📚 Simple Electronic Structure | 简单电子结构
Understanding how electrons are arranged in atoms is the foundation of A-Level Chemistry. This article explains the simple electronic structure from energy levels and sub-shells to electron configurations, using Cambridge terminology and exam-focused examples.
理解电子在原子中如何排布是 A-Level 化学的基础。本文用剑桥考试术语和考点示例，讲解从能级、亚层到电子构型的简单电子结构。
1. Atomic number and electrons | 原子序数与电子
ber of electrons. For example, carbon has Z = 6, so a neutral carbon atom has 6 electrons.
每个中性原子的电子数等于质子数。原子序数 Z 既标识元素，也代表中性原子的电子总数。例如，碳的 Z = 6，所以中性碳原子有 6 个电子。
2. Principal quantum shells | 主量子层
These shells are often called energy levels. The higher the value of n, the further the shell is from the nucleus and the higher its energy.
电子占据主量子层，记作 n = 1、2、3、4 等。这些层常被称为能级。n 值越大，电子层离核越远，能量也越高。
The maximum number of electrons a shell can hold is given by 2n².
一个电子层最多可容纳的电子数由 2n² 给出。
| Shell | 层 (n) | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Maximum electrons | 最多电子数 | 2 | 8 | 18 | 32 |
3. Sub-shells: s, p, d | 亚层：s、p、d
Each principal shell contains one or more sub-shells. The first shell has only an s sub-shell; the second has s and p; the third has s, p and d; the fourth has s, p, d and f. Within the same shell, sub-shell energies follow the order s < p < d < f.
每个主层包含一个或多个亚层。第一层只有 s 亚层；第二层有 s 和 p；第三层有 s、p 和 d；第四层有 s、p、d 和 f。同一主层内，亚层能量顺序为 s < p < d < f。
4. Orbitals and their shapes | 轨道及其形状
A sub-shell is made up of orbitals. An orbital is a region of space where there is a high probability of finding an electron. An s orbital is spherical; p orbitals are dumbbell-shaped and occur in three orientations, labelled px, py and pz; d orbitals have five orientations and more complex shapes.
亚层由轨道组成。轨道是电子出现概率较高的空间区域。s 轨道为球形；p 轨道呈哑铃形，有三个取向，记作 px、py 和 pz；d 轨道有五个取向，形状更复杂。
| Sub-shell | 亚层 | Number of orbitals | 轨道数 | Maximum electrons | 最多电子数 |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
5. Filling rules | 填充规则
Three rules control how electrons fill orbitals. Aufbau principle: electrons fill the lowest available energy levels first. Pauli exclusion principle: each orbital can hold a maximum of two electrons, and these two electrons must have opposite spins. Hund's rule: when electrons fill degenerate orbitals in the same sub-shell, they occupy empty orbitals singly before pairing up.
电子的填充由三条规则控制。构造原理：电子优先填入最低可用能级。泡利不相容原理：每个轨道最多可容纳两个电子，且这两个电子自旋相反。洪特规则：当电子填充同一亚层中的简并轨道时，先单独占据空轨道，再配对。
For A-Level, the common filling order up to 4p is: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p.
在 A-Level 中，到 4p 为止的常见填充顺序为：1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p。
6. Writing electron configurations | 电子构型的书写
Electron configurations are written by listing sub-shells with superscript electron counts. For example, sodium has 11 electrons and the configuration 1s² 2s² 2p⁶ 3s¹. For heavier atoms, a noble-gas shorthand can be used: chlorine, with 17 electrons, is [Ne] 3s² 3p⁵.
电子构型通过列出亚层并附上上标电子数来表示。例如钠有 11 个电子，构型为 1s² 2s² 2p⁶ 3s¹。对于较重的原子，可使用稀有气体简写：氯有 17 个电子，简写为 [Ne] 3s² 3p⁵。
| Element | 元素 | Atomic number | 原子序数 | Electron configuration | 电子构型 |
|---|---|---|
| H | 1 | 1s¹ |
| He | 2 | 1s² |
| Li | 3 | 1s² 2s¹ |
| Be | 4 | 1s² 2s² |
| B | 5 | 1s² 2s² 2p¹ |
| C | 6 | 1s² 2s² 2p² |
| N | 7 | 1s² 2s² 2p³ |
| O | 8 | 1s² 2s² 2p⁴ |
| F | 9 | 1s² 2s² 2p⁵ |
| Ne | 10 | 1s² 2s² 2p⁶ |
7. Electronic structure and the Periodic Table | 电子结构与周期表
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