A-Level Chemistry: Periodic Trends in Ionisation Energy | A-Level化学：电离能在周期表中的递变规律

📚 A-Level Chemistry: Periodic Trends in Ionisation Energy | A-Level化学：电离能在周期表中的递变规律

Ionisation energy is one of the most important periodic properties in A-Level chemistry. Understanding how and why it changes across the periodic table allows you to predict atomic structure, bonding and reactivity.

电离能是A-Level化学中最重要的周期性性质之一。理解它在周期表中如何变化以及为何变化，能帮助你预测原子结构、成键方式和反应活性。

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1. Definition of Ionisation Energy | 电离能的定义

The first ionisation energy (IE₁) is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms in the ground state, forming one mole of gaseous unipositive ions.

第一电离能（IE₁）是指从一摩尔基态气态原子中移走一摩尔电子，生成一摩尔气态一价正离子所需的最低能量。

For an atom M, the process is written as:

对于原子M，该过程可写为：

M(g) → M⁺(g) + e⁻

The second ionisation energy (IE₂) is the energy required to remove one mole of electrons from one mole of gaseous M⁺ ions:

第二电离能（IE₂）是指从一摩尔气态M⁺离子中移走一摩尔电子所需的能量：

M⁺(g) → M²⁺(g) + e⁻

All ionisation energies are positive because energy must be supplied to overcome the electrostatic attraction between the electron and the nucleus.

所有电离能均为正值，因为必须提供能量以克服电子与原子核之间的静电吸引。

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

The magnitude of an ionisation energy depends on four main factors. The first is nuclear charge: the greater the number of protons in the nucleus, the stronger the attraction for the outer electron, and the higher the ionisation energy.

电离能的大小主要取决于四个因素。第一是核电荷：核内质子数越多，对外层电子的吸引越强，电离能越高。

The second factor is atomic radius. If the outermost electron is farther from the nucleus, the electrostatic attraction is weaker, so the ionisation energy is lower.

第二个因素是原子半径。最外层电子离核越远，静电吸引越弱，电离能越低。

The third factor is electron shielding. Inner-shell electrons repel the outer electron and partly cancel the pull of the nucleus, reducing the effective nuclear charge experienced by the outer electron. More shielding means lower ionisation energy.

第三个因素是电子屏蔽。内层电子排斥外层电子，部分抵消原子核的吸引力，从而降低外层电子感受到的有效核电荷。屏蔽越强，电离能越低。

The fourth factor is spin-pair repulsion. When an electron is removed from an orbital that already contains two electrons, the repulsion between the paired electrons slightly reduces the energy needed to remove one of them.

第四个因素是自旋配对排斥。当要从一个已经有两个电子的轨道中移走一个电子时，配对电子之间的斥力会略微降低移走其中一个所需能量。

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3. General Trend Across a Period | 同周期的总体趋势

Across a period from left to right, protons are added to the nucleus, and electrons are added to the same outer shell. The nuclear charge increases, the atomic radius decreases, and shielding from inner electrons remains roughly constant.

同一周期从左到右，原子核增加质子，电子也加到同一外壳层。核电荷增大，原子半径减小，而内层电子的屏蔽效应大致不变。

As a result, the effective nuclear charge on the outer electrons increases. The outer electrons are pulled more strongly toward the nucleus, so the first ionisation energy generally increases across each period.

因此，外层电子感受到的有效核电荷增大。

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

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