📚 Periodicity: Trends in the Periodic Table | 周期性：周期表中的趋势
In AQA A Level Chemistry, periodicity is the repeating pattern of physical and chemical properties across a period. By studying Period 3 elements from sodium to argon, you can link electronic structure to trends in atomic radius, first ionisation energy, melting point, electronegativity and electrical conductivity.
在 AQA A Level 化学中，周期性是指元素性质沿着周期呈现的重复变化规律。通过研究从钠到氩的第三周期元素，你可以把电子结构与原子半径、第一电离能、熔点、电负性和导电性等趋势联系起来。
1. Defining Periodicity and the Modern Periodic Table | 定义周期性与现代周期表
The modern periodic table arranges elements in order of increasing atomic number, not relative atomic mass. Horizontal rows are called periods and vertical columns are called groups. Elements in the same group have the same outer-shell electron configuration, so they show similar chemical behaviour.
现代周期表按原子序数递增排列元素，而不是按相对原子质量。横行为周期，纵列为族。同族元素具有相同的外层电子构型，因此表现出相似的化学性质。
The periodic table is divided into s, p, d and f blocks. The block name tells you which sub-shell is being filled by the highest-energy electrons. For AQA, you mainly need the s-block, p-block and the d-block transition metals.
周期表分为 s 区、p 区、d 区和 f 区。区的名称表示最高能级电子所填充的亚层。对 AQA 考试而言，主要需要掌握 s 区、p 区和 d 区过渡金属。
2. Electronic Structure and Block Classification | 电子结构与分区
An element is placed in a block according to its highest-energy sub-shell. For example, sodium has the electron configuration 1s² 2s² 2p⁶ 3s¹, so the 3s sub-shell is the outermost occupied sub-shell and sodium is in the s-block.
元素根据其最高能级电子所在的亚层划分区块。例如，钠的电子构型为 1s² 2s² 2p⁶ 3s¹，因此 3s 亚层是最外层占据的亚层，钠属于 s 区。
Aluminium has the configuration 1s² 2s² 2p⁶ 3s² 3p¹, so it is a p-block element because its highest-energy electron enters a 3p orbital. Transition metals such as scandium fill the 3d sub-shell, making them d-block elements.
铝的电子构型为 1s² 2s² 2p⁶ 3s² 3p¹，由于最高能级电子进入 3p 轨道，所以铝是 p 区元素。钪等过渡金属填充 3d 亚层，因此属于 d 区元素。
Na: 1s² 2s² 2p⁶ 3s¹ | Al: 1s² 2s² 2p⁶ 3s² 3p¹
3. Atomic Radius Across Period 3 | 第三周期原子半径趋势
Atomic radius decreases across Period 3 from sodium to chlorine. Each added proton increases the nuclear charge, while the added electron enters the same principal energy level, so shielding by inner shells remains approximately constant.
从钠到氯，第三周期原子半径逐渐减小。每增加一个质子，核电荷增加，而新增电子进入同一主能级，内层屏蔽作用基本保持不变。
The stronger attraction from the nucleus pulls the outer electrons closer, causing a smaller atomic radius. Argon is usually excluded from simple radius comparisons because it exists as isolated atoms.
原子核对最外层电子的吸引力增强，使外层电子被拉得更近，因此原子半径变小。氩通常不参与简单的半径比较，因为它以单原子形式存在。
4. First Ionisation Energy: General Rise | 第一电离能：总体上升
ion is X(g) → X⁺(g) + e⁻. Across Period 3, the general trend is an increase from Na to Ar.
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，方程式为 X(g) → X⁺(g) + e⁻。在第三周期中，从 Na 到 Ar，第一电离能总体呈上升趋势。
This rise is explained by the increasing nuclear charge and decreasing atomic radius, which strengthen the attraction between the nucleus and the outer electron. Shielding remains similar because the electrons are added to the same shell.
这一上升趋势可由核电荷增加和原子半径减小来解释，它们都增强原子核对外层电子的吸引力。由于电子加在同一电子层，屏蔽作用相近。
5. Dips in First Ionisation Energy at Al and S | 铝和硫的第一电离能下降
There are two important deviations from the general rise in first ionisation energy. Aluminium has a lower first ionisation energy than magnesium because the electron removed from aluminium is in a higher-energy 3p orbital, whereas magnesium loses an electron from a 3s orbital.
第一电离能的总体上升中有两个重要例外。铝的第一电离能低于镁，因为铝失去的电子位于能量较高的 3p 轨道，而镁失去的是 3s 轨道的电子。
Sulfur has a lower first ionisation energy than phosphorus because sulfur has paired electrons in one of its 3p orbitals. The repulsion between paired electrons makes it easier to remove one electron from sulfur.
硫的第一电离能低于磷，因为硫的其中一个 3p 轨道中有一对成对电子。成对电子之间的排斥作用使硫更容易失去一个电子。
Mg: 1s² 2s² 2p⁶ 3s² | Al: 1s² 2s² 2p⁶ 3s² 3p¹
P: 1s² 2s² 2p⁶ 3s² 3p³ | S: 1s² 2s² 2p⁶ 3s² 3p⁴
6. Melting Point Trend Across Period 3 | 第三周期熔点趋势
The melting points of Period 3 elements show a clear pattern linked to structure. Sodium, magnesium and aluminium are metals; their melting points increase from Na to Al because the metal ions have increasing charge and decreasing ionic radius, giving stronger metallic bonding with more delocalised electrons.
第三周期元素的熔点表现出与结构密切相关的规律。钠、镁和铝是金属；从 Na 到 Al 熔点升高，因为金属离子电荷增加、离子半径减小，金属键更强，离域电子更多。
Silicon has a giant covalent structure, so a very high temperature is needed to break the strong covalent bonds throughout the lattice. This gives silicon the highest melting point in Period 3.
硅具有巨型共价结构，因此需要很高的温度才能破坏整个晶格中的强共价键。这使硅成为第三周期中熔点最高的元素。
Phosphorus, sulfur and chlorine form simple molecular structures. Their melting points are much lower because only weak van der Waals forces exist between molecules. Sulfur S₈ has a higher melting point than phosphorus P₄ because S₈ molecules are larger and have stronger van der Waals forces.
磷、硫和氯形成简单分子结构。由于分子间只存在微弱的范德华力，它们的熔点要低得多。硫 S₈ 的熔点高于白磷 P₄，因为 S₈ 分子更大，范德华力更强。
Argon is monatomic with only very weak London dispersion forces between atoms, so it has the lowest melting point in Period 3.
氩是单原子分子，原子间只存在极弱的伦敦色散力，因此它在第三周期中熔点最低。
7. Electronegativity Across Period 3 | 第三周期电负性
Electronegativity is the power of an atom to attract the bonding pair of electrons in a covalent bond. Across Period 3, electronegativity generally increases from sodium to chlorine.
电负性是指原子在共价键中吸引成键电子对的能力。在第三周期中，从钠到氯，电负性总体上升。
The increase occurs because nuclear charge increases and atomic radius decreases. A smaller atom with a larger nuclear charge attracts the shared electron pair more strongly. Chlorine has a high electronegativity, while sodium has a low electronegativity.
电负性上升的原因是核电荷增加、原子半径减小。半径更小且核电荷更大的原子对共用电子对的吸引力更强。氯的电负性较高，而钠的电负性较低。
Argon is not assigned an electronegativity value under standard AQA comparisons because it does not readily form covalent bonds.
在 AQA 标准比较中，氩通常不赋予电负性数值，因为它不容易形成共价键。
8. Electrical Conductivity and Structure | 导电性与结构
Electrical conductivity across Period 3 moves from metallic conductors to a semiconductor to insulators. Na, Mg and Al conduct electricity because their delocalised electrons are free to move through the metallic lattice.
第三周期的导电性从金属导体过渡到半导体再到绝缘体。Na、Mg 和 Al 能够导电，因为它们的离域电子可以在金属晶格中自由移动。
Silicon is a semiconductor because its giant covalent structure contains electrons that can be promoted to conduct under certain conditions, but not as freely as in metals. Phosphorus, sulfur, chlorine and argon are non-conductors in their standard states.
硅是半导体，因为其巨型共价结构中的电子在一定条件下可以被激发而
Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图