Simple Electronic Structure | 简单电子结构

📚 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

Every neutral atom has the same number of electrons as protons. The atomic number Z identifies the element and, in a neutral atom, the total number 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

Electrons occupy principal quantum shells labelled n = 1, 2, 3, 4 and so on. 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。

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。

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。

7. Electronic structure and the Periodic Table | 电子结构与周期表

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