📚 Patterns in Ionisation Energies in the Periodic Table | 元素周期表中电离能的规律
First ionisation energies provide direct experimental evidence for electron arrangement in atoms. They show periodic repeating patterns across the Periodic Table, with general rises along periods and falls down groups, interrupted by important anomalies that reveal sub-shell structure. Understanding these patterns is essential for A-level Chemistry.
第一电离能提供了原子中电子排布的直接实验证据。它们在周期表中呈现周期性重复的规律，沿周期总体升高、沿族向下降低，并被揭示亚壳层结构的重要异常所打断。理解这些规律对 A-level 化学至关重要。
1. Defining Ionisation Energy | 电离能的定义
The first ionisation energy, usually written as I₁, is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous singly charged positive ions. It is measured in kJ mol⁻¹.
第一电离能通常写作 I₁，是指从一摩尔气态原子中移走一摩尔电子，形成一摩尔气态带一个正电荷的离子所需的能量。它以 kJ mol⁻¹ 为单位。
X(g) → X⁺(g) + e⁻
For sodium, the process is Na(g) → Na⁺(g) + e⁻. Ionisation energies are always endothermic because energy must be supplied to overcome the electrostatic attraction between the nucleus and the electron being removed.
对钠而言，该过程为 Na(g) → Na⁺(g) + e⁻。电离能总是吸热的，因为必须提供能量来克服原子核与被移走电子之间的静电吸引。
2. Key Factors Controlling Ionisation Energy | 影响电离能的主要因素
The magnitude of an ionisation energy depends mainly on three factors: nuclear charge, distance of the outer electron from the nucleus, and shielding by inner-shell electrons. The actual attraction experienced by the outer electron is often described by the effective nuclear charge.
电离能大小主要取决于三个因素：核电荷、外层电子与原子核的距离以及内层电子的屏蔽作用。外层电子实际感受到的吸引力通常用有效核电荷来描述。
- Higher nuclear charge increases attraction and therefore raises ionisation energy.
- 核电荷越大，吸引力越强，因此电离能越高。
- Greater distance between the outer electron and the nucleus weakens attraction and lowers ionisation energy.
- 外层电子与原子核的距离越大，吸引力越弱，电离能越低。
- More inner-shell shielding reduces the effective nuclear charge felt by the outer electron and lowers ionisation energy.
- 内层屏蔽越多，外层电子感受到的有效核电荷越小，电离能越低。
When comparing two atoms, all three factors must be considered together. An increase in nuclear charge alone does not automatically mean a higher ionisation energy if shielding or distance also changes significantly.
比较两个原子时，必须同时考虑这三个因素。如果屏蔽或距离也发生显著变化，单凭核电荷增加并不能自动推出电离能升高。
3. First Ionisation Energy Across Period 2 | 第二周期第一电离能的横向变化
Across Period 2 from lithium to neon, the first ionisation energy generally increases. Nuclear charge increases from +3 to +10, while the inner 1s² shell provides roughly constant shielding. The outer electrons are added to the same n = 2 shell, so atomic radius decreases and the outer electron is held more tightly.
第二周期从锂到氖，第一电离能总体升高。核电荷从 +3 增加到 +10，而内层 1s² 的屏蔽大致不变。外层电子都进入同一个 n = 2 壳层，因此原子半径减小，外层电子被更牢固地吸引。
| Element | Electronic configuration | First ionisation energy / kJ mol⁻¹ |
|---|---|---|
| Li | 1s² 2s¹ | 519 |
| Be | 1s² 2s² | 900 |
| B | 1s² 2s² 2p¹ | 799 |
| C | 1s² 2s² 2p² | 1086 |
| N | 1s² 2s² 2p³ | 1402 |
| O | 1s² 2s² 2p⁴ | 1314 |
| F | 1s² 2s² 2p⁵ | 1681 |
| Ne | 1s² 2s² 2p⁶ | 2081 |
The data show a clear upward trend from Li to Ne, but there are two important falls: from Be to B and from N to O. These falls are not exceptions to quantum theory; they provide evidence for sub-shell structure and electron pairing.
数据表明从 Li 到 Ne 有明显的上升趋势，但存在两处重要下降：从 Be 到 B 以及从 N 到 O。这些下降并不是量子理论的例外，而是为亚壳层结构和电子成对提供了证据。
4. First Ionisation Energy Across Period 3 | 第三周期第一电离能的横向变化
Period 3 repeats the same general pattern because electrons are added to the n = 3 shell while the inner shells shield the outer electron. The first ionisation energy rises from Na to Ar, with falls from Mg to Al and P to S.
第三周期重复相同的总体规律，因为电子添加到 n = 3 壳层，而内层电子屏蔽外层电子。第一电离能从 Na 到 Ar 升高，但从 Mg 到 Al、P 到 S 存在下降。
| Element | Electronic configuration | First ionisation energy / kJ mol⁻¹ |
|---|---|---|
| Na | 1s² 2s² 2p⁶ 3s¹ | 496 |
| Mg | 1s² 2s² 2p⁶ 3s² | 738 |
| Al | 1s² 2s² 2p⁶ 3s² 3p¹ | 577 |
| Si | 1s² 2s² 2p⁶ 3s² 3p² | 786 |
| P | 1s² 2s² 2p⁶ 3s² 3p³ | 1012 |
| S | 1s² 2s² 2p⁶ 3s² 3p⁴ | 1000 |
| Cl | 1s² 2s² 2p⁶ 3s² 3p⁵ | 1251 |
| Ar | 1s² 2s² 2p⁶ 3s² 3p⁶ | 1521
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