📚 Experimental Evidence for Electronic Structure in A-Level Chemistry | A-Level化学：电子结构的实验证据
The modern model of the atom describes electrons as occupying quantised energy levels, subshells and orbitals. This model is not purely theoretical: it is supported by a range of experimental observations. In this article, we examine the key experimental evidence that led chemists to accept the current electronic structure, focusing on emission spectra, ionisation energies and related phenomena.
现代原子模型将电子描述为占据量子化的能级、亚层和轨道。这一模型并非纯理论，而是由一系列实验观察所支持。在本文中，我们将考查导致化学家接受当前电子结构的关键实验证据，重点关注发射光谱、电离能及相关现象。
1. Atomic Emission Spectra: Line Spectra | 原子发射光谱：线状光谱
When a gaseous element is excited by heat or an electric discharge, it emits light. Passing this light through a prism or diffraction grating produces a spectrum. Unlike a continuous rainbow, the atomic emission spectrum consists of discrete, sharp lines of specific wavelengths. Each line corresponds to a specific energy change as an excited electron falls from a higher energy level to a lower one.
当气态元素通过加热或放电被激发时，它会发出光。让这束光通过棱镜或衍射光栅便得到光谱。与连续的彩虹不同，原子发射光谱由一系列离散、尖锐的特定波长谱线组成。每一条谱线对应着一个激发电子从较高能级跃迁到较低能级时发生的特定能量变化。
The simplest emission spectrum is that of hydrogen. The lines are grouped into series, such as the Lyman series (ultraviolet), Balmer series (visible) and Paschen series (infrared). The existence of only certain wavelengths shows that only certain energy changes are allowed, which implies quantised energy levels.
最简单的发射光谱是氢原子光谱。谱线分为若干线系，例如莱曼系（紫外）、巴尔末系（可见）和帕邢系（红外）。只存在某些特定波长表明只有某些能量变化是允许的，这隐含了能级的量子化。
2. The Balmer Series and the Rydberg Equation | 巴尔末系与里德堡方程
In 1885, Johann Balmer found that the wavelengths of the visible hydrogen lines could be described by a simple empirical equation. Later, the general form became known as the Rydberg equation:
1885年，约翰·巴尔末发现氢原子可见光谱线的波长可以用一个简单的经验公式描述。后来，其一般形式被称为里德堡方程：
1/λ = Rₕ (1/n₁² − 1/n₂²)
Here λ is the wavelength, Rₕ is the Rydberg constant for hydrogen, and n₁ and n₂ are positive integers with n₂ > n₁. The fact that n₂ takes only whole-number values provides strong evidence for discrete energy levels. A single electron moving between quantised orbits (or energy levels) produces exactly the observed line spectrum.
此处 λ 是波长，Rₕ 是氢的里德堡常数，n₁ 和 n₂ 是正整数且 n₂ > n₁。n₂ 只能取整数值这一事实为分立能级的存在提供了强有力的证据。单个电子在量子化轨道（即能级）之间跃迁正好产生观测到的线状光谱。
3. Continuous Spectra versus Line Spectra | 连续光谱与线状光谱的对比
A continuous spectrum, such as white light from a hot solid, contains all wavelengths without gaps. In contrast, an atomic line spectrum contains only specific wavelengths. The difference is crucial: continuous spectra arise from unquantised energy changes, while line spectra arise from transitions between fixed energy levels.
连续光谱，例如炽热固体发出的白光，包含所有波长而无间隔。相比之下，原子线状光谱只包含特定波长。这一差异至关重要：连续光谱来自非量子化的能量变化，而线状光谱来自固定能级之间的跃迁。
- Continuous spectrum: all wavelengths present; produced by hot solids or liquids.
- Line spectrum: only certain wavelengths present; produced by excited atoms in the gas phase.
- Each element has a unique line spectrum, acting as a “fingerprint” for identification.
- 连续光谱：所有波长都存在；由炽热固体或液体产生。
- 线状光谱：只存在某些波长；由气相中受激发的原子产生。
- 每种元素都有独特的线状光谱，如同用于鉴别的”指纹”。
4. Ionisation Energies as Evidence for Electron Shells | 电离能作为电子层的证据
The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. For a given element, successive ionisation energies can be measured: IE₁, IE₂, IE₃, etc. If an atom simply had freely moving electrons, these values would increase only gradually. Instead, experiments show abrupt jumps at certain points, which indicate that electrons are arranged in distinct shells.
第一电离能是指从一摩尔气态原子中移走一摩尔电子形成一摩尔气态 1+ 离子所需的能量。对于给定元素，可以测量逐级电离能：IE₁、IE₂、IE₃ 等。如果原子中电子只是自由运动，这些数值只会逐渐增大。然而，实验表明在特定位置会出现突然的跃升，这表明电子排列在不同的壳层中。
For example, the successive ionisation energies of sodium show a large jump between the first and second ionisation energy. This demonstrates that the first electron is much easier to remove than the others, consistent with one outer electron far from the nucleus, while the remaining electrons are in a tighter inner shell.
例如，钠的逐级电离能在第一与第二电离能之间出现一个巨大跃升。这表明第一个电子比其他电子容易移走得多，与一个远离原子核的外层电子以及其余电子位于更紧密的内壳层这一图像一致。
5. Sharp Jumps in Successive Ionisation Energies | 逐级电离能中的骤增
Consider the successive ionisation energies of an element. Removing electrons from the same shell requires increasing energy because the nuclear charge is unchanged while electron-electron repulsion decreases. However, once a whole shell has been removed, the next electron comes from an inner shell that is much closer to the nucleus and experiences a much larger effective nuclear charge. This causes a sudden, large increase in ionisation energy.
考虑一种元素的逐级电离能。从同一壳层移走电子需要越来越多的能量，因为核电荷不变而电子间排斥力减小。然而，一旦整个壳层被移走，下一个电子将来自更靠近原子核的内壳层，受到显著更大的有效核电荷作用。这导致电离能出现突然的、大幅的增大。
For carbon (1s² 2s² 2p²), the ionisation energies increase steadily from IE₁ to IE₄, then jump sharply at IE₅ because the fifth electron must be removed from the 1s shell. The number of electrons in the outer shell is thus revealed by counting the ionisation energies before the sharp jump.
对于碳（1s² 2s² 2p²），从 IE₁ 到 IE₄ 电离能平稳增大，然后在 IE₅ 处急剧跃升，因为第五个电子必须从 1s 壳层移走。通过计数骤升之前的电离能数目，就可以揭示外层电子数。
6. Periodic Trends in First Ionisation Energies | 第一电离能的周期性趋势
If we plot first ionisation energy against atomic number, a clear periodic pattern emerges. Values increase across a period from left to right, and decrease down a group. This pattern strongly supports the idea of principal quantum shells (energy levels) being filled in a regular order, with electrons in inner shells shielding the outer electrons from the full nuclear charge.
如果将第一电离能对原子序数作图，会呈现清晰的周期图案。同周期从左到右数值增大，同族从上到下数值减小。这种图案强有力地支持主量子壳层（能级）按规则顺序填充的观点，并且内层电子对外层电子屏蔽了部分核电荷。
The general increase across Period 2 (Li to Ne) reflects an increasing nuclear charge and similar shielding, so outer electrons are held more strongly. The sharp drop from Ne to Na corresponds to the start of a new shell at a much larger distance from the nucleus.
第二周期（Li 到 Ne）的总体增大反映了核电荷增加而屏蔽效应相近，因此外层电子被更牢固地吸引。从 Ne 到 Na 的急剧下降对应新壳层的开始，该壳层距原子核远得多。
7. Anomalous Trends: Be vs B and N vs O | 反常趋势：Be 与 B、N 与 O
The first ionisation energy of boron is slightly lower than that of beryllium. In beryllium, the outer electron is in a 2s orbital, while in boron it is in a 2p orbital. The 2p orbital is slightly higher in energy and more effectively shielded by the 2s electrons, so it is easier to remove. This provides evidence for the existence of subshells (s and p) within the same principal quantum level.
硼的第一电离能略低于铍。在铍中，外层电子处于 2s 轨道；而在硼中，外层电子处于 2p 轨道。2p 轨道能级略高，且被 2s 电子更有效地屏蔽，因此更容易移走。这为主量子层内存在亚层（s 和 p）提供了证据。
Similarly, nitrogen has a higher first ionisation energy than oxygen. Nitrogen has a half-filled 2p subshell (2p³), which is especially stable due to exchange energy. In oxygen, the fourth 2p electron must pair up in an already occupied orbital, and electron-electron repulsion makes it easier to remove. These subtle variations can only be understood with a model that includes orbitals and spin pairing.
类似地，氮的第一电离能高于氧。氮具有半充满的 2p 亚层（2p³），由于交换能而格外稳定。在氧中，第四个 2p 电子必须进入已经占据的轨道配对，电子间排斥力使其更容易被移走。这些细微变化只有通过包含轨道和自旋配对的模型才能理解。
8. Flame Tests and Electron Transitions | 焰色试验与电子跃迁
When a metal salt is placed in a Bunsen flame, characteristic colours are observed. The heat energy promotes electrons to higher energy levels. When these electrons return to lower levels, they emit photons of specific wavelengths. For example, lithium gives a crimson flame, sodium gives yellow, and copper gives blue-green.
当金属盐放入本生灯火焰中时，会观察到特征颜色。热能促使电子跃迁到较高能级。当这些电子返回较低能级时，会发射特定波长的光子。例如，锂呈深红色焰，钠呈黄色焰，铜呈蓝绿色焰。
These colours correspond to differences between energy levels. The fact that only certain colours are seen means only certain energy gaps exist. Flame tests therefore provide simple, visible evidence for quantised electron energy levels in atoms.
这些颜色对应于能级之间的差值。只能看到某些颜色，意味着只存在某些能隙。因此焰色试验为原子中量子化电子能级提供了简单、可见的证据。
9. Evidence from Photoelectron Spectroscopy | 光电子能谱的证据
In photoelectron spectroscopy (PES), X-rays or ultraviolet photons eject electrons from atoms, and the kinetic energies of these ejected electrons are measured. The binding energy of each electron can then be calculated. The spectrum shows distinct peaks for electrons in different subshells. For example, sodium shows a peak at a very high binding energy for 1s electrons, and a small peak at a very low binding energy for the 3s electron.
在光电子能谱（PES）中，用 X 射线或紫外光子将电子从原子中击出，并测量这些出射电子的动能，从而计算每个电子的结合能。谱图对不同亚层中的电子显示出清晰的峰。例如，钠在很高结合能处显示 1s 电子的峰，在很低结合能处显示 3s 电子的一个小峰。
The number of peaks in a PES spectrum tells us how many different energy levels or subshells are occupied. The relative peak areas reflect the number of electrons in each subshell. This technique offers direct experimental evidence for the arrangement of electrons into s, p and d subshells.
PES 谱中峰的数量告诉我们有多少不同的能级或亚层被占据。峰的相对面积反映每个亚层中的电子数。该技术为电子在 s、p 和 d 亚层中的排布提供了直接的实验证据。
10. Ionisation Energy Data and Effective Nuclear Charge | 电离能数据与有效核电荷
The magnitude of each ionisation energy depends on the effective nuclear charge (Z_eff) experienced by the electron being removed. Across a period, Z_eff increases because additional electrons enter the same shell and shielding by inner electrons remains almost constant. Therefore ionisation energy generally increases.
每个电离能的大小取决于被移走电子所感受到的有效核电荷（Z_eff）。同一周期中，Z_eff 增大，因为额外电子进入同一壳层而内层电子的屏蔽几乎不变，因此电离能通常增大。
Down a group, although nuclear charge increases, the outer electrons are further from the nucleus and are shielded by more inner electron shells. These two factors outweigh the increased nuclear charge, so the first ionisation energy decreases. Such trends are only measurable experimentally, and they allow us to infer the shell structure of the atom.
同族向下，虽然核电荷增大，但外层电子距离核更远且被更多内层电子壳层屏蔽。这两个因素超过了核电荷增大的影响，所以第一电离能减小。这些趋势只能通过实验测量获得，并使我们得以推断原子的壳层结构。
11. Summary: Connecting Evidence to the Model | 总结：将证据与模型联系起来
The quantum mechanical model of atomic structure did not arise from theory alone. It was developed alongside experimental observations. Line spectra show that electrons occupy quantised energy levels. Sharp jumps in successive ionisation energies reveal principal shells. Small irregularities in ionisation energies reveal subshells and orbital stability. Flame tests and photoelectron spectroscopy further confirm the existence of discrete energy levels.
原子结构的量子力学模型并非仅从理论产生，而是与实验观察同步发展。线状光谱表明电子占据量子化能级。逐级电离能的骤升揭示主壳层。电离能上的细微不规则揭示亚层和轨道稳定性。焰色试验和光电子能谱进一步证实分立能级的存在。
For A-Level examination purposes, you should be able to explain how each piece of evidence supports the modern model, interpret ionisation energy graphs, and identify anomalies such as Be/B and N/O. Remember to use precise language: “quantised energy levels”, “subshell”, “effective nuclear charge” and “electron shielding”.
在 A-Level 考试中，你应该能够解释每项证据如何支持现代模型，解读电离能图表，并识别 Be/B 和 N/O 等反常现象。记住使用精确术语：”量子化能级”、”亚层”、”有效核电荷”和”电子屏蔽”。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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