IB Chemistry: Electron Configuration Rules & Writing Techniques | IB化学：电子排布规律与书写技巧

📚 IB Chemistry: Electron Configuration Rules & Writing Techniques | IB化学：电子排布规律与书写技巧

Electron configuration is one of the first deep ideas you meet in IB Chemistry, but it is also the silent key to many later topics: periodicity, bonding, redox, and even organic mechanisms. Mastering how electrons occupy orbitals gives you a mental map of the atom and makes exam questions about ionisation energy, atomic radius and magnetic properties far simpler.

电子排布是 IB 化学中最先遇到的重要概念之一，但它也是理解后续许多主题的关键：周期性、化学键、氧化还原，甚至有机反应机理。掌握电子如何占据轨道，能让你在脑海中建立原子的地图，也使涉及电离能、原子半径和磁性等考题变得轻松很多。

1. Why Electron Configurations Matter | 为什么电子排布重要

Electron configurations describe the distribution of electrons among the atomic orbitals. They explain why lithium is reactive, why neon is inert, why chromium has a special configuration, and why transition-metal ions are coloured. In IB Chemistry, you are expected to write electron configurations for atoms and ions of the first 36 elements, and to use them to explain periodic trends.

电子排布描述的是电子在原子轨道中的分布情况。它可以解释为什么锂活泼、氖惰性、铬有特殊的排布方式，以及为什么过渡金属离子有颜色。在 IB 化学中，你需要能够为前 36 号元素的原子和离子写出电子排布，并利用它们解释周期律。

This article will guide you through the underlying rules, the standard notation, common exceptions, and the exact steps that earn full marks in exam questions.

这篇文章将带你梳理背后的规律、标准书写方式、常见例外，以及在考试中拿满分的具体步骤。

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2. The Quantum Mechanical Model and Orbitals | 量子力学模型与轨道

In the quantum model, electrons do not travel in fixed circular paths. Instead, they exist in orbitals — regions of space where the probability of finding an electron is high. Each orbital is defined by quantum numbers, and the most useful one for IB is the principal quantum number, n, which relates to the main energy level and the average distance from the nucleus.

在量子力学模型中，电子并不沿固定圆形轨道运动。它们存在于“轨道”中——也就是找到电子概率较高的空间区域。每个轨道由量子数定义，对 IB 最重要的就是主量子数 n，它对应主能层以及与原子核的平均距离。

Orbitals are grouped into subshells labelled s, p, d and f. Their capacities are fixed:

轨道按亚层分类，标记为 s、p、d 和 f。它们的容量是固定的：

Subshell	Number of orbitals	Maximum electrons
s	1	2
p	3	6
d	5	10
f	7	14

Remember the 2, 6, 10, 14 pattern: it tells you how many electrons can fit into each subshell.

记住 2、6、10、14 这个规律：它告诉你每个亚层最多能容纳多少电子。

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3. The Three Fundamental Rules | 三大基本规则

All electron configurations obey three fundamental rules. The first is the Aufbau principle: electrons occupy the lowest available energy orbitals first. The second is the Pauli exclusion principle: no two electrons in the same atom can have the same set of four quantum numbers, so an orbital holds at most two electrons and they must have opposite spins.

所有电子排布都遵循三大基本规则。第一是构造原理：电子优先占据能量最低的可用轨道。第二是泡利不相容原理：同一原子中不能有四个量子数完全相同的两个电子，因此一个轨道最多容纳两个电子，且它们的自旋方向必须相反。

The third is Hund’s rule: when filling orbitals of the same energy (degenerate orbitals), electrons spread out one by one before pairing up. This is because electron-electron repulsion is minimised when electrons occupy separate orbitals, and it also explains why nitrogen has three unpaired electrons.

第三是洪特规则：当填充多个简并轨道（即能量相同的轨道）时，电子会先分别进入不同轨道、自旋平行，之后才配对。原因是电子分别占据不同轨道时相互排斥最小；这也解释了为什么氮原子有三个未成对电子。

Aufbau: fill lowest energy first → Pauli: max 2 per orbital, opposite spins → Hund: spread before pairing

构造原理：先填最低能量 → 泡利原理：每轨道最多 2 个电子且自旋相反 → 洪特规则：先分散占据再配对

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4. The Aufbau Order and Orbital Filling Sequence | 构造原理与轨道填充顺序

The energy order of orbitals is not simply 1, 2, 3, 4 by shell number. Because of electron-electron repulsion and nuclear charge, the 4s orbital has a lower energy than the 3d orbital in neutral atoms. The standard filling order for the first 36 elements is:

轨道的能量顺序并不是简单按壳层 1、2、3、4 排列。由于电子间排斥和核电荷的影响，在电中性原子中 4s 轨道的能量低于 3d 轨道。前 36 号元素的标准填充顺序为：

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p

An easy way to reconstruct this order is the (n+l) rule. For an orbital with principal quantum number n and azimuthal quantum number l, the energy increases with n+l; if two orbitals have the same n+l, the one with lower n fills first.

一个简单的方法是 (n+l) 规则。对给定的轨道，若主量子数为 n、角量子数为 l，则能量随 n+l 增大而升高；若两个轨道的 n+l 相同，则 n 较小的先填充。

For example, 4s has n+l = 4 + 0 = 4, while 3d has n+l = 3 + 2 = 5. Since 4 is smaller than 5, 4s fills first.

例如，4s 的 n+l = 4 + 0 = 4，而 3d 的 n+l = 3 + 2 = 5。因为 4 小于 5，所以 4s 先被填充。

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5. Notation: Full, Condensed, and Orbital Diagrams | 书写方式：完整、简写与轨道图

There are three common ways to write an electron configuration. The full form lists every occupied subshell, with the number of electrons in that subshell written as a superscript. For example, calcium is:

电子排布有三种常见书写方式。完整式列出所有被占据的亚层，并把该亚层的电子数写成上标。例如钙：

Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Ca：1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

The condensed form uses the previous noble gas as a core, then continues from there. For calcium, the noble gas core is [Ar], so the condensed configuration is [Ar] 4s². This saves time and focuses attention on the outer electrons.

简写式使用前一个稀有气体作为原子实，然后从其后继续书写。钙的原子实是 [Ar]，所以简写式为 [Ar] 4s²。这样可以节省时间，并把注意力集中在最外层电子上。

The orbital diagram uses boxes or lines to represent orbitals and arrows for electrons. An upward arrow represents one spin direction and a downward arrow represents the opposite spin. Always follow Hund’s rule when drawing these diagrams.

轨道图用方框或横线表示轨道，用箭头表示电子。向上箭头表示一种自旋方向，向下箭头表示另一种自旋方向。画图时一定要遵循洪特规则。

Orbital diagram for nitrogen: 2p³ = ↑ ↑ ↑ (one arrow in each p orbital)

氮的轨道图：2p³ = ↑ ↑ ↑（三个 p 轨道各有一个箭头）

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6. Exceptions: Cr and Cu | 轨道排布中的例外：铬和铜

If we follow the Aufbau rule mechanically, chromium would be [Ar] 4s² 3d⁴ and copper would be [Ar] 4s² 3d⁹. But experimental data show the actual configurations are:

如果机械地按照构造原理，铬似乎应该是 [Ar] 4s² 3d⁴，铜似乎应该是 [Ar] 4s² 3d⁹。但实验数据给出的实际排布是：

Cr: [Ar] 4s¹ 3d⁵

Cu: [Ar] 4s¹ 3d¹⁰

The reason is that half-filled (d⁵) and fully-filled (d¹⁰) subshells have extra stability due to exchange energy and reduced electron-electron repulsion. This small energy difference is enough to promote one electron from 4s into the 3d subshell.

原因是半充满（d⁵）和全充满（d¹⁰）亚层具有额外稳定性，这来源于交换能的增加和电子间排斥的降低。这个较小的能量差足以将一个 4s 电子激发到 3d 亚层。

You should mention this exception in exam answers when asked about Cr and Cu. IB examiners expect you to know these two examples, and sometimes also Mo (molybdenum) as an analogous case, though it is not required for first 36 elements.

在考试中回答关于 Cr 和 Cu 的题目时，你应该提到这一例外。IB 考官希望你知道这两个例子；有时钼 Mo 也作为类似案例出现，但它不在前 36 号元素的要求范围内。

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7. Writing Configurations for Ions | 离子的电子排布

For ions, electrons are removed from the highest energy occupied orbital. Here is a common trap: in neutral transition-metal atoms, 4s fills before 3d, but when a cation forms, the 4s electrons are removed first. The reason is that in positive ions, the 3d orbital becomes lower in energy than the 4s.

对于离子，电子从能量最高的被占据轨道移除。这里有一个常见陷阱：在电中性过渡金属原子中，4s 先于 3d 填充，但形成阳离子时，先移除的是 4s 电子。原因是正离子中 3d 轨道能量变得低于 4s。

For a positive ion, first write the neutral atom configuration, then remove electrons from the highest n value, not the last subshell in the written sequence. In practice, this means removing 4s electrons before 3d electrons for transition metals.

对于阳离子，先写出中性原子的电子排布，然后从主量子数 n 最大的轨道移除电子，而不是从书写顺序中最后一个亚层移除。对于过渡金属，这意味着先移除 4s 电子，再移除 3d 电子。

Fe: [Ar] 4s² 3d⁶ → Fe²⁺: [Ar] 3d⁶ → Fe³⁺: [Ar] 3d⁵

Fe：[Ar] 4s² 3d⁶ → Fe²⁺：[Ar] 3d⁶ → Fe³⁺：[Ar] 3d⁵

For anions, simply add electrons to the available orbitals following the Aufbau principle. For example, O²⁻ has the same electron configuration as Ne.

对于阴离子，只需按构造原理把电子填入可用轨道。例如 O²⁻ 的电子排布与 Ne 相同。

O²⁻: 1s² 2s² 2p⁶

O²⁻：1s² 2s² 2p⁶

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8. Valence Electrons and Core Notation | 价电子与原子实简写

The valence electrons are the electrons in the highest occupied principal energy level, and for transition metals they also include the partially-filled d subshell. These electrons are responsible for chemical behaviour and bonding.

价电子是指最高占据主能层中的电子；对过渡金属来说，还包括部分填充的 d 亚层电子。这些电子决定了化学行为和成键方式。

For main-group elements, the group number equals the number of valence electrons for groups 1, 2 and 13–18 (with 18 for group 18). For example, chlorine is in group 17 and has seven valence electrons: 3s² 3p⁵.

对主族元素而言，第 1、2 族和 13–18 族的族数等于价电子数（第 18 族为 8，He 为 2）。例如氯在 17 族，有 7 个价电子：3s² 3p⁵。

Core notation is not just a shortcut; it makes the pattern of valence electrons obvious. When comparing elements, [Ne] 3s¹ for sodium and [Ne] 3s² 3p¹ for aluminium immediately show the electron gain/loss tendencies.

原子实简写不仅是节省时间的手段，更重要的是让价电子规律一目了然。比较元素时，Na 写为 [Ne] 3s¹、Al 写为 [Ne] 3s² 3p¹，得失电子的趋势立刻变得清楚。

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9. Periodic Trends Linked to Configuration | 电子排布与周期律的联系

Electron configuration is the foundation of the periodic table. Elements in the same group have the same number of valence electrons and therefore similar chemical properties. The period number tells you the principal quantum number of the highest occupied shell.

电子排布是元素周期表的基础。同一主族的元素具有相同的价电子数，因此化学性质相似。周期数告诉你最高占据壳层的主量子数。

Moving across a period, electrons fill the same shell while the nuclear charge increases. The result is a decrease in atomic radius, an increase in first ionisation energy, and an increase in electronegativity. These trends are all easier to explain when you can see that the outer shell is unchanged while the positive charge increases.

在同一周期从左向右移动，电子填充同一个壳层，而核电荷增加。结果是原子半径减小、第一电离能增大、电负性增大。当你看到最外层壳层不变而正电荷增加时，这些趋势就更容易解释了。

For transition metals, the similar properties arise because the 4s is filled before the 3d starts, and the differences between elements mostly involve inner 3d electrons. Thus atomic radii and ionisation energies change very slowly across the d-block.

过渡金属性质相似，是因为 4s 先填满而后 3d 再填充，元素之间的差别主要体现在内层 3d 电子上。因此 d 区元素从左到右原子半径和电离能变化很缓慢。

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10. Common Mistakes and How to Avoid Them | 常见错误与避坑指南

One common mistake is pairing electrons in degenerate orbitals before all orbitals have one electron. For carbon, the correct 2p configuration is ↑ ↑ with two unpaired electrons, not ↑↓ with a closed pair. Drawing the latter violates Hund’s rule and loses marks.

常见错误之一是在所有简并轨道都各有一个电子之前就配对。碳的 2p 排布应该是两个未成对电子 ↑ ↑，而不是直接配对 ↑↓。画出后者违反了洪特规则，会被扣分。

Another mistake is writing 3d before 4s in a neutral atom’s full configuration sequence. In neutral atoms of the first 36 elements, IB usually expects the order by increasing energy: 4s comes before 3d. When you form ions, however, the 3d configuration is written before considering 4s removal.

另一个错误是在中性原子的完整排布序列中把 3d 写在 4s 前面。对于前 36 号元素的中性原子，IB 通常期望按能量升序书写：4s 出现在 3d 之前。但形成离子时，需要先移除 4s 电子，再写出 3d 部分。

• Wrong: K as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹ — that is not how potassium fills. The last electron goes into 4s, so the configuration is [Ar] 4s¹.
• 错例：K 写成 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹ —— 钾的最后一个电子应进入 4s，因此是 [Ar] 4s¹。
• Wrong: S²⁻ written as 1s² 2s² 2p⁶ 3s² 3p⁴ + 2 electrons in 3d? No, adding electrons to a negative ion still follows Aufbau: the 3p subshell simply becomes 3p⁶.
• 错例：S²⁻ 写成 1s² 2s² 2p⁶ 3s² 3p⁴ 后又把两个电子放进 3d？不对，阴离子加电子依然遵循构造原理：3p 亚层变成 3p⁶ 即可。

Always check the total number of electrons: for a neutral atom, it equals the atomic number; for an ion, add or subtract the charge.

始终检查电子总数：电中性原子中，电子数等于原子序数；离子中，需要加上或减去电荷数。

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11. Worked Examples and Quick Checks | 例题精讲与快速检查

Let us write the full configuration for titanium, Z = 22. Count out the order: 1s², 2s², 2p⁶, 3s², 3p⁶: that is 18 electrons. The next two electrons go into 4s, then the remaining two into 3d. The full configuration is:

我们来写钛（Z = 22）的完整电子排布。按顺序数：1s²、2s²、2p⁶、3s²、3p⁶ 共有 18 个电子。接下来两个电子进入 4s，剩下两个进入 3d。完整排布为：

Ti: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d²

Ti：1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d²

Now check the total: 2+2+6+2+6+2+2 = 22. Good. The condensed form is [Ar] 4s² 3d². For Ti²⁺, remove the two 4s electrons first, giving [Ar] 3d².

检查总数：2+2+6+2+6+2+2 = 22，正确。简写式为 [Ar] 4s² 3d²。对于 Ti²⁺，先移除两个 4s 电子，得到 [Ar] 3d²。

Another useful quick check is to count unpaired electrons. Nickel, Ni: [Ar] 4s² 3d⁸. The 3d subshell has five orbitals; with eight electrons, the first five occupy one each, then three pair up. This leaves two unpaired electrons.

另一个有用的快速检查是数未成对电子。镍 Ni：[Ar] 4s² 3d⁸。3d 亚层有五个轨道；八个电子中，前五个各占一个轨道，剩下三个必须配对。这样会留下两个未成对电子。

Ni 3d⁸: ↑↓ ↑↓ ↑↓ ↑ ↑

Ni 3d⁸：↑↓ ↑↓ ↑↓ ↑ ↑

This kind of diagram helps you answer questions about paramagnetism and magnetic moment quickly and accurately.

这类轨道图能帮助你快速而准确地回答关于顺磁性和磁矩的问题。

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12. Summary and Exam Tips | 总结与考试技巧

On the IB exam, electron configuration questions reward clarity, order and consistency. Start by writing the noble gas core if the element is beyond neon, then fill subshells in the Aufbau order. Always respect Pauli and Hund when drawing orbital diagrams.

在 IB 考试中，电子排布题得分的关键是清晰、有序、一致。若元素在氖之后，先从稀有气体原子实开始写，然后按构造原理填充亚层。画轨道图时始终遵守泡利原理和洪特规则。

For transition elements, remember the 4s/3d energy inversion: fill 4s first for neutral atoms, but remove 4s first for cations. Keep Cr and Cu exceptions fresh in memory, and always check the total electron count.

对于过渡元素，记住 4s/3d 能量反转：中性原子先填 4s，阳离子先移除 4s。时刻记得 Cr 和 Cu 的例外，并总是检查电子总数。

Finally, connect configuration to the periodic table. If you can see that elements in the same group have matching valence configurations, you will also understand their similar chemistry and the periodic trends.

最后，把电子排布与元素周期表联系起来。如果你能看到同族元素具有相同的价电子构型，你也就理解了它们相似的化学性质以及周期律。

With these rules and the worked examples in mind, you can approach any IB electron configuration question with confidence.

掌握了这些规则和上述例题，你就可以自信地应对任何 IB 电子排布问题。

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