📚 Mastering the Periodic Table | 掌握元素周期表
The periodic table is a cornerstone of chemistry and a key topic in the Edexcel IGCSE Science syllabus. It organises all known elements according to their atomic structure and properties, allowing students to predict chemical behaviour and reactions. This revision guide breaks down the essential knowledge into digestible sections, with bilingual explanations to reinforce understanding.
元素周期表是化学的基石，也是 Edexcel IGCSE 科学大纲中的核心主题。它按照原子的结构和性质排列所有已知元素，帮助我们预测化学行为和反应。这篇复习指南将关键知识分解为易于理解的章节，并配以中英双语解释，以强化学习效果。
1. The History of the Periodic Table | 元素周期表的历史
In the 1860s, Dmitri Mendeleev arranged the known elements in order of increasing atomic weight and grouped them by similar chemical properties. He left gaps for undiscovered elements and predicted their properties with remarkable accuracy.
19世纪60年代，德米特里·门捷列夫按照原子量递增的顺序排列已知元素，并根据相似的化学性质进行分组。他为尚未发现的元素留下空位，并惊人准确地预测了这些元素的性质。
Later, Henry Moseley established that elements are better arranged by atomic number (proton number) rather than atomic weight. This modern periodic law corrected discrepancies in Mendeleev’s table and gave each element a unique, sequential position.
后来，亨利·莫塞莱确定元素应按原子序数（质子数）而非原子量排列。这一现代周期律修正了门捷列夫表中的不一致之处，使每个元素都有唯一且连续的位置。
2. Atomic Structure and Electron Configuration | 原子结构与电子排布
An atom consists of a nucleus containing protons and neutrons, surrounded by electrons in shells (energy levels). The atomic number Z equals the number of protons, while the mass number A equals the total number of protons and neutrons.
原子由包含质子和中子的原子核以及围绕核运动的电子壳层（能级）组成。原子序数 Z 等于质子数，质量数 A 等于质子数和中子数之和。
Electrons occupy shells in order of increasing energy. For example, sodium has the electronic configuration 2.8.1. The outermost shell is called the valence shell, and its electrons determine how the atom reacts chemically.
电子按能量递增的顺序填充壳层。例如，钠的电子排布为 2.8.1。最外层称为价电子层，其电子决定原子的化学性质。
3. Arrangement of Elements: Periods and Groups | 元素排列：周期与族
The modern periodic table places elements in order of increasing atomic number. Horizontal rows are called periods, and vertical columns are called groups (or families). Each period corresponds to the filling of a new electron shell.
现代元素周期表按原子序数递增排列元素。横的行为“周期”，纵的列为“族”。每个周期对应一个新的电子壳层开始填充。
Elements in the same group have the same number of outer-shell electrons, so they exhibit similar chemical properties. Group number often indicates the number of valence electrons for main-group elements, which is essential for predicting ion charge and bonding.
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。主族元素的族号通常表示价电子数，这对预测离子电荷和成键方式至关重要。
4. Group 1: Alkali Metals | 第1族：碱金属
Group 1 elements (Li, Na, K, Rb, Cs, Fr) are soft, highly reactive metals with low density. They all have one electron in their outer shell, which they readily lose to form a 1+ ion, e.g. Na → Na⁺ + e⁻.
第1族元素（Li、Na、K、Rb、Cs、Fr）是柔软的、高反应性的金属，密度低。它们最外层只有一个电子，容易失去该电子形成 +1 离子，例如 Na → Na⁺ + e⁻。
Reactivity increases down the group. As atomic radius increases, the outer electron is farther from the nucleus and more easily lost. These metals react vigorously with water, producing hydrogen gas and an alkaline metal hydroxide solution, for example 2Na + 2H₂O → 2NaOH + H₂.
同一族中反应性自上而下增强。随着原子半径增大，最外层电子离核更远，更容易失去。这些金属与水剧烈反应，产生氢气和碱性的金属氢氧化物溶液，例如 2Na + 2H₂O → 2NaOH + H₂。
5. Group 7 (17): Halogens | 第7族（17）：卤素
Group 7 elements (F, Cl, Br, I, At) are non-metals with seven valence electrons. They tend to gain one electron to form a 1− ion, such as Cl + e⁻ → Cl⁻. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid.
第7族元素（F、Cl、Br、I、At）是非金属，具有七个价电子。它们倾向于获得一个电子形成 −1 离子，例如 Cl + e⁻ → Cl⁻。室温下，氟和氯是气体，溴是液体，碘是固体。
Reactivity decreases down the group because the incoming electron is placed further from the nucleus and experiences greater shielding. Chlorine can displace bromine from potassium bromide solution: Cl₂ + 2KBr → 2KCl + Br₂. This shows that chlorine is more reactive than bromine.
同一族中反应性自上而下减弱，因为新进入的电子离核更远，屏蔽效应更强。氯能从溴化钾溶液中置换出溴：Cl₂ + 2KBr → 2KCl + Br₂。这说明氯的反应性强于溴。
6. Group 8 (18): Noble Gases | 第8族（18）：稀有气体
Group 8 (Group 0) elements include helium, neon, argon, krypton, xenon and radon. They have a full outer shell of electrons, giving them a stable configuration. As a result, they are monatomic and chemically inert under ordinary conditions.
第8族（0族）元素包括氦、氖、氩、氪、氙、氡。它们的最外层电子已经满额，因此具有稳定构型。所以它们在常温下为单原子气体，化学性质极不活泼。
Although unreactive, noble gases have applications based on their physical properties. Argon is used in welding as an inert shield, and helium is used in balloons and airships because it is less dense than air and non-flammable.
虽然稀有气体不活泼，但其物理性质仍有很多应用。氩在焊接中用作惰性保护气；氦用于气球和飞艇，因其比空气轻且不可燃。
7. Trends Across a Period | 同一周期中的递变规律
Going from left to right across a period, the atomic number increases and electrons fill the same outer shell. The number of protons increases, so the nuclear charge increases. The outer electrons are pulled closer to the nucleus, causing a decrease in atomic radius.
在一个周期内从左到右，原子序数增大，电子填充同一个最外层壳层。质子数增加导致核电荷增大。最外层电子被更强烈地拉向原子核，原子半径减小。
This greater nuclear attraction also results in a higher first ionisation energy and higher electronegativity. Metallic character decreases; elements change from metals to metalloids to non-metals, reaching a noble gas at the end of the period.
更强的核吸引导致第一电离能和电负性增大。金属性减弱，元素从金属过渡到类金属再到非金属，周期末尾以稀有气体结束。
8. Trends Down a Group | 同一族中的递变规律
Moving down a group, each element gains an extra electron shell. The atomic radius increases, and the outermost electrons are both further from the nucleus and more effectively shielded by inner shells. This reduces the attraction between the nucleus and valence electrons.
在同一族中自上而下，每个元素增加一个电子壳层。原子半径增大，最外层电子离核更远，同时受到内层电子的有效屏蔽。这减弱了原子核与价电子之间的吸引。
Consequently, metallic elements lose electrons more easily, so ionisation energy decreases. For non-metals such as the halogens, the tendency to gain an electron decreases, making them less reactive down the group.
因此，金属元素更容易失去电子，电离能降低。对于卤素等非金属，获得电子的倾向减弱，因而反应性自上而下降低。
9. Metals, Non-metals and Metalloids | 金属、非金属与类金属
Metals are found on the left and center of the periodic table. They typically conduct electricity and heat well, are malleable and ductile, and form positive ions. Their physical properties arise from the sea of delocalised electrons in metallic bonding.
金属位于周期表的左侧和中前部。它们通常导电导热性能好，具有延展性和可锻性，并能形成正离子。这些物理性质源于金属键中自由电子的“电子海”模型。
Non-metals are found on the upper right of the periodic table. They conduct poorly, are brittle in solid form, and often form covalent bonds or negative ions. Metalloids, such as silicon and germanium, have intermediate properties and are used as semiconductors.
非金属位于周期表右上方。它们导电性差，固体时易脆，通常形成共价键或负离子。类金属如硅、锗性质介于金属和非金属之间，常用作半导体材料。
10. Ions and Ionic Bonding | 离子与离子键
Ions are atoms or groups of atoms that have gained or lost electrons. Metals lose electrons to form positive cations, while non-metals gain electrons to form negative anions. The charges are directly linked to the group number: Group 1 → 1⁺, Group 2 → 2⁺, Group 6 → 2⁻, Group 7 → 1⁻.
离子是得到或失去电子的原子或原子团。金属失去电子形成正离子（阳离子），非金属得到电子形成负离子（阴离子）。电荷与族号密切相关：第1族 → +1，第2族 → +2，第6族 → −2，第7族 → −1。
Ionic bonding is the electrostatic attraction between oppositely charged ions. For example, sodium and chlorine form sodium chloride: Na loses one electron to become Na⁺, and Cl gains it to become Cl⁻. In the solid state, ions are arranged in a giant lattice structure, which gives ionic compounds high melting points and electrical conductivity when molten or dissolved.
离子键是带相反电荷离子之间的静电吸引力。例如钠和氯形成氯化钠：钠失去一个电子成为 Na⁺，氯获得该电子成为 Cl⁻。固态时离子排列成巨大晶格结构，使离子化合物具有高熔点，并在熔融或溶解状态下导电。
11. Covalent Bonding and Molecules | 共价键与分子
When non-metal atoms bond, they often share electrons to achieve a full outer shell. Each shared pair of electrons forms a covalent bond. For example, hydrogen molecules have a single H–H bond, and oxygen molecules have a double O=O bond.
当非金属原子成键时，它们通常通过共享电子达到满外层。每一对共享电子形成一个共价键。例如，氢分子具有单键 H–H，氧分子具有双键 O=O。
Covalent substances may form simple molecules with weak intermolecular forces, leading to low melting and boiling points. Alternatively, they can form giant covalent structures, such as diamond or silicon dioxide, where atoms are linked in a network and the material is hard and has a high melting point.
共价物质可以形成简单分子，分子间作用力弱，因而熔沸点低。也可以形成巨大共价结构，如金刚石或二氧化硅，原子以网状连接，材料坚硬且熔点高。
12. Using the Periodic Table for Chemical Equations | 利用周期表配平化学方程式
The periodic table helps determine ion charges and oxidation states, which are essential for writing correct chemical formulas. For example, magnesium forms Mg²⁺ and chlorine forms Cl⁻, so the formula of magnesium chloride is MgCl₂, not MgCl.
元素周期表帮助我们确定离子电荷和化合价，这是书写正确化学式的关键。例如，镁形成 Mg²⁺，氯形成 Cl⁻，因此氯化镁的化学式是 MgCl₂，而不是 MgCl。
Balancing equations also relies on atomic numbers and symbols. Conservation of mass requires equal numbers of atoms of each element on both sides. For instance, the reaction between calcium and oxygen is written as 2Ca + O₂ → 2CaO, with the coefficient 2 placed before Ca and CaO to balance the equation.
配平方程式还依赖于原子序数和元素符号。质量守恒要求反应前后每种元素的原子数相等。例如，钙与氧的反应写作 2Ca + O₂ → 2CaO，在 Ca 和 CaO 前加上系数 2 以配平。
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