📚 AS Inorganic Chemistry: Periodicity, Group 2 and Group 7 | AS 无机化学：周期性、第2族与第7族
This revision guide covers the core AS inorganic chemistry topics for the Oxford AQA International A-level specification: periodicity, Group 2 and Group 7. It explains key trends, reactions and tests you need to apply in topic tests and exam questions.
本复习指南涵盖牛津 AQA 国际 A-level 考试大纲中 AS 无机化学的核心主题：周期性、第2族和第7族。它解释了你需要在单元测验和考试题目中应用的关键规律、反应和检验方法。
1. Periodicity and the Periodic Table | 周期性与元素周期表
Periodicity is the repeating pattern of physical and chemical properties across a period. In the Periodic Table, elements are arranged by increasing atomic number, and elements with similar outer-shell electron configurations fall into the same group.
周期性是指元素性质在同一周期中呈现的重复变化规律。在元素周期表中，元素按原子序数递增排列，外层电子排布相似的元素位于同一族。
For AS inorganic chemistry, you must be able to classify elements into s, p and d blocks using their highest-energy electrons: s-block elements have outer electrons in an s orbital, p-block elements in a p orbital, and d-block elements in a d orbital.
在 AS 无机化学中，你必须能够根据元素最高能级电子所在的轨道将元素分为 s 区、p 区和 d 区：s 区元素的最外层电子在 s 轨道，p 区元素在 p 轨道，d 区元素在 d 轨道。
2. Atomic Radius Trend Across Period 3 | 第3周期原子半径的变化趋势
Across Period 3 (Na to Ar), atomic radius decreases. The number of protons increases, so nuclear charge increases, while electrons are added to the same outer shell. This pulls the outer electrons closer to the nucleus.
沿着第3周期（钠到氩），原子半径逐渐减小。质子数增多，核电荷数增大，而电子都填充在同一外层电子层中。这使得外层电子被更强烈地拉向原子核。
Shielding from inner electrons stays roughly constant across the period, so the increased nuclear charge is not fully cancelled. As a result, the force of attraction on the outer electrons rises.
同一周期内层电子的屏蔽作用基本不变，因此核电荷的增加不会被完全抵消。结果，外层电子受到的吸引力增强。
3. First Ionisation Energy Trends | 第一电离能的变化趋势
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. Across a period, first ionisation energy generally increases, but with drops between certain pairs, such as Mg to Al and P to S.
第一电离能是指从一摩尔气态原子中移走一摩尔电子形成一摩尔气态 1+ 离子所需的能量。沿周期从左到右，第一电离能总体上升，但某些相邻元素之间会出现下降，例如 Mg 到 Al、P 到 S。
The drop from Mg (3s²) to Al (3p¹) occurs because the 3p electron is higher in energy and slightly shielded by the 3s electrons, so it is easier to remove. The drop from P (3p³) to S (3p⁴) occurs because sulfur has paired electrons in one 3p orbital, and electron-electron repulsion makes it easier to remove the paired electron.
从 Mg (3s²) 到 Al (3p¹) 电离能下降，因为 3p 电子能量更高且受到 3s 电子的一定屏蔽，因此较易移走。从 P (3p³) 到 S (3p⁴) 下降，因为硫的一个 3p 轨道中有一对成对电子，电子间排斥使移走这对电子中的一个更容易。
4. Electronegativity and Melting Points | 电负性与熔点
Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. Across Period 3, electronegativity increases from sodium to chlorine because nuclear charge increases and atomic radius decreases, making the nucleus better able to attract the bonding electrons.
电负性是指原子在共价键中吸引成键电子对的能力。沿着第3周期从钠到氯，电负性增大，因为核电荷增加而原子半径减小，使原子核更容易吸引成键电子。
Melting points across Period 3 show a general pattern linked to structure: Na, Mg and Al are metallic with high melting points (Al higher than Mg because Al has more delocalised electrons and a smaller ion); Si is a giant covalent structure with a very high melting point; P₄, S₈ and Cl₂ are simple molecular with low melting points; Ar is monatomic with the lowest.
第3周期元素的熔点与其结构相关：Na、Mg、Al 为金属，熔点较高（Al 高于 Mg，因为 Al 的离域电子更多且离子半径更小）；Si 为巨型共价结构，熔点非常高；P₄、S₈ 和 Cl₂ 为简单分子，熔点低；Ar 为单原子分子，熔点最低。
5. Group 2: Physical Properties and Trends |
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