Ionisation Energy and Periodicity | 电离能与周期性

📚 Ionisation Energy and Periodicity | 电离能与周期性

Ionisation energy is one of the most powerful diagnostic tools in Edexcel A-Level Chemistry. It explains why elements behave the way they do and gives direct evidence for the existence of electron shells and subshells. In this article, we will unpack the definition, trends, exceptions and exam techniques needed to master this topic.

电离能是 Edexcel A-Level 化学中最具诊断价值的概念之一。它可以解释元素为何表现出特定的化学行为，并为电子层和电子亚层的存在提供直接证据。本文将详细解析电离能的定义、变化趋势、例外情况以及考试答题技巧，帮助你彻底掌握这一主题。

1. Defining First Ionisation Energy | 第一电离能的定义

The first ionisation energy of an element is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.

第一电离能是指从一摩尔气态原子中移除一摩尔电子，形成一摩尔带一个单位正电荷的气态离子所需的能量。

The equation for the first ionisation energy of sodium is written as follows.

钠的第一电离能方程式如下。

Na(g) → Na⁺(g) + e⁻

It is essential to remember that all species must be in the gas phase, and the value is expressed in kilojoules per mole (kJ mol⁻¹). Ionisation always requires an input of energy, so the process is endothermic.

必须记住，所有物种都必须处于气态，并且电离能的单位是千焦每摩尔 (kJ mol⁻¹)。电离过程总是需要吸收能量，因此该过程是吸热的。

2. The Three Key Factors | 影响电离能的三大因素

Three factors determine the size of an ionisation energy: nuclear charge, distance from the nucleus, and shielding by inner electrons.

决定电离能大小的三个因素分别是：核电荷数、电子离核的距离以及内层电子的屏蔽作用。

A higher nuclear charge pulls outer electrons more strongly and raises ionisation energy. A greater distance reduces the attraction and lowers ionisation energy. Shielding occurs when inner shell electrons repel the outer electron, reducing the effective nuclear attraction.

核电荷数越高，对外层电子的吸引越强，电离能越大。距离越远，吸引力越弱，电离能越小。当内层电子排斥外层电子时，就会产生屏蔽作用，从而降低有效核吸引。

These three factors allow you to explain almost all trends and exceptions in ionisation energy across the periodic table.

掌握这三个因素，你就能解释周期表中几乎所有电离能的变化趋势和例外情况。

3. General Trend Across a Period | 同周期从左到右的总体趋势

Across a period from left to right, first ionisation energy generally increases. The nuclear charge increases as protons are added, while electrons are added to the same outer shell.

在同一周期中，从左到右第一电离能总体呈上升趋势。随着质子数增加，核电荷增大，而新增电子进入同一个外层电子层。

Because the outer electrons are approximately the same distance from the nucleus and experience similar shielding, the increasing nuclear charge dominates. This makes the outer electrons more tightly held.

由于外层电子与原子核的距离大致相同，且受到相似的屏蔽作用，不断增加的核电荷起主导作用，因此外层电子被吸引得更牢固。

For example, sodium has a much lower first ionisation energy than chlorine because chlorine has a greater nuclear charge but only two inner shells, compared with sodium which also has two inner shells.

例如，钠的第一电离能远低于氯，因为氯的核电荷数更大，而它们都只有两个内层电子层。

4. The Dips Across Period 2 and Period 3 | 第二、第三周期中的“凹陷”

There are two important dips in the general increase across Period 2 and Period 3. The first occurs between Group 2 and Group 13, for example from beryllium to boron or from magnesium to aluminium.

在第二和第三周期中，总体上升趋势会出现两个重要的“凹陷”。第一个出现在第 2 族和第 13 族之间，例如从铍到硼或从镁到铝。

In boron and aluminium, the outermost electron is in a p subshell rather than an s subshell. A p electron has slightly more energy than an s electron in the same shell and is also slightly further from the nucleus, so it is easier to remove.

在硼和铝中，最外层电子位于 p 亚层而不是 s 亚层。同一电子层中 p 电子的能量略高于 s 电子，且离核稍远，因此更容易被移除。

The second dip occurs between Group 15 and Group 16, for example from nitrogen to oxygen or from phosphorus to sulfur. In oxygen and sulfur, the outer p subshell contains a paired electron, whereas in nitrogen and phosphorus it contains three unpaired electrons.

第二个凹陷出现在第 15 族和第 16 族之间，例如从氮到氧或从磷到硫。在氧和硫中，外层 p 亚层含有一对成对电子，而在氮和磷中则含有三个未成对电子。

The repulsion between paired electrons in the same orbital makes it easier to remove one of them, so the ionisation energy drops slightly.

同一轨道中成对电子之间的排斥作用使移除其中一个电子更加容易，因此电离能略有下降。

5. General Trend Down a Group | 同族从上到下的总体趋势

Down a group, first ionisation energy decreases. Although nuclear charge increases, the outer electron is in a shell further from the nucleus and experiences more shielding from additional inner shells.

在同一族中，从上到下第一电离能降低。尽管核电荷数增加，但外层电子所处的电子层离核更远，并且受到更多新增内层电子的屏蔽。

The effect of increased distance and shielding outweighs the effect of increased nuclear charge, so the outer electron is held less strongly.

距离增大和屏蔽增强的影响超过了核电荷增大的影响，因此外层电子被原子核吸引得更弱。

For example, the first ionisation energy of caesium is much lower than that of lithium because caesium has many more inner shells and its outer electron is much further from the nucleus.

例如，铯的第一电离能远低于锂，因为铯的内层电子层多得多，而且它的外层电子离核远得多。

6. Successive Ionisation Energies | 逐级电离能

The second ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions. Successive ionisation energies always increase because the remaining electrons are held more tightly by the same nuclear charge.

第二电离能是指从一摩尔气态 1+ 离子中移除一摩尔电子，形成一摩尔气态 2+ 离子所需的能量。逐级电离能总是增大，因为相同核电荷对剩余电子的吸引变得更强。

The pattern of successive ionisation energies gives clear evidence for electron shells. A large jump occurs when an electron is removed from a new inner shell that is much closer to the nucleus.

逐级电离能的变化模式为电子层的存在提供了清晰证据。当从更靠近原子核的新内层移除电子时，会出现一个大幅跳跃。

For sodium, the first ionisation energy is relatively low, but the second ionisation energy is much larger because the second electron is removed from the 2p subshell, which is much closer to the nucleus and has much less shielding.

对钠而言，第一电离能相对较低，但第二电离能要大得多，因为第二个电子从 2p 亚层中移除，该亚层离核更近且屏蔽作用小得多。

7. Using Successive Ionisation Energies to Identify an Element | 利用逐级电离能推断元素

You can predict the group of an element from the pattern of its successive ionisation energies. The position of the largest jump tells you how many electrons are in the outer shell.

你可以根据逐级电离能的变化模式预测元素所在的族。最大跳跃出现的位置会告诉你外层有多少个电子。

For example, if the largest jump occurs between the third and fourth ionisation energies, the element has three outer electrons and is in Group 13.

例如，如果最大的跳跃出现在第三和第四电离能之间，则该元素有三个外层电子，位于第 13 族。

Element	Outer electrons	Largest jump occurs	Group
Sodium	1	Between 1st and 2nd	Group 1
Magnesium	2	Between 2nd and 3rd	Group 2
Aluminium	3	Between 3rd and 4th	Group 13

Always look for the biggest rise in the list, not just any increase, because every successive ionisation energy is normally higher than the previous one.

一定要寻找列表中最大的上升，而不是任意一个增大，因为通常每一级电离能都会比前一级更高。

8. Periodicity of Atomic Radius and Ionic Radius | 原子半径与离子半径的周期性

Atomic radius decreases across a period and increases down a group. These trends are explained by the same factors of nuclear charge, distance and shielding.

原子半径在同一周期中从左到右减小，在同一族中从上到下增大。这些趋势同样可以用核电荷、距离和屏蔽三个因素来解释。

Ionic radius follows an additional rule: positive ions are smaller than their parent atoms because they lose an outer shell, while negative ions are larger because they gain electrons and electron-electron repulsion increases.

离子半径还有一条额外规律：正离子比其对应的原子小，因为它们失去了一个外层电子层；负离子比其对应的原子大，因为它们获得电子，电子间排斥增强。

For example, Na⁺ is much smaller than Na, while Cl⁻ is larger than Cl. Isoelectronic ions such as N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ decrease in radius as nuclear charge increases because all have the same electron configuration.

例如，Na⁺ 比 Na 小得多，而 Cl⁻ 比 Cl 大。像 N³⁻、O²⁻、F⁻、Na⁺、Mg²⁺ 和 Al³⁺ 这些等电子离子的半径随着核电荷增大而减小，因为它们具有相同的电子构型。

9. Periodicity of Electronegativity | 电负性的周期性

Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. It follows a similar pattern to ionisation energy.

电负性是指原子在共价键中吸引成键电子对的能力。它的变化规律与电离能相似。

Electronegativity increases across a period because the nuclear charge increases and atomic radius decreases, so the bonding pair is attracted more strongly. It decreases down a group because the bonding pair is further from the nucleus and more shielded.

电负性在同一周期中从左到右增大，因为核电荷增大且原子半径减小，成键电子对被吸引得更强。它沿同一族从上到下减小，因为成键电子对离核更远且受到更多屏蔽。

Fluorine is the most electronegative element, with a value of 4.0 on the Pauling scale. Noble gases are usually excluded from electronegativity comparisons because they do not form covalent bonds easily.

氟是电负性最强的元素，其鲍林标度值为 4.0。稀有气体通常不参与电负性比较，因为它们不易形成共价键。

10. Common Exam Pitfalls | 常见考试失分点

One common mistake is saying that ionisation energy decreases across a period because shielding increases. Shielding remains approximately constant across a period because electrons are added to the same outer shell, not a new inner shell.

一个常见错误是认为电离能在同一周期中因屏蔽增强而下降。事实上，屏蔽作用在同一周期中大致保持不变，因为新增电子进入同一个外层电子层，而不是新的内层。

Another pitfall is confusing first ionisation energy with electronegativity. Ionisation energy refers to gaseous atoms losing electrons, while electronegativity refers to atoms in a covalent bond attracting a shared pair.

另一个易错点是把第一电离能与电负性混为一谈。电离能涉及气态原子失去电子，而电负性涉及共价键中的原子吸引共用电子对。

Always state that the value refers to one mole of gaseous atoms and one mole of electrons. Missing the gas state or the mole specification loses marks in definition questions.

答题时一定要写明该值针对一摩尔气态原子和一摩尔电子。在定义题中遗漏气态或摩尔条件会扣分。

11. Worked Edexcel-Style Example | Edexcel 风格例题解析

A student obtains the following successive ionisation energies for element X: 578, 1817, 2745, 11577, 14842 kJ mol⁻¹. Identify the group of element X and justify your answer.

某学生测得元素 X 的逐级电离能为：578、1817、2745、11577、14842 kJ mol⁻¹。请判断元素 X 所在的族并说明理由。

The largest jump occurs between the third and fourth ionisation energies. This means the fourth electron is removed from a new inner shell, so the atom has three outer electrons.

最大的跳跃出现在第三和第四电离能之间。这说明第四个电子是从新的内层中移除的，因此该原子有 3 个外层电子。

Element X is therefore in Group 13. The actual element is aluminium, which has the electron configuration 1s² 2s² 2p⁶ 3s² 3p¹.

因此元素 X 位于第 13 族。该元素实际上是铝，其电子构型为 1s² 2s² 2p⁶ 3s² 3p¹。

You should always mention the largest jump and link it directly to the number of outer electrons when answering this type of question.

在回答此类题目时，你应当始终提到最大的跳跃，并将其直接与外层电子数联系起来。

12. Summary and Revision Checklist | 总结与复习清单

Define first ionisation energy with gas state and mole terms.
用气态和摩尔条件定义第一电离能。
Explain trends across periods and down groups using nuclear charge, distance and shielding.
用核电荷、距离和屏蔽解释同周期和同族的趋势。
Identify the two dips across Period 2 and Period 3 using subshell structure and electron pairing.
利用亚层结构和电子成对识别第二、第三周期中的两个凹陷。
Use successive ionisation energy jumps to determine the group of an unknown element.
利用逐级电离能的大幅跳跃判断未知元素所在的族。
Compare atomic radius, ionic radius and electronegativity trends across the periodic table.
比较周期表中原子半径、离子半径和电负性的变化趋势。

Mastering ionisation energy gives you a reliable framework for explaining periodicity, chemical reactivity and electronic structure across the Edexcel specification.

掌握电离能这一主题，你就拥有了一个可靠的框架，可以用来解释 Edexcel 课程体系中的周期性、化学反应性和电子结构。

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Ionisation Energy and Periodicity | 电离能与周期性