Patterns in Ionisation Energies in the Periodic Table | 周期表中的电离能模式

📚 Patterns in Ionisation Energies in the Periodic Table | 周期表中的电离能模式

Ionisation energy is a fundamental concept in A-Level Chemistry that helps explain the reactivity and electronic structure of elements. Understanding how ionisation energies vary across periods and down groups provides crucial insights into electron configurations and the periodic trends governed by atomic structure. This article explores the patterns, underlying reasons, and exceptions in ionisation energies across the Periodic Table.

电离能是A-Level化学中的一个基本概念，有助于解释元素的反应活性和电子结构。理解电离能在同一周期和同一族中的变化规律，可以深入了解电子排布以及由原子结构决定的周期性趋势。本文将探讨周期表中电离能的模式、根本原因及异常情况。

1. Introduction to Ionisation Energy | 电离能简介

The first ionisation energy (IE₁) is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms to produce one mole of gaseous ions with a single positive charge. The equation is: X(g) → X⁺(g) + e⁻. Ionisation energies are always endothermic (positive values) because energy must be supplied to overcome the electrostatic attraction between the nucleus and the electron being removed. They are measured in kJ mol⁻¹ under standard conditions.

第一电离能（IE₁）的定义是：从一摩尔气态原子中移走一摩尔电子，生成一摩尔带一个正电荷的气态离子所需的能量。方程式为：X(g) → X⁺(g) + e⁻。电离能总是吸热的（正值），因为必须提供能量来克服原子核与被移除电子之间的静电引力。电离能在标准条件下以 kJ mol⁻¹ 为单位进行测量。

2. Factors Influencing Ionisation Energy | 影响电离能的因素

Three main factors determine the magnitude of ionisation energy: nuclear charge, distance of the outermost electron from the nucleus (atomic radius), and shielding by inner electrons. Greater nuclear charge increases attraction, raising IE. Larger atomic radius reduces attraction, lowering IE. More inner-shell shielding also reduces the effective nuclear charge experienced by the outer electron, thus lowering IE.

决定电离能大小的三个主要因素是：核电荷、最外层电子与原子核的距离（原子半径）和内层电子的屏蔽作用。核电荷越大，吸引力越强，电离能越高。原子半径越大，吸引力越弱，电离能越低。内层电子屏蔽越多，外层电子感受到的有效核电荷越小，因此电离能越低。

3. General Trend Across a Period | 同一周期内的总体趋势

As you move from left to right across a period (e.g., from sodium to argon in Period 3), the first ionisation energy generally increases. This is because the nuclear charge increases, while electrons are added to the same outer shell, so shielding remains similar. The outer electrons are pulled closer to the nucleus, reducing atomic radius and increasing the energy required to remove an electron.

在同一周期中从左向右移动时（例如从第三周期的钠到氩），第一电离能总体呈现增加趋势。这是因为核电荷增加，而电子被添加到同一外层，因此屏蔽效应相似。外层电子被更紧地吸引向原子核，原子半径减小，移走电子所需的能量增加。

4. Anomalies in Period 2: Be vs B and N vs O | 第二周期中的异常：Be 与 B 及 N 与 O

There are two noticeable dips in the first ionisation energy trend across Period 2. Between beryllium (Be: 1s²2s²) and boron (B: 1s²2s²2p¹), IE₁ drops because the electron removed from boron comes from a 2p orbital, which is higher in energy and slightly more shielded than the 2s orbital in Be. The second dip occurs between nitrogen (N: 1s²2s²2p³) and oxygen (O: 1s²2s²2p⁴). Nitrogen has a half-filled 2p subshell with extra stability, so its IE₁ is higher. In oxygen, one 2p orbital must contain a pair of electrons, resulting in electron-electron repulsion that makes it easier to remove an electron, thus lowering IE₁.

第二周期的第一电离能趋势中有两个明显的下降。在铍（Be: 1s²2s²）和硼（B: 1s²2s²2p¹）之间，IE₁ 下降，因为从硼中移走的电子来自 2p 轨道，该轨道的能量比 Be 中的 2s 轨道更高，并受到稍多的屏蔽。第二个下降出现在氮（N: 1s²2s²2p³）和氧（O: 1s²2s²2p⁴）之间。氮具有半充满的 2p 亚层，具有额外的稳定性，因此其 IE₁ 较高。在氧中，一个 2p 轨道必须容纳一对电子，产生电子-电子排斥，使移走一个电子更容易，从而降低了 IE₁。

5. Anomalies in Period 3: Mg vs Al and P vs S | 第三周期中的异常：Mg 与 Al 及 P 与 S

Similar anomalies appear in Period 3. Magnesium (Mg: 1s²2s²2p⁶3s²) has a higher IE₁ than aluminium (Al: 1s²2s²2p⁶3s²3p¹) because the electron removed from Al is in a higher-energy 3p orbital, which is easier to remove than an electron from the 3s orbital of Mg. Between phosphorus (P: …3s²3p³) and sulfur (S: …3s²3p⁴), IE₁ drops. Phosphorus has a stable half-filled 3p subshell, whereas sulfur has a paired electron in one of the 3p orbitals, causing repulsion and a lower ionisation energy.

第三周期也出现了类似的异常现象。镁（Mg: 1s²2s²2p⁶3s²）的第一电离能高于铝（Al: 1s²2s²2p⁶3s²3p¹），因为从 Al 中移走的电子处于能量更高的 3p 轨道，比从 Mg 的 3s 轨道移走电子更容易。在磷（P: …3s²3p³）和硫（S: …3s²3p⁴）之间，IE₁ 下降。磷具有稳定的半充满 3p 亚层，而硫在其中一个 3p 轨道上有一对电子，电子之间的排斥导致电离能降低。

6. Trend Down a Group | 同一族内的趋势

Down any group, the first ionisation energy decreases. Although nuclear charge increases, the outer electrons are in shells further from the nucleus, so atomic radius increases significantly. In addition, there are more inner shells of electrons, which increase shielding. This results in a weaker attraction between the nucleus and the outermost electron, making it easier to remove and lowering IE₁. For example, IE₁ decreases from lithium to potassium.

在任一族中向下移动时，第一电离能降低。尽管核电荷增加，但外层电子位于离核更远的电子层上，因此原子半径显著增大。此外，内层电子壳层更多，屏蔽效应增强。这导致原子核与最外层电子之间的吸引力减弱，使电子更容易被移走，从而降低了 IE₁。例如，从锂到钾，第一电离能依次降低。

7. Second and Successive Ionisation Energies | 第二电离能及后续电离能

Second ionisation energy (IE₂) is the energy to remove an electron from a singly charged gaseous ion: X⁺(g) → X²⁺(g) + e⁻. Successive ionisation energies for an element always increase because after each electron removal, the ion becomes more positively charged, the remaining electrons are held more tightly, and the ionic radius decreases. For example, for magnesium, IE₁ = 738, IE₂ = 1451, IE₃ = 7733 kJ mol⁻¹. The huge jump after removing the two outer electrons reflects moving to an inner principal quantum shell.

第二电离能（IE₂）是从带一个正电荷的气态离子中移走一个电子所需的能量：X⁺(g) → X²⁺(g) + e⁻。同一元素的各级电离能总是递增的，因为每移走一个电子，离子的正电荷增加，剩余电子被束缚得更紧，并且离子半径减小。例如，镁的电离能：IE₁ = 738, IE₂ = 1451, IE₃ = 7733 kJ mol⁻¹。在移走两个外层电子后出现巨大跃升，这反映了移除内层主量子壳层的电子。

8. Evidence for Electron Shells from Successive Ionisation Energies | 从连续电离能得到电子层的证据

Plotting successive ionisation energies for an element provides evidence for the existence of principal quantum shells. Large jumps in the graph occur when an electron is removed from a shell closer to the nucleus. For instance, consider sodium (1s²2s²2p⁶3s¹). The first IE is low (removing 3s¹). The second IE is much higher because it removes an electron from the 2p⁶ sub‑shell. A similar huge jump occurs for magnesium after the second ionisation, indicating that the third electron comes from the 2p level. These jumps align with the electron configurations and confirm the shell model.

绘制某一元素的连续电离能图可以为原子主量子壳层的存在提供证据。当从更靠近原子核的壳层移走电子时，图上会出现大幅跳跃。例如钠（1s²2s²2p⁶3s¹）。第一电离能较低（移

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