📚 Explain Trends in First Ionisation Energy | 解释第一电离能变化趋势
Many AQA A-level Chemistry 'explain' questions ask you to account for trends in first ionisation energy across Period 3 and down groups. This article breaks down the key definitions, factors, exceptions and exam technique you need.
许多AQA A-level化学解释题要求说明第一电离能沿第三周期和族内的变化趋势。本文分解你需要掌握的关键定义、影响因素、例外情况和答题技巧。
1. What Is First Ionisation Energy? | 什么是第一电离能
The first ionisation energy (IE) is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
第一电离能是指从一摩尔气态原子中移除一摩尔电子，形成一摩尔气态1+离子所需的能量。
For any element X, this can be written as:
对于任意元素X，可写成：
X(g) → X⁺(g) + e⁻
Ionisation is always endothermic, so the enthalpy change is positive.
电离始终是吸热过程，因此焓变为正值。
2. The Three Core Factors | 三个核心因素
Three main factors control ionisation energy: nuclear charge, atomic radius and shielding. Greater nuclear charge pulls electrons more strongly, increasing IE; larger radius and more inner-shell shielding reduce the attraction, decreasing IE.
三个主要因素控制电离能：核电荷、原子半径和屏蔽效应。核电荷越大，对电子吸引越强，电离能越高；半径越大、内层屏蔽越多，吸引减弱，电离能降低。
A fourth factor is spin-pair repulsion: two electrons in the same orbital repel each other, so a paired electron is easier to remove.
第四个因素是自旋成对排斥：同一轨道中的两个电子相互排斥，因此成对电子更容易被移除。
3. General Trend Across Period 3 | 第三周期总体趋势
Across Period 3 from Na to Ar, first ionisation energy generally increases. Nuclear charge increases from 11 to 18 protons while the outer electrons enter the same principal energy level, so shielding remains similar and atomic radius decreases. The outer electron feels a stronger effective nuclear charge.
从Na到Ar沿第三周期，第一电离能总体上升。核电荷从11个质子增加到18个，而最外层电子进入同一主能层，因此屏蔽效应相近，原子半径减小。最外层电子感受到的有效核电荷增强。
This makes the outer electron progressively harder to remove.
这使得最外层电子越来越难被移除。
4. Dips at Aluminium and Sulfur | 铝和硫处的下降
The overall rise is interrupted by falls at aluminium and sulfur. Aluminium has the electron configuration 1s² 2s² 2p⁶ 3s² 3p¹. Its outermost electron is removed from a 3p orbital, which is higher in energy than the 3s orbital, so less energy is required than for magnesium's 3s electron.
总体上升趋势在铝和硫处出现下降。铝的电子构型为1s² 2s² 2p⁶ 3s² 3p¹。其最外层电子从3p轨道移除，3p轨道能量高于3s轨道，因此所需能量比镁的3s电子少。
Sulfur has the configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In sulfur, one 3p orbital contains a pair of electrons, whereas in phosphorus all 3p orbitals are singly occupied. The paired electrons repel each other, making one easier to remove, so sulfur's IE is lower than phosphorus's.
硫的电子构型为1s² 2s² 2p⁶ 3s² 3p⁴。在硫中，一个3p轨道含有一对电子，而磷的3p轨道均为单电子占据。成对电子相互排斥，使其中一个电子更容易被移除，因此硫的电离能低于磷。
5. Trend Down a Group | 族内自上而下趋势
Going down Group 2 from Be to Ba, first ionisation energy decreases. Although nuclear charge increases, the outer electron is in a higher principal energy level further from the nucleus. Additional inner shells increase shielding, so the effective nuclear attraction on the outer electron falls.
沿第2族从Be到Ba自上而下，第一电离能降低。尽管核电荷增加，但最外层电子位于更高的主能层，离核更远。增加的内部壳层增强了屏蔽效应，因此作用在最外层电子上的有效核吸引减弱。
This outweighs the increased nuclear charge, so less energy is required to remove the outer electron.
这超过了核电荷增加的影响，因此移除最外层电子所需能量更少。
6. Successive Ionisation Energies | 逐级电离能
Successive ionisation energies for the same element always increase. After the first electron is removed, the ion has a greater positive charge and fewer electrons, so the remaining electrons are held more tightly. Removing another electron requires more energy.
同一元素的逐级电离能总是增加。第一个电子被移除后，离子的正电荷更大、电子更少，因此剩余电子被更紧密地束缚。移除下一个电子需要更多能量。
Large jumps in successive ionisation energies indicate that the electron is being removed from a new inner shell. For example, a jump between the second and third ionisation energies suggests an element in Group 2.
逐级电离能中的大幅跃升表明电子开始从新的内层壳层移除。例如，第二与第三电离能之间的跃升提示该元素属于第2族。
7. How to Structure an 'Explain' Answer | 如何组织解释题答案
In an AQA 'explain' question, start by stating which electrons are being removed and from which orbitals. Then compare the atomic radius and shielding, and identify whether either atom or ion has paired electrons. Link each point back to the attraction between the nucleus and the outer electron.
在AQA解释题中，首先说明移除的是哪个电子以及来自哪个轨道。然后比较原子半径和屏蔽效应，确定任一原子或离子是否存在成对电子。将每一点与原子核和最外层电子之间的吸引联系起来。
Use phrases such as 'therefore the outer electron is less strongly attracted, so the ionisation energy is lower.'
使用诸如“因此最外层电子受到较弱吸引，所以电离能较低”的表述。
8. Common Mistakes and AQA Marking Points | 常见错误与AQA评分要点
A common mistake is claiming that shielding increases across a period. Across a period, electrons are added to the same outer shell, so shielding remains approximately constant. Another error is forgetting the aluminium and sulfur dips when describing the overall trend.
常见错误是声称屏蔽效应沿周期增加。沿周期电子加入同一最外层壳层，因此屏蔽效应大致保持不变。另一个错误是在描述总体趋势时忘记铝和硫处的下降。
AQA mark schemes also expect you to mention spin-pair repulsion for sulfur and the higher energy of the 3p electron for aluminium, rather than simply saying 'aluminium is an exception'.
AQA评分方案还期望你针对硫提到自旋成对排斥，针对铝提到3p电子能量较高，而不是简单地说“铝是一个例外”。
9. Worked Example Question | 例题示范
Question: Explain why the first ionisation energy of oxygen is lower than that of nitrogen.
例题：解释为什么氧的第一电离能低于氮。
Nitrogen has the configuration 1s² 2s² 2p³, with three unpaired electrons in separate 2p orbitals. Oxygen has 1s² 2s² 2p⁴, so one 2p orbital must contain a pair of electrons. The paired electrons repel each other, so less energy is required to remove one of them. Therefore oxygen's first ionisation energy is lower than nitrogen's.
氮的电子构型为1s² 2s² 2p³，三个2p轨道各有未成对电子。氧的电子构型为1s² 2s² 2p⁴，因此一个2p轨道必须含有一对电子。成对电子相互排斥，因此移除其中一个电子所需能量较少。所以氧的第一电离能低于氮。
This is a classic 'explain the exception' question and tests the spin-pair repulsion idea.
这是经典的“解释例外”题，考查自旋成对排斥概念。
10. Exam Practice Tips | 备考练习提示
Practice by writing out electron configurations for Period 3 elements and predicting the IE trend with reasons. Use past AQA questions that ask for 'explain' rather than just 'state'. Draw a rough sketch of the Period 3 IE graph and annotate the aluminium and sulfur dips.
练习写出第三周期元素的电子构型，并预测电离能趋势及原因。使用AQA历年真题中要求“解释”而非仅仅“陈述”的题目。画出第三周期电离能趋势简图，并标注铝和硫处的下降。
Check your answers against the mark scheme for precise phrases such as 'same shielding', 'paired electron repulsion' and 'higher energy 3p orbital'.
对照评分方案检查答案，注意诸如“相同的屏蔽效应”“成对电子排斥”和“能量更高的3p轨道”等准确表述。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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