Atomic Structure | 原子结构

📚 Atomic Structure | 原子结构

Atomic structure is the foundation of chemistry: it explains why elements behave differently, how ions form, and why ionisation energies follow periodic trends. In the Cambridge A-Level syllabus, you must link subatomic particles, isotopes, mass spectrometry and electronic configuration to observable chemical properties.

原子结构是化学的基础：它解释了为什么元素性质不同、离子如何形成，以及电离能为什么呈现周期性变化。剑桥 A-Level 大纲要求把亚原子粒子、同位素、质谱和电子排布与可观察的化学性质联系起来。

1. The Nuclear Atom | 核型原子

The nuclear model describes the atom as a tiny, dense, positively charged nucleus surrounded by negatively charged electrons. Most of the volume of the atom is empty space, but almost all of its mass is concentrated in the nucleus.

核型原子模型把原子描述为一个极小、致密、带正电的原子核，周围环绕着带负电的电子。原子的大部分体积是空的，但几乎全部质量都集中在原子核中。

The nucleus contains protons and neutrons, which are collectively called nucleons. The number of electrons in a neutral atom equals the number of protons, so overall charge is zero.

原子核含有质子和中子，它们统称为核子。中性原子中的电子数等于质子数，因此总电荷为零。

2. Subatomic Particles | 亚原子粒子

The three subatomic particles are the proton, neutron and electron. Their relative charges and masses are shown below.

三种亚原子粒子分别是质子、中子和电子。它们的相对电荷和相对质量如下表所示。

Particle / 粒子	Relative charge / 相对电荷	Relative mass / 相对质量	Position / 位置
proton / 质子	+1	1	nucleus / 原子核
neutron / 中子	0	1	nucleus / 原子核
electron / 电子	−1	1/1836	around nucleus / 核外

Protons and neutrons have almost the same mass, while an electron is much lighter, about 1/1836 of the mass of a proton. The charge on a proton is exactly equal and opposite to the charge on an electron.

质子和中子的质量几乎相同，而电子轻得多，质量约为质子的 1/1836。质子的电荷与电子的电荷大小完全相等、符号相反。

3. Nuclide Notation and Isotopes | 核素符号与同位素

The atomic number Z is the number of protons in the nucleus and determines the identity of the element. The mass number A is the total number of protons and neutrons, so the number of neutrons is A − Z.

原子序数 Z 是原子核中的质子数，它决定了元素的种类。质量数 A 是质子数与中子数之和，因此中子数等于 A − Z。

For example, ²³⁵U has 92 protons and 143 neutrons, while ¹²C has 6 protons and 6 neutrons. Nuclide notation shows the mass number as a superscript before the symbol.

例如，²³⁵U 有 92 个质子和 143 个中子，而 ¹²C 有 6 个质子和 6 个中子。核素符号把质量数作为元素符号前的上标。

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties because electron arrangement is unchanged, but physical properties such as mass, density and diffusion rate may differ.

同位素是同一种元素中质子数相同但中子数不同的原子。由于电子排布不变，它们的化学性质相同，但质量、密度和扩散速率等物理性质可能不同。

4. Relative Atomic Mass | 相对原子质量

Relative atomic mass, Aᵣ, is the weighted average mass of an atom of an element relative to 1/12 of the mass of one atom of carbon-12. Carbon-12 is assigned a relative atomic mass of exactly 12.

相对原子质量 Aᵣ 是元素一个原子的加权平均质量与一个碳-12 原子质量的 1/12 的比值。碳-12 的相对原子质量被规定为 12。

Aᵣ = Σ (isotopic mass × % abundance) / 100

The calculated Aᵣ may not be an integer because it is an average over the natural isotopic mixture. For example, chlorine consists of 75% ³⁵Cl and 25% ³⁷Cl.

计算得到的 Aᵣ 可能不是整数，因为它是天然同位素混合物的平均值。例如，氯由 75% 的 ³⁵Cl 和 25% 的 ³⁷Cl 组成。

Aᵣ(Cl) = (35 × 75 + 37 × 25) / 100 = 35.5

5. Mass Spectrometry | 质谱法

A mass spectrometer is used to determine the relative isotopic masses and relative abundances of isotopes in a sample. It produces a mass spectrum with peaks at different mass-to-charge ratios, m/z.

质谱仪用于测定样品中同位素的相对同位素质量和相对丰度。它会产生质谱图，在不同质荷比 m/z 处出现峰。

The main stages are vaporisation, ionisation, acceleration, deflection and detection. In ionisation, gaseous atoms are bombarded with high-energy electrons to form positive ions by removing one electron.

主要步骤包括气化、电离、加速、偏转和检测。在电离过程中，气态原子被高能电子轰击，失去一个电子形成正离子。

In deflection, a magnetic field bends the paths of the positive ions. Lighter ions and ions with a higher charge are deflected more, so the instrument separates isotopes by m/z ratio.

在偏转过程中，磁场使正离子的路径弯曲。较轻的离子和带电荷较多的离子偏转更大，因此仪器按 m/z 比分离同位素。

6. Electron Shells and Subshells | 电子壳层与亚壳层

Electrons occupy shells labelled by the principal quantum number n = 1, 2, 3 and so on. The higher the shell number, the further the electron is from the nucleus and the higher its energy.

电子占据由主量子数 n = 1、2、3 等标记的壳层。壳层数越大，电子离原子核越远，能量越高。

Each shell contains subshells labelled s, p, d and f. An s subshell has 1 orbital, a p subshell has 3 orbitals, a d subshell has 5 orbitals, and an f subshell has 7 orbitals.

每个壳层包含标记为 s、p、d、f 的亚壳层。s 亚壳层有 1 个轨道，p 亚壳层有 3 个轨道，d 亚壳层有 5 个轨道，f 亚壳层有 7 个轨道。

Each orbital can hold a maximum of two electrons, so s, p, d and f subshells can hold 2, 6, 10 and 14 electrons respectively.

每个轨道最多容纳两个电子，因此 s、p、d、f 亚壳层分别最多容纳 2、6、10 和 14 个电子。

7. Orbitals and Electron Spin | 轨道与电子自旋

An atomic orbital is a region of space around the nucleus where there is a high probability of finding an electron. It is not a fixed circular path but a three-dimensional probability region.

原子轨道是原子核周围空间中出现电子概率很高的区域。它不是固定的圆形路径，而是三维的概率区域。

An s orbital is spherical in shape. A p orbital has a dumbbell shape, and the three p orbitals are arranged at right angles along the x, y and z axes.

s 轨道呈球形。p 轨道呈哑铃形，三个 p 轨道分别沿 x、y、z 轴成直角排列。

Electrons have a property called spin, with values +½ and −½, often shown as ↑ and ↓. Two electrons in the same orbital must have opposite spins, which is called spin pairing.

电子具有称为自旋的性质，取值为 +½ 和 −½，通常用 ↑ 和 ↓ 表示。同一轨道中的两个电子必须自旋相反，这称为自旋配对。

8. Filling Rules for Electrons | 电子填充规则

The Aufbau principle states that electrons occupy the lowest energy orbitals first. The common order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and so on.

构造原理指出，电子首先占据能量最低的轨道。常见顺序为 1s、2s、2p、3s、3p、4s、3d、4p 等。

The Pauli exclusion principle states that an orbital can hold a maximum of two electrons, and these two electrons must have opposite spins.

泡利不相容原理指出，一个轨道最多容纳两个电子，且这两个电子必须自旋相反。

Hund’s rule states that electrons fill degenerate orbitals, such as the three p orbitals, singly with parallel spins before pairing. For nitrogen, the configuration is 1s² 2s² 2pₓ¹ 2p_y¹ 2p_z¹.

洪特规则指出，电子在填充简并轨道（例如三个 p 轨道）时，先以平行自旋单独占据每个轨道，然后再配对。氮的排布为 1s² 2s² 2pₓ¹ 2p_y¹ 2p_z¹。

9. Electronic Configurations | 电子排布

Electronic configuration lists the subshells occupied by electrons, with the number of electrons shown as a superscript. For example, oxygen is 1s² 2s² 2p⁴.

电子排布列出电子占据的亚壳层，电子数以上标表示。例如，氧的电子排布为 1s² 2s² 2p⁴。

Shorthand notation uses the previous noble gas to represent inner shells. For iron, the full configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s², which can be written as [Ar] 3d⁶ 4s².

简写电子排布用前一个稀有气体表示内层电子。铁的完整排布为 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²，也可写作 [Ar] 3d⁶ 4s²。

Chromium and copper show exceptions because a half-filled or completely

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Atomic Structure | 原子结构