Atomic Structure | 原子结构

📚 Atomic Structure | 原子结构

The atom is the fundamental building block of matter, and understanding its internal structure is the foundation of all chemistry. This topic explores the subatomic particles, the arrangement of electrons, and the concepts of isotopes and relative mass that underpin the AQA A-Level specification.

原子是物质的基本组成单位，理解其内部结构是学习一切化学的基础。本专题探讨亚原子粒子、电子排布以及同位素和相对质量等重要概念，这些内容正是 AQA A-Level 大纲的基础。

1. The Subatomic Particles | 亚原子粒子

An atom consists of a central nucleus surrounded by electrons. The nucleus contains protons and neutrons, collectively called nucleons. The properties of these particles are crucial to understanding atomic structure.

原子由位于中心的原子核和围绕核运动的电子构成。原子核中包含质子和中子，统称为核子。这些粒子的性质对于理解原子结构至关重要。

Proton: positive charge (+1), relative mass 1.
质子：带正电荷（+1），相对质量为 1。
Neutron: neutral charge (0), relative mass 1.
中子：不带电荷（0），相对质量为 1。
Electron: negative charge (-1), relative mass 1/1840 (negligible).
电子：带负电荷（-1），相对质量约为 1/1840（可忽略不计）。

proton = +1, mass 1 | neutron = 0, mass 1 | electron = −1, mass ≈ 0

2. Relative Mass and Charge | 相对质量和电荷

Rather than using absolute masses of particles, which are extremely small, chemists use relative values. The relative mass of a proton is taken as 1, and relative charges are compared to the elementary charge.

由于粒子的绝对质量极其微小，化学家使用相对值来表示。质子的相对质量被定义为 1，相对电荷则以基本电荷为基准进行比较。

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	−1	1/1840

Notice that protons and neutrons have almost identical masses, while electrons are thousands of times lighter. This explains why the nucleus contains almost all of the atom’s mass.

注意：质子与中子的质量几乎相同，而电子比它们轻数千倍。这解释了为什么原子核几乎集中了原子的全部质量。

3. Atomic Number and Mass Number | 原子序数与质量数

The atomic number (Z) is the number of protons in the nucleus. The mass number (A) is the total number of protons and neutrons. The standard notation for an element is:

原子序数（Z）是原子核中的质子数。质量数（A）是质子数和中子数的总和。元素的表示方法如下：

⁴⁰₁₈Ar

Using the example ⁴⁰₁₈Ar, the bottom number (18) is the atomic number, and the top number (40) is the mass number. Therefore, the number of neutrons is 40 − 18 = 22.

以 ⁴⁰₁₈Ar 为例，下方的数字（18）是原子序数，上方的数字（40）是质量数。因此，中子数为 40 − 18 = 22。

Since atoms are electrically neutral, the number of electrons equals the number of protons. In an ion, the electron count changes but the proton number does not.

由于原子呈电中性，电子数等于质子数。在离子中，电子数目会改变，但质子数不变。

4. Isotopes | 同位素

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers.

同位素是指同一种元素的原子，具有相同的质子数但不同的中子数。因此它们具有相同的原子序数，但质量数不同。

Isotopes have identical chemical properties because chemical behaviour depends on the electron configuration, which is the same.
同位素具有相同的化学性质，因为化学行为取决于电子排布，而电子排布相同。
Physical properties such as density and diffusion rate may differ slightly due to mass differences.
物理性质如密度和扩散速率可能因质量差异而略有不同。

Example: carbon-12 and carbon-14 are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.

例如：碳-12 和碳-14 是碳的同位素。两者都有 6 个质子，但碳-12 有 6 个中子，而碳-14 有 8 个中子。

5. Relative Atomic Mass and Relative Isotopic Mass | 相对原子质量与相对同位素质量

Relative isotopic mass is the mass of one isotope compared to 1/12 of the mass of carbon-12. Relative atomic mass (Aᵣ) is the weighted mean mass of an atom of an element compared to 1/12 of the mass of carbon-12.

相对同位素质量是指某一种同位素的质量与碳-12 质量的 1/12 之比。相对原子质量（Aᵣ）是指元素原子的平均质量与碳-12 质量的 1/12 之比，且考虑了同位素的相对丰度。

The formula for calculating Aᵣ is:

计算 Aᵣ 的公式如下：

Aᵣ = (Σ isotopic mass × relative abundance) / total abundance

For example, naturally occurring chlorine consists of 75.8% ³⁵Cl and 24.2% ³⁷Cl. The Aᵣ is (35 × 75.8 + 37 × 24.2) / 100 = 35.5.

例如，天然存在的氯由 75.8% 的 ³⁵Cl 和 24.2% 的 ³⁷Cl 组成。其 Aᵣ = (35 × 75.8 + 37 × 24.2) / 100 = 35.5。

6. Mass Spectrometry | 质谱法

Mass spectrometry is a technique used to measure the relative masses and abundances of isotopes. It is also used to determine the relative atomic mass of an element.

质谱法是一种用于测量同位素相对质量和相对丰度的技术，还可用于测定元素的相对原子质量。

The key stages in a simple mass spectrometer are:

简易质谱仪的主要阶段包括：

Ionisation: atoms are ionised by removing an electron, usually using high-energy electrons.
离子化：通常使用高能电子轰击原子，使其失去一个电子而形成正离子。
Acceleration: ions are accelerated by an electric field to give them the same kinetic energy.
加速：在电场中加速离子，使它们获得相同的动能。
Deflection: ions are deflected by a magnetic field; lighter ions are deflected more than heavier ions.
偏转：利用磁场使离子发生偏转；较轻的离子偏转程度大于较重的离子。
Detection: ions strike a detector, producing a current proportional to their abundance.
检测：离子撞击检测器，产生与丰度成正比的电流。

The resulting mass spectrum shows peaks at each isotopic mass, with peak heights indicating relative abundance.

生成的质谱图在每个同位素质量处显示一个峰，峰的高度表示其相对丰度。

7. Calculating Aᵣ from Mass Spectra | 从质谱图计算相对原子质量

Using the mass spectrum of an element, we can determine the relative atomic mass by multiplying each isotopic mass by its relative abundance and summing.

利用元素的质谱图，我们可以通过将每个同位素质量乘以其相对丰度并求和来计算相对原子质量。

Example: A mass spectrum of element X shows three peaks:

示例：元素 X 的质谱图显示三个峰：

⁷⁰X: abundance 20%
⁷⁰X：丰度 20%
⁷²X: abundance 50%
⁷²X：丰度 50%
⁷⁴X: abundance 30%
⁷⁴X：丰度 30%

Aᵣ = (70 × 20 + 72 × 50 + 74 × 30) / 100 = 72.4

Always check that your final answer lies between the smallest and largest isotopic masses, and is close to the most abundant isotope if it is very abundant.

务必检查计算出的 Aᵣ 是否处于最轻和最重同位素质量之间；如果某同位素丰度极高，Aᵣ 应接近该同位素的质量。

8. Electronic Configuration – Shells and Subshells | 电子排布 – 电子层与亚层

Electrons are arranged in shells (principal energy levels) around the nucleus. Each shell can hold up to 2n² electrons, where n is the shell number. Shells are divided into subshells: s, p, d, and f.

电子围绕原子核分布在电子层（主能级）中。每个电子层最多可容纳 2n² 个电子，其中 n 为电子层序数。电子层又可进一步分为 s、p、d、f 亚层。

s subshell: 1 orbital, max 2 electrons
s 亚层：包含 1 个轨道，最多容纳 2 个电子
p subshell: 3 orbitals, max 6 electrons
p 亚层：包含 3 个轨道，最多容纳 6 个电子
d subshell: 5 orbitals, max 10 electrons
d 亚层：包含 5 个轨道，最多容纳 10 个电子
f subshell: 7 orbitals, max 14 electrons
f 亚层：包含 7 个轨道，最多容纳 14 个电子

The order of filling is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Note that the 4s subshell fills before 3d because it has a lower energy.

电子填充顺序为 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p。注意4s 亚层先于 3d 填充，因为它的能量更低。

9. Principles of Electron Filling | 电子填充的原理

Three important rules govern electron configuration:

电子排布遵循三条重要规则：

Aufbau principle: electrons fill the lowest available energy orbitals first.
构造原理（Aufbau）：电子首先填充能量最低的可用轨道。
Pauli exclusion principle: each orbital holds a maximum of two electrons with opposite spins.
泡利不相容原理：每个轨道最多容纳两个自旋相反的电子。
Hund’s rule: electrons occupy degenerate orbitals singly before pairing up.
洪特规则：电子在简并轨道中先单独占据，然后才配对。

Example: The electron configuration of nitrogen (N, Z=7) is 1s² 2s² 2p³. The 2p electrons occupy three separate p orbitals singly before pairing occurs.

示例：氮（N，Z=7）的电子排布为 1s² 2s² 2p³。2p 的 3 个电子会先分别占据 3 个 p 轨道，然后才发生配对。

10. First Ionisation Energy | 第一电离能

First ionisation energy 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：

X(g) → X⁺(g) + e⁻

Factors affecting ionisation energy:

影响电离能的因素包括：

Atomic radius: larger radius means the outer electron is further from the nucleus, so ionisation energy is lower.
原子半径：半径越大，外层电子离核越远，电离能越低。
Nuclear charge: more protons means a greater attractive force, so ionisation energy is higher.
核电荷数：质子数越多，吸引力越强，电离能越高。
Electron shielding: inner electrons shield outer electrons from the full nuclear charge, reducing ionisation energy.
电子屏蔽效应：内层电子对外层电子产生屏蔽，削弱核电荷的吸引，从而降低电离能。

11. Successive Ionisation Energies | 逐级电离能

Successive ionisation energies refer to the energies needed to remove the 1st, 2nd, 3rd, etc., electrons from the same atom. A large jump in ionisation energy indicates that the electron is being removed from a new, inner shell.

逐级电离能是指从同一原子上依次移除第 1 个、第 2 个、第 3 个等电子所需的能量。电离能出现巨大突跃，表明电子来自更内层的电子壳层。

For sodium (1s² 2s² 2p⁶ 3s¹), the 1st ionisation energy is relatively low, but the 2nd is much higher because the second electron is removed from the 2p subshell, which is closer to the nucleus and more strongly attracted.

对于钠（1s² 2s² 2p⁶ 3s¹），第一电离能相对较低，但第二电离能显著升高，因为第二个电子是从更靠近原子核的 2p 亚层中移出的，受到的吸引力更强。

This data helps us determine the number of outer shell electrons and infer the group number of an element.

通过分析逐级电离能的突跃，可以帮助我们判断元素的最外层电子数目，进而推测其所在的主族。

12. Periodicity in Ionisation Energy | 电离能的周期性

First ionisation energy generally increases across a period and decreases down a group. These trends are explained by the factors of nuclear charge, atomic radius, and shielding.

第一电离能总体上在同一周期从左到右逐渐增大，在同一个族中自上而下逐渐减小。这些趋势可以通过核电荷数、原子半径和屏蔽效应来解释。

Across a period: nuclear charge increases and radius decreases, so outer electrons are held more tightly.
在同一周期内：核电荷增加，半径减小，外层电子受到更强的束缚。
Down a group: atomic radius increases and shielding increases, outweighing the effect of increased nuclear charge.
在同一族中：原子半径增大，屏蔽效应增强，这些因素超过核电荷增加的影响。

There are small drops in the trend across a period, e.g. from Mg to Al and from P to S, which are due to subshell changes and electron pairing. These details are often tested in exams.

同一周期中会出现小幅下降，例如从 Mg 到 Al 以及从 P 到 S，原因在于亚层变化和电子配对。这些细节在考试中经常出现。

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Atomic Structure | 原子结构