但未说明正电荷如何分布或电子如何运动。
The model was soon challenged by experiments involving alpha particle scattering, which required a completely different arrangement of mass and charge inside the atom.
该模型很快受到α粒子散射实验的挑战，这些实验要求对原子内部的质量与电荷分布作出完全不同的解释。
5. Rutherford’s Nuclear Model and the Gold Foil Experiment | 卢瑟福核模型与金箔实验
In 1909, Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden performed the famous gold foil experiment. They directed a narrow beam of alpha particles (helium nuclei, charge 2+) at a very thin sheet of gold foil and observed the scattering of these particles on a fluorescent screen.
1909年，卢瑟福与同事盖革和马斯登进行了著名的金箔实验。他们将一束狭窄的α粒子(氦原子核，带2+电荷)射向极薄的金箔，并在荧光屏上观察这些粒子的散射情况。
Most alpha particles passed straight through the foil, but a small number were deflected at large angles, and about 1 in 8000 rebounded back. This was completely unexpected under the plum pudding model, which predicted that the spread-out positive charge would cause only small deflections.
大多数α粒子径直穿透金箔，但少数粒子以大角度偏转，约8000个粒子中有1个反弹回来。这在葡萄干布丁模型下是完全意外的，因为该模型预测弥散的正电荷只会引起小角度偏转。
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Most alpha particles passed through undeflected, indicating that the atom is mostly empty space.
大多数α粒子未偏转通过，说明原子内部大部分是空间。
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Some alpha particles were deflected, suggesting a concentrated positive charge that repels the positive alpha particles.
部分α粒子发生偏转，表明存在集中的正电荷，能排斥带正电的α粒子。
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A very few alpha particles bounced back, implying a very small, dense and massive nucleus at the centre.
极少数α粒子反弹，说明原子中心存在一个体积很小、密度极大且质量集中的原子核。
Rutherford therefore proposed the nuclear model: an atom consists of a tiny, dense, positively charged nucleus containing virtually all the mass, surrounded by electrons moving through mostly empty space. This model is a cornerstone of modern atomic theory.
因此卢瑟福提出核模型：原子由微小、致密、带正电的原子核构成，核内几乎集中了全部质量，电子围绕核在绝大部分空虚的空间中运动。该模型是现代原子理论的基石。
6. Bohr’s Model of the Hydrogen Atom | 玻尔的氢原子模型
Rutherford’s model could not explain why electrons do not spiral into the nucleus or why atoms emit distinct line spectra. In 1913, Niels Bohr proposed a quantised model for hydrogen: electrons move in fixed circular orbits called energy levels, each with a specific energy. An electron can move between levels only by absorbing or emitting a photon of energy exactly equal to the energy difference.
卢瑟福模型无法解释电子为何不落入原子核，也无法解释原子为何产生分明线状光谱。1913年，玻尔为氢原子提出量子化模型：电子在称为能级的固定圆形轨道上运动，每个能级具有特定能量。电子只能通过吸收或发射能量差的光子来跃迁。
ΔE = hν = hc / λ
Here, ΔE is the energy difference between two levels, h is Planck’s constant, ν is the photon frequency, c is the speed of light, and λ is the photon wavelength. The hydrogen emission spectrum consists of a series of sharp lines because only certain photon energies are allowed by quantised energy levels.
其中ΔE是两个能级间的能量差，h为普朗克常数，ν为光子频率，c为光速，λ为光子波长。氢原子发射光谱由一系列锐线组成，因为量化的能级只允许特定能量的光子存在。
The Bohr model successfully predicted the wavelengths of hydrogen spectral lines, especially in the visible region. However, it failed for multi-electron atoms and could not explain the splitting of spectral lines in magnetic fields. This limitation motivated the development of the quantum mechanical model.
玻尔模型成功预测了氢原子谱线波长，特别是可见光区。但它无法适用于多电子原子，也无法解释磁场中谱线的分裂。这一局限推动了量子力学模型的发展。
7. Wave-Particle Duality and the Quantum Mechanical Model | 波粒二象性与量子力学模型
In 1924, Louis de Broglie proposed that all matter has wave-like properties, with wavelength given by:
1924年，德布罗意提出所有物质都具有波动性，其波长关系为：
λ = h / (mv)
where m is mass and v is velocity. For macroscopic objects, the wavelength is so tiny it is undetectable, but for an electron, the wavelength is comparable to atomic dimensions. This wave-like behaviour explains why electrons occupy stable, standing-wave orbits rather than collapsing into the nucleus.
其中m为质量，v为速度。宏观物体的波长小到无法检测，但电子的波长与原子的尺寸相近。这种波动性解释了电子为何以驻波形式占据稳定轨道，而不是掉入原子核。
Werner Heisenberg later formulated the uncertainty principle, which states that it is impossible to know simultaneously the exact position and exact momentum of an electron:
海森堡随后提出不确定性原理，指出无法同时精确知道电子的位置和动量：
Δx · Δp ≥ h / (4π)
This principle means that we cannot describe electrons as following defined paths. Instead, we use probability distributions called atomic orbitals. Erwin Schrödinger developed the wave equation that describes these orbitals; the square of the wave function relates to the probability of finding an electron in a given region.
这一原理意味着我们不能将电子描述为沿确定路径运动。相反，我们使用称为原子轨道的概率分布。薛定谔建立了描述这些轨道的波动方程；波函数的平方与在特定区域找到电子的概率相关。
8. Atomic Orbitals and Quantum Numbers | 原子轨道与量子数
In the quantum mechanical model, each electron in an atom is described by four quantum numbers. The principal quantum number (n) indicates the main energy level and average distance from the nucleus. The azimuthal quantum number (l) defines the shape of the orbital, and the magnetic quantum number (mₗ) defines its orientation in space. The spin quantum number (mₛ) describes whether an electron spins up or down.
在量子力学模型中，每个电子由四个量子数描述。主量子数(n)表示主能级和离核的平均距离；角量子数(l)决定轨道形状；磁量子数(mₗ)决定轨道在空间的方向；自旋量子数(mₛ)描述电子自旋向上或向下。
| Orbital | 轨道 | Shape | 形状 | Maximum Electrons | 最多电子数 |
|---|---|---|
| s | Spherical | 球形 | 2 |
| p | Dumbbell | 哑铃形 | 6 |
| d | Cloverleaf / complex | 三叶草形/复杂形 | 10 |
In the first energy level (n = 1), there is only one s orbital. In the second level (n = 2), there are one s and three p orbitals. The third level contains one s, three p and five d orbitals. This ordering explains the aufbau principle, Hund’s rule and the Pauli exclusion principle, which are essential for writing electron configurations in IB Chemistry.
第一能级(n=1)只有一个s轨道。第二能级(n=2)含一个s轨道和三个p轨道。第三能级含一个s、三个p和五个d轨道。这种排布解释了构造原理、洪特规则和泡利不相容原理，这些对书写IB化学中的电子构型至关重要。
9. Structure and Stability of the Nucleus | 原子核的结构与稳定性
The nucleus contains protons and neutrons, collectively known as nucleons. Because protons repel one another electrostatically, an additional attractive force is required to hold the nucleus together. This short-range force, called the strong nuclear force, acts between all nucleons and is much stronger than electrostatic repulsion at nuclear distances.
原子核由质子和中子组成，统称为核子。由于质子间存在静电排斥力，必须有额外的吸引力将核子束缚在一起。这种短程力称为强核力，作用于所有核子之间，在核尺度上远强于静电排斥力。
As atomic number increases, the number of neutrons needed to provide stability grows faster than the number of protons, because neutrons add attractive nucleons without increasing electrostatic repulsion. Stable nuclei therefore tend to have a neutron-to-proton ratio greater than 1 for heavy elements.
随着原子序数增加，维持稳定所需的中子数增长速度快于质子数，因为中子只增加核力吸引力而不增加静电排斥。因此重元素的稳定核通常具有大于1的中子/质子比。
If a nucleus is unstable, it undergoes radioactive decay to become more stable. The three common types of radiation are alpha (α, helium nucleus), beta (β, electron or positron) and gamma (γ, high-energy electromagnetic radiation). Alpha decay reduces the mass number by 4 and the atomic number by 2; beta decay transforms a neutron into a proton, or a proton into a neutron.
若原子核不稳定，会通过放射性衰变趋于稳定。常见辐射有三种：α(氦原子核)、β(电子或正电子)和γ(高能电磁辐射)。α衰变使质量数减4、原子序数减2；β衰变将中子转变为质子，或将质子转变为中子。
10. Mass Spectrometry and Isotopic Abundance | 质谱法与同位素丰度
Mass spectrometry is a key experimental technique used to determine atomic masses and isotopic abundances. In a mass spectrometer, a vaporised sample is ionised to form positive ions. These ions are accelerated by an electric field and then deflected by a magnetic field. The amount of deflection depends on the mass-to-charge ratio (m/z) of each ion.
质谱法是测定原子质量与同位素丰度的关键实验技术。在质谱仪中，气化样品被电离形成正离子。离子经电场加速后，在磁场中发生偏转。偏转程度取决于离子的质荷比(m/z)。
Lighter ions are deflected more than heavier ions, so ions with different masses arrive at different positions on a detector. The resulting mass spectrum shows peaks corresponding to each isotope. The height or area of each peak indicates the relative abundance of each isotope.
较轻离子比重离子偏转更多，不同质量的离子到达检测器的不同位置。所得质谱图显示对应各同位素的峰，峰高或峰面积表示各同位素的相对丰度。
From this data, the relative atomic mass (Aᵣ) can be calculated as a weighted average:
依据这些数据，相对原子质量(Aᵣ)可按加权平均计算：
Aᵣ = Σ (isotopic mass × fractional abundance)
For example, chlorine has two stable isotopes: ³⁵Cl with 75.77% abundance and ³⁷Cl with 24.23% abundance. Using these values, the relative atomic mass of chlorine is about 35.5. This value appears on the periodic table and explains why chlorine’s atomic mass is not a whole number.
例如，氯有两种稳定同位素：³⁵Cl丰度75.77%，³⁷Cl丰度24.23%。用这些数值计算，氯的相对原子质量约为35.5。该值出现在元素周期表中，也解释了氯原子质量为何不是整数。
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