📚 4.9.1 Atomic Structure | 4.9.1 原子结构
Understanding the structure of the atom is fundamental to both chemistry and physics in the Edexcel IGCSE Science curriculum. This topic, often labelled 4.9.1 in the Physics specification and covered within Principles of Chemistry, explores the subatomic particles, how they are arranged, and how atoms are represented symbolically. Mastery of this content is essential for explaining chemical bonding, radioactivity, and properties of elements.
理解原子结构是Edexcel IGCSE科学课程中化学和物理的基础。这一主题通常在物理大纲中标为4.9.1，并在化学原理部分涉及，探讨亚原子粒子、它们的排列方式以及原子的符号表示。掌握这些内容对于解释化学键、放射性和元素性质至关重要。
1. Fundamental Particles – Protons, Neutrons and Electrons | 基本粒子——质子、中子和电子
All atoms consist of three types of subatomic particles: protons, neutrons and electrons. Protons and neutrons are located in the nucleus, while electrons orbit the nucleus in shells. The table below summarises their relative masses and charges.
所有原子都由三种亚原子粒子组成：质子、中子和电子。质子和中子位于原子核内，而电子在核外的电子层中运动。下表总结了它们的相对质量和电荷。
| Particle / 粒子 | Relative mass / 相对质量 | Relative charge / 相对电荷 | Location / 位置 |
|---|---|---|---|
| Proton / 质子 | 1 | +1 | Nucleus / 原子核 |
| Neutron / 中子 | 1 | 0 | Nucleus / 原子核 |
| Electron / 电子 | 1/1836 (negligible) | -1 | Shells around nucleus / 核外电子层 |
The relative mass of an electron is so small that it is often ignored when calculating the mass of an atom. The nucleus therefore contains virtually all the mass.
电子的相对质量极小，在计算原子质量时常被忽略。因此，原子核几乎包含了原子的全部质量。
2. The Nuclear Model of the Atom | 原子的核模型
According to the nuclear model, an atom consists of a small, dense nucleus containing protons and neutrons, surrounded by electrons moving in discrete energy levels (shells). The nucleus is positively charged because of the protons, while the electrons create a negative cloud that balances the charge in a neutral atom.
根据核模型，原子由一个致密的小原子核（包含质子和中子）以及在其周围分立的能级（电子层）中运动的电子构成。原子核因质子而带正电荷，电子则形成负电云，在中性原子中平衡电荷。
The diameter of the nucleus is approximately 1/10,000 of the diameter of the atom, meaning most of the atom is empty space. This model explains why alpha particles in Rutherford's scattering experiment mostly passed straight through gold foil, with very few being deflected.
原子核的直径大约是原子直径的万分之一，这意味着原子内部大部分是空的。这一模型解释了为何在卢瑟福散射实验中，绝大多数α粒子径直穿过金箔，只有极少数发生偏转。
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 of an atom. It determines the identity of the element. For a neutral atom, the number of electrons equals the atomic number. The mass number (A) is the total number of protons and neutrons in the nucleus.
原子序数（Z）是原子核中质子的数目，它决定了元素的种类。对于中性原子，电子数等于原子序数。质量数（A）是原子核中质子与中子数之和。
carbon atom with A = 12 and Z = 6 has 6 protons, 6 electrons and 6 neutrons (12 - 6 = 6).
利用这些数字，中子数可以通过质量数减去原子序数求得：中子数 = A - Z。例如，质量数为12、原子序数为6的碳原子有6个质子、6个电子和6个中子（12 - 6 = 6）。
4. Atomic Symbol Notation | 原子符号表示法
In the Edexcel IGCSE, atoms are often represented using standard notation: the mass number is written as a superscript before the element symbol, and the atomic number as a subscript. For example, the most common isotope of uranium is shown as ²³⁵₉₂U. The lower number is the atomic number (92 protons), and the upper number is the mass number (235 nucleons).
在Edexcel IGCSE中，原子常使用标准符号表示：质量数以元素符号前的上标形式书写，原子序数作为下标。例如，最常见的铀同位素表示为²³⁵₉₂U。下标是原子序数（92个质子），上标是质量数（235个核子）。
When writing symbols, ensure the mass number is always larger; the atomic number can be omitted if the element is clearly identified. For chlorine-35, you may write ³⁵₁₇Cl or simply ³⁵Cl. In calculations, it is vital to correctly interpret such notation.
书写符号时，务必确保质量数总是更大；若元素已明确，原子序数可省略。对于氯-35，可以写作³⁵₁₇Cl或简写³⁵Cl。在计算中，正确解读此种符号至关重要。
5. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Hence, isotopes have the same atomic number but different mass numbers. For instance, hydrogen has three isotopes: protium (¹H), deuterium (²H), and tritium (³H), with 0, 1 and 2 neutrons respectively.
同位素是具有相同质子数但中子数不同的同种元素原子。因此，同位素的原子序数相同，但质量数不同。例如，氢有三种同位素：氕（¹H）、氘（²H）和氚（³H），分别有0、1和2个中子。
Isotopes of an element have identical chemical properties because they have the same electron arrangement. However, their physical properties, such as density and rate of diffusion, can differ slightly due to the difference in mass. Some isotopes are radioactive (radioisotopes), such as ¹⁴C and ²³⁵U.
同一元素的同位素化学性质相同，因为它们的电子排布相同。然而，由于质量不同，其物理性质（如密度和扩散速率）可能略有差异。一些同位素具有放射性（放射性同位素），例如¹⁴C和²³⁵U。
6. Relative Atomic Mass (Aᵣ) | 相对原子质量（Aᵣ）
. It is calculated using the formula: Aᵣ = Σ (isotopic mass × percentage abundance) / 100.
元素的相对原子质量是该元素一个原子的加权平均质量，相对于一个碳-12原子质量的1/12。由于元素以同位素混合物形式存在，Ar通常不是整数。其计算公式为：Aᵣ = Σ（同位素质量 × 百分丰度）/ 100。
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