📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
Understanding atomic structure and how elements are arranged in the Periodic Table is central to CIE IGCSE Chemistry. These ideas explain why elements react, how they bond, and why trends occur across periods and down groups. This article summarises the key syllabus points for revision and exam practice.
理解原子结构以及元素在周期表中的排列方式是 CIE IGCSE 化学的核心内容。这些概念可以解释元素为什么会发生反应、如何成键，以及为什么在周期表中横向和纵向会出现趋势。本文概述了教学大纲中的关键考点，便于复习和备考。
1. Subatomic Particles | 亚原子粒子
All atoms contain three types of subatomic particle: protons, neutrons and electrons. Protons carry a positive charge, electrons carry a negative charge, and neutrons have no charge. The protons and neutrons are located in the nucleus, so the nucleus is positively charged and contains almost all the mass of the atom. Electrons orbit the nucleus in shells and have very little mass.
所有原子都含有三种亚原子粒子：质子、中子和电子。质子带正电，电子带负电，中子不带电。质子和中子位于原子核内，因此原子核带正电并集中了几乎全部原子质量。电子在核外的电子层中运动，质量非常小。
The relative masses and relative charges of these particles can be summarised as follows:
这些粒子的相对质量和相对电荷可以归纳如下：
- Proton: relative mass 1, relative charge +1 — 质子：相对质量 1，相对电荷 +1。
- Neutron: relative mass 1, relative charge 0 — 中子：相对质量 1，相对电荷 0。
- Electron: relative mass 1/1836, relative charge -1 — 电子：相对质量 1/1836，相对电荷 -1。
Atoms are electrically neutral because the number of protons equals the number of electrons. If an atom gains or loses electrons, it becomes a charged particle called an ion.
原子呈电中性，因为质子数等于电子数。如果原子得到或失去电子，就变成带电的粒子，称为离子。
2. Atomic Number and Mass Number | 原子序数与质量数
The atomic number, often written as Z, is the number of protons in the nucleus of an atom. It defines which element an atom is. The mass number, often written as A, is the total number of protons plus neutrons in the nucleus. For a neutral atom, the number of electrons equals the number of protons.
原子序数通常写作 Z，是原子核内质子的数目，它决定了元素种类。质量数通常写作 A，是原子核内质子数与中子数之和。对于中性原子，电子数等于质子数。
For example, a sodium atom has 11 protons and 12 neutrons. Its atomic number is 11 and its mass number is 23. The number of neutrons can be found using the relationship below.
例如，一个钠原子有 11 个质子和 12 个中子。它的原子序数是 11，质量数是 23。中子数可以用下面的关系式求出。
number of neutrons = mass number A - atomic number Z
For ions, the number of protons stays fixed, but the number of electrons changes. A positive ion has fewer electrons than protons; a negative ion has more electrons than protons.
对于离子，质子数保持固定，但电子数发生变化。正离子比质子数少几个电子；负离子比质子数多几个电子。
3. Isotopes | 同位素
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers. Isotopes have identical chemical properties because they have the same electron arrangement, but they may differ in physical properties such as density and mass.
同位素是同一种元素中质子数相同但中子数不同的原子。因此它们的原子序数相同，但质量数不同。同位素具有相同的化学性质，因为它们的电子排布相同，但物理性质如密度和质量可能不同。
A common example is chlorine. Naturally occurring chlorine consists of about 75% chlorine-35 and 25% chlorine-37. Both isotopes have 17 protons, but they have 18 and 20 neutrons respectively.
一个常见的例子是氯。天然存在的氯大约由 75% 的氯-35 和 25% 的氯-37 组成。两种同位素都有 17 个质子，但分别有 18 个和 20 个中子。
The relative atomic mass of an element is the weighted average mass of its isotopes compared with 1/12 of the mass of a carbon-12 atom. It can be calculated from isotopic abundances using the formula below.
元素的相对原子质量是其同位素质量的加权平均值，与碳-12 原子质量的 1/12 相比。它可以通过下面的公式从同位素丰度计算得出。
Relative atomic mass = Σ (isotopic mass × percentage abundance) ÷ 100
4. Electronic Configuration | 电子排布
Electrons are arranged in shells around the nucleus. For the first 20 elements at IGCSE level, the first shell can hold up to 2 electrons, the second shell up to 8, and the third shell up to 8. Electrons fill the lowest energy shells first before moving to higher shells.
电子按壳层排列在原子核外。对 IGCSE 范围的前 20 号元素，第一层最多容纳 2 个电子，第二层最多容纳 8 个，第三层最多容纳 8 个。电子先填充能量较低的壳层，再进入较高壳层。
The electronic configuration is written as a series of numbers separated by commas or full stops. For example, hydrogen is 1, helium is 2, sodium is 2,8,1, and argon is 2,8,8.
电子排布用一系列数字表示，数字之间用逗号或点分隔。例如，氢是 1，氦是 2，钠是 2,8,1，氩是 2,8,8。
The number of electrons in the outer shell is equal to the group number for Groups I to VII. This is why elements in the same group have similar chemical reactions.
对第 I 族到第 VII 族，最外层电子数等于族序数。这就是为什么同一族的元素具有相似的化学反应。
5. The Periodic Table: Organisation | 元素周期表：编排方式
The Periodic Table arranges elements in order of increasing atomic number. Elements are placed in vertical columns called groups and horizontal rows called periods. Elements in the same group have the same number of outer-shell electrons and therefore similar chemical properties.
元素周期表按原子序数递增的顺序排列元素。元素排成纵列（族）和横排（周期）。同一族的元素具有相同的最外层电子数，因此化学性质相似。
The group number helps predict ion charge and reactivity. For example, Group I elements form +1 ions, Group II elements form +2 ions, Group VI elements form -2 ions, and Group VII elements form -1 ions.
族序数有助于预测离子电荷和反应活性。例如，第 I
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