📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表
This article covers the core ideas of atomic structure and the periodic table required for the Edexcel IGCSE Science specification. You will learn about the particles that make up atoms, how electrons are arranged, and why elements in the same group share similar chemical properties.
本篇文章涵盖 Edexcel IGCSE 科学课程大纲中关于原子结构和元素周期表的核心内容。你将了解组成原子的粒子、电子的排布方式，以及为什么同一族的元素具有相似的化学性质。
1. The Atom | 原子的组成
An atom is the smallest particle of an element that can take part in a chemical reaction. It consists of a central nucleus surrounded by electrons.
原子是元素的最小粒子，能够参与化学反应。它由一个位于中心的原子核以及围绕核运动的电子组成。
The nucleus contains protons and neutrons and is very small compared to the whole atom.
Electrons move around the nucleus in energy levels or shells.
Most of the atom is empty space.
原子核包含质子和中子，与整个原子相比非常小。
电子在能级或电子壳层中围绕原子核运动。
原子内部大部分是空旷的空间。
2. Subatomic Particles | 亚原子粒子
Protons, neutrons and electrons are called subatomic particles. Their properties are summarised below.
质子、中子和电子被称为亚原子粒子。它们的性质总结如下。
| Particle | Charge | Mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | -1 | 1/1840 |
Protons and neutrons have approximately the same mass. Electrons are extremely light and have negligible mass compared with protons and neutrons.
质子和中子的质量大约相同。电子的质量极轻，与质子、中子相比可以忽略不计。
3. Atomic Number and Mass Number | 原子序数与质量数
The atomic number (Z) is the number of protons in an atom. The mass number (A) is the total number of protons and neutrons.
原子序数（Z）是原子中质子的数目。质量数（A）是质子和中子的总数。
Atomic number = number of protons
原子序数 = 质子数
Mass number = protons + neutrons
质量数 = 质子数 + 中子数
In a neutral atom, the number of electrons equals the number of protons. The number of neutrons can be found using this equation:
在电中性的原子中，电子数等于质子数。中子的数目可以用下面这个方程求得：
Neutrons = mass number − atomic number
中子数 = 质量数 − 原子序数
4. Isotopes | 同位素
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.
同位素是同一元素的不同原子，它们具有相同的质子数，但中子数不同。它们的原子序数相同，但质量数不同。
Example: chlorine-35 and chlorine-37 are isotopes of chlorine.
Chemical properties of isotopes are the same because the electron arrangement is identical.
Physical properties such as melting point and density may differ slightly.
例如：氯-35 和 氯-37 是氯元素的同位素。
同位素的化学性质相同，因为电子排布完全一致。
物理性质如熔点、密度可能略有不同。
5. Electron Configuration | 电子排布
Electrons occupy shells around the nucleus. The first shell can hold up to 2 electrons, the second shell up to 8, and the third shell up to 8 in the first 20 elements.
电子在原子核周围占据不同的电子壳层。第一层最多容纳 2 个电子，第二层最多 8 个，第三层在前 20 号元素中最多 8 个。
For example, sodium has 11 electrons. Its electron configuration is 2, 8, 1. The arrangement of electrons determines how the atom reacts chemically.
例如，钠有 11 个电子，它的电子排布是 2, 8, 1。电子的排列方式决定了原子如何发生化学反应。
The reactivity of an atom is mainly controlled by the number of electrons in its outermost shell.
原子的反应活性主要由它的最外层电子数决定。
6. The Periodic Table | 元素周期表
In the modern periodic table, elements are arranged in order of increasing atomic number. Rows are called periods and columns are called groups.
在现代元素周期表中，元素按照原子序数递增的顺序排列。横行称为周期，纵列称为族。
Group number tells you how many electrons are in the outer shell for elements in groups 1, 2 and 3.
Period number tells you how many occupied electron shells an atom has.
Group 1 elements have 1 outer electron, Group 2 have 2, and Group 7 have 7.
对于第 1、2、3 族元素，族号表示最外层电子数。
周期号表示原子中占用电子壳层的数目。
第 1 族元素有 1 个最外层电子，第 2 族有 2 个，第 7 族有 7 个。
7. Group Trends | 族规律
Elements in the same group have similar chemical properties. This is because they have the same number of outer electrons.
同一族的元素具有相似的化学性质，这是因为它们的最外层电子数相同。
Group 1 metals, also called alkali metals, become more reactive down the group. Their melting points decrease and they are soft.
第 1 族金属也叫碱金属，从上到下反应性增强，熔点降低，而且质地较软。
Group 7 elements, called halogens, become less reactive down the group. Their melting points increase.
第 7 族元素称为卤素，从上到下反应性减弱，熔点升高。
8. Ions and Ionic Bonds | 离子与离子键
Atoms can lose or gain electrons to form charged particles called ions. Metals tend to lose electrons and become positive ions. Non-metals tend to gain electrons and become negative ions.
原子可以失去或获得电子，从而形成带电粒子，称为离子。金属倾向于失去电子，成为正离子；非金属倾向于获得电子，成为负离子。
An ionic bond is the electrostatic attraction between oppositely charged ions. For example, sodium chloride contains Na⁺ and Cl⁻ ions.
离子键是带相反电荷离子之间的静电吸引力。例如，氯化钠含有 Na⁺ 和 Cl⁻ 离子。
Sodium atom (Na) → sodium ion (Na⁺) + electron
钠原子（Na）→ 钠离子（Na⁺）+ 电子
9. Covalent Bonds | 共价键
A covalent bond forms when two non-metal atoms share a pair of electrons. Each shared pair produces one covalent bond.
当两个非金属原子共用一对电子时，就形成共价键。每共用一对电子形成一个共价键。
Hydrogen gas (H₂) has one covalent bond between two hydrogen atoms.
Water (H₂O) has two O–H covalent bonds.
Methane (CH₄) has four C–H covalent bonds.
氢气（H₂）的两个氢原子之间有一个共价键。
水（H₂O）有两个 O–H 共价键。
甲烷（CH₄）有四个 C–H 共价键。
Covalent substances can exist as simple molecules or giant covalent structures such as diamond and graphene.
共价物质可以以简单分子或巨型共价结构存在，例如金刚石和石墨烯。
10. Relative Atomic Mass | 相对原子质量
The relative atomic mass (Aᵣ) of an element is the weighted mean mass of its atoms compared with 1/12 of the mass of a carbon-12 atom.
相对原子质量（Aᵣ）是元素的平均原子质量与一个碳-12原子质量的1/12相比的值。
For elements with isotopic abundance data, the relative atomic mass is calculated using the equation:
对于具有同位素丰度数据的元素，相对原子质量使用以下公式计算：
Relative atomic mass = (mass × abundance) total / 100
相对原子质量 =（质量 × 丰度）总和 / 100
Example: chlorine has two main isotopes: ⁷⁵Cl is 75% and ³⁷Cl is 25% abundance. Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5
示例：氯有两种主要同位素：⁷⁵Cl 占 75%，³⁷Cl 占 25%。Aᵣ = (35 × 75 + 37 × 25) / 100 = 35.5
11. Symbols and Formulae | 符号与化学式
Every element is represented by a chemical symbol. The first letter is always capitalised and the second letter, if present, is lowercase.
每种元素都有一个化学符号。第一个字母总是大写，第二个字母如果存在则小写。
Examples: H, He, C, Na, Cl, Mg
示例：H、He、C、Na、Cl、Mg
Chemical formulae show the ratio of atoms in a compound. For example, CaCO₃ contains one calcium atom, one carbon atom and three oxygen atoms.
化学式表示化合物中原子比例。例如 CaCO₃ 含有一个钙原子、一个碳原子和三个氧原子。
12. Key Exam Tips | 实用考试技巧
In Edexcel IGCSE exams, you should be able to calculate protons, neutrons and electrons from mass and atomic numbers. You should also be able to draw and interpret dot-and-cross diagrams for simple molecules.
在 Edexcel IGCSE 考试中，你应该能够根据质量数和原子序数计算质子、中子、电子的数目，还应能够绘制并解读简单分子的电子点叉图。
Always check whether the atom is charged. Positive ions have fewer electrons than protons.
When writing electronic configuration, write the shell numbers in order, e.g. 2,8,3 for aluminium.
Use the word 'electrostatic' when describing ionic or metallic bonding.
始终检查原子是否带电荷。正离子的电子数少于质子数。
书写电子排布时，按壳层顺序写，例如铝为 2,8,3。
描述离子键或金属键时，使用"静电"一词。
Make sure you revise the properties of groups 1 and 7, and that you can explain trends in reactivity using electron shells.
务必修习第 1 族和第 7 族的性质，并能够用电子壳层解释反应性的变化规律。
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