📚 A-Level AQA Chemistry 2.1 Periodicity | A-Level AQA 化学 2.1 周期性
Periodicity is the study of repeating patterns in the properties of elements across the Periodic Table. In AQA A-Level Chemistry, Section 2.1 focuses on Period 3 and considers how atomic radius, ionisation energy, melting point and electronegativity change across a period, as well as the acid-base behaviour of the oxides and chlorides of the Period 3 elements.
周期性是研究元素周期表中元素性质重复性规律的重要内容。在 AQA A-Level 化学 2.1 节中，我们以第三周期为中心，探讨原子半径、电离能、熔点和电负性在一个周期内的变化规律，以及第三周期元素氧化物和氯化物的酸碱性行为。
1. The Periodic Table: Periods and Blocks | 1. 元素周期表：周期与区
The Periodic Table is arranged by increasing atomic number. Elements in the same period have the same number of electron shells, while elements in the same group have the same number of outer-shell electrons. AQA A-Level Chemistry expects you to classify elements into s, p and d blocks according to the subshell that is being filled.
元素周期表按照原子序数递增排列。同一周期的元素具有相同的电子层数，同一主族的元素具有相同的最外层电子数。AQA A-Level 化学要求你能够根据正在填充的亚层，将元素划分为 s 区、p 区和 d 区。
s-block: the final electron enters an s orbital (Groups 1 and 2).
s 区：最后一个电子进入 s 轨道（第 1、2 主族）。
p-block: the final electron enters a p orbital (Groups 3 to 0, including the noble gases).
p 区：最后一个电子进入 p 轨道（第 3 主族至 0 族，包括稀有气体）。
d-block: the final electron enters a d orbital (the transition metals).
d 区：最后一个电子进入 d 轨道（过渡金属）。
2. Period 3 Elements and Their Properties | 2. 第三周期元素及其性质
Period 3 contains the elements from sodium to argon: Na, Mg, Al, Si, P, S, Cl and Ar. Across this period, the number of protons increases from 11 to 18, and the number of occupied electron shells remains three. This explains many of the trends in physical and chemical properties.
第三周期包含从钠到氩的元素：Na、Mg、Al、Si、P、S、Cl 和 Ar。沿着这一周期，质子数从 11 增加到 18，而占据的电子层数保持为三层。这解释了许多物理和化学性质的变化趋势。
The outer-shell configuration changes from 3s¹ in sodium to 3s²3p⁶ in argon. The presence of a full 3s and 3p sub-shell in argon gives it a stable octet and makes it very unreactive.
最外层电子排布从钠的 3s¹ 变为氩的 3s²3p⁶。氩具有填满的 3s 和 3p 亚层，形成稳定的八隅体结构，因此非常不活泼。
3. Atomic Radius Across Period 3 | 3. 第三周期原子半径的变化趋势
Atomic radius decreases across Period 3 from sodium to chlorine. Although the number of electron shells stays the same, the nuclear charge increases by one proton at each step. The electrons are pulled closer to the nucleus because the increasing positive charge is not offset by extra shielding within the same shell.
从钠到氯，原子半径逐渐减小。虽然电子层数不变，但核电荷数每步增加一个质子。由于在同一层内增加额外电子不能带来更多的屏蔽效应，增大的正电荷会把电子拉得更靠近原子核。
Trend: Na > Mg > Al > Si > P > S > Cl
Noble gases are usually considered separately because atomic radius is measured as the van der Waals radius, which is larger than covalent radius for the same shell.
稀有气体通常单独讨论，因为原子半径测量的是范德华半径，同一电子层下它比共价半径更大。
4. First Ionisation Energy Across a Period | 4. 第一电离能沿周期的变化
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. The general trend across Period 3 is an increase from sodium to argon, because nuclear charge increases and atomic radius decreases.
第一电离能是指从一摩尔气态原子中移走一摩尔电子，形成一摩尔气态正一价离子所需的能量。第三周期从钠到氩的总体趋势是电离能增大，因为核电荷增加且原子半径减小。
General trend: Na < Mg < Al? No — ionisation energy increases overall, but there are two drops.
The first drop occurs between Mg and Al. Magnesium has a 3s² outer configuration, while aluminium has the outer configuration 3s²3p¹. The 3p electron in aluminium is higher in energy and slightly more shielded by the 3s electrons, so it is easier to remove.
第一次下降发生在镁和铝之间。镁的最外层是 3s²，而铝的最外层是 3s²3p¹。铝的 3p 电子能量较高，并且受到 3s 电子的额外屏蔽，因此更容易被移走。
The second drop occurs between phosphorus and sulfur. Phosphorus has a half-filled 3p³ sub-shell, which is relatively stable due to exchange energy. Sulfur's fourth p electron must pair with an existing electron in a 3p orbital, and the resulting electron-electron repulsion makes sulfur's first ionisation energy slightly lower than phosphorus.
第二次下降发生在磷和硫之间。磷具有半充满的 3p³ 亚层，由于交换能而相对稳定。硫的第四个 p 电子必须与已有的 3p 轨道电子配对，产生的电子-电子排斥作用使硫的第一电离能略低于磷。
Succeeding ionisation energies are important too. The jump in ionisation energy when removing an electron from a full inner shell, or when breaking into the next shell, helps explain the stable oxidation states of Period 3 elements.
后续各级电离能也很重要。当从填满的内层或更内层移走电子时，电离能会出现急剧跃升，这有助于解释第三周期元素的稳定氧化态。
5. Melting Points Across Period 3 | 5. 第三周期熔点的变化
The melting points across Period 3 show a pattern that depends on the type of bonding and structure. You should be able to explain each value in terms of metallic bonding, giant covalent structure, simple molecular structure, and noble gas monatomic structure.
第三周期熔点的变化模式取决于键型和结构类型。你需要能够从金属键、巨型共价结构、简单分子结构和稀有气体单原子结构的角度解释每个数值。
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Sodium, magnesium and aluminium are metals. Melting point increases from Na to Al because the number of delocalised electrons per atom increases and the ionic charge increases, giving stronger metallic bonding.
钠、镁和铝是金属。从 Na 到 Al 熔点升高，因为每个原子提供的离域电子数增多，离子电荷增大，金属键增强。
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Silicon has a giant covalent structure with strong covalent bonds in a diamond-like lattice. Its melting point is very high.
硅具有巨型共价结构，在类似金刚石的晶格中充满强共价键，因此熔点非常高。
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Phosphorus, sulfur and chlorine exist as simple molecules, e.g. P₄, S₈ and Cl₂. Their melting points are much lower because only weak van der Waals forces need to be overcome.
磷、硫和氯以简单分子形式存在，例如 P₄、S₈ 和 Cl₂。它们的熔点低得多，因为只需要克服微弱的范德华力。
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Argon is a monatomic gas with only very weak London forces, giving it the lowest melting point in the period.
氩是单原子气体，仅存在极弱的伦敦力，因此其熔点是全周期最低的。
Note that sulfur's melting point is higher than phosphorus and chlorine because S₈ molecules are larger and have more electrons, increasing the strength of the van der Waals forces between molecules.
注意硫的熔点高于磷和氯，因为 S₈ 分子更大，电子更多，分子间范德华力更强。
6. Electronegativity | 6. 电负性
Electronegativity is the power of an atom to attract the electron density in a covalent bond. Across Period 3, electronegativity increases from sodium to chlorine because nuclear charge increases and atomic radius decreases, so the nucleus attracts bonding electrons more strongly.
电负性是指原子在共价键中吸引电子密度的能力。沿第三周期，从钠到氯电负性增大，因为核电荷增大且原子半径减小，原子核对成键电子的吸引更强。
Na < Mg < Al < Si < P < S < Cl
Argon is normally given no electronegativity value because it does not usually form covalent bonds. Sodium and magnesium are metals with low electronegativity, so they form ionic compounds with non-metals. Chlorine has the highest electronegativity in Period 3, so it forms polar covalent bonds with most non-metals and ionic bonds with metals such as sodium.
氩通常没有电负性数值，因为它一般不形成共价键。钠和镁是金属，电负性低，因此与非金属形成离子化合物。氯在第三周期中电负性最高，因此与大多数非金属形成极性共价键，与钠等金属形成离子键。
7. Period 3 Oxides: Acidic, Basic and Amphoteric | 7. 第三周期氧化物：酸性、碱性与两性
The oxides of Period 3 show a clear change from basic to acidic across the period. This trend is linked to electronegativity and the type of bonding present.
第三周期氧化物的性质沿周期表现出从碱性到酸性的明显变化。这一趋势与电负性和键的类型密切相关。
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e ionic oxides that dissolve in water to form alkaline solutions, e.g. Na₂O + H₂O → 2NaOH.
Na₂O 和 MgO 是离子型氧化物，溶于水形成碱性溶液，例如 Na₂O + H₂O → 2NaOH。
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Al₂O₃ is amphoteric. It reacts with both acids and strong bases, for example with HCl and with NaOH.
Al₂O₃ 是两性氧化物。它既能与酸反应，也能与强碱反应，例如与 HCl 和 NaOH 反应。
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SiO₂ is weakly acidic and does not react with water, but it reacts with strong alkalis such as NaOH to form silicates.
SiO₂ 呈弱酸性，不与水反应，但能与 NaOH 等强碱反应生成硅酸盐。
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P₄O₁₀, SO₂ and SO₃ are acidic oxides. They react with water to form acids: phosphoric acid, sulfurous acid and sulfuric acid.
P₄O₁₀、SO₂ 和 SO₃ 是酸性氧化物。它们与水反应生成酸：磷酸、亚硫酸和硫酸。
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Cl₂O is also acidic and reacts with water to form chloric(I) acid, HOCl.
Cl₂O 也是酸性氧化物，与水反应生成氯(I)酸，即 HOCl。
→ MgO (basic) → Al₂O₃ (amphoteric) → SiO₂ (weakly acidic) → P₄O₁₀, SO₂, SO₃, Cl₂O (acidic)
As electronegativity increases across the period, the element-oxygen bond becomes more covalent and the oxide becomes more acidic. Ionic oxides tend to give hydroxide ions in water, while covalent oxides react with water to produce H⁺ ions.
随着电负性沿周期增大，元素与氧之间的键变得更加共价，氧化物也变得更具有酸性。离子型氧化物在水中产生氢氧根离子，而共价型氧化物与水反应生成 H⁺ 离子。
8. Period 3 Chlorides | 8. 第三周期氯化物
The chlorides of Period 3 also show a trend in bonding and acid-base behaviour. Sodium and magnesium chlorides are ionic solids; aluminium chloride is covalent; and the chlorides of silicon, phosphorus, sulfur and chlorine are simple covalent molecules.
第三周期氯化物在键型和酸碱行为上也表现出趋势。钠和镁的氯化物是离子固体；氯化铝是共价化合物；硅、磷、硫和氯的氯化物则是简单共价分子。
Sodium chloride, NaCl, is a giant ionic lattice with a high melting point. Its aqueous solution is neutral because both Na⁺ and Cl⁻ are poor at reacting with water to change pH.
氯化钠 NaCl 是巨型离子晶格，熔点较高。其水溶液呈中性，因为 Na⁺ 和 Cl⁻ 都不易与水反应改变 pH 值。
Magnesium chloride, MgCl₂, is also ionic, but the Mg²⁺ ion in water undergoes slight hydrolysis, making the solution slightly acidic.
氯化镁 MgCl₂ 也是离子化合物，但 Mg²⁺ 在水中会发生轻微水解，使溶液略微呈酸性。
Aluminium chloride, AlCl₃, is covalent in the gas phase and exists as a dimer Al₂Cl₆. In water, it hydrolyses strongly to produce acidic solutions, often with the formation of aluminium hydroxide.
氯化铝 AlCl₃ 在气相中是共价化合物，以二聚体 Al₂Cl₆ 形式存在。在水中强烈水解产生酸性溶液，通常伴随氢氧化铝生成。
9. Reactions of Oxides and Chlorides with Water | 9. 氧化物和氯化物与水的反应
You need to know the equations for the reactions of Period 3 oxides and chlorides with water, and be able to describe the pH of the resulting solutions.
你需要掌握第三周期氧化物和氯化物与水反应的方程式，并能够描述所得溶液的 pH。
| Substance | Equation with water | pH / nature |
| Na₂O | Na₂O + H₂O → 2NaOH | Strongly alkaline, pH > 13 |
| MgO | MgO + H₂O → Mg(OH)₂ | Alkaline, pH ≈ 9 |
| Al₂O₃ | Insoluble in water; reacts with acid and alkali | Amphoteric |
| SiO₂ | No reaction with water | Insoluble, weakly acidic |
| P₄O₁₀ | P₄O₁₀ + 6H₂O → 4H₃PO₄ | Strongly acidic |
| SO₂ | SO₂ + H₂O ⇌ H₂SO₃ | Acidic (weak) |
| SO₃ | SO₃ + H₂O → H₂SO₄ | Strongly acidic, pH ≈ 1 |
| NaCl | Dissolves; no hydrolysis | Neutral |
| MgCl₂ | Mg²⁺ + 2H₂O ⇌ Mg(OH)₂ + 2H⁺ | Slightly acidic |
| AlCl₃ | Al³⁺ + 3H₂O ⇌ Al(OH)₃ + 3H⁺ | Acidic |
| SiCl₄ | SiCl₄ + 2H₂O → SiO₂ + 4HCl | Acidic, steamy fumes |
| PCl₅ | PCl₅ + 4H₂O → H₃PO₄ + 5HCl | Strongly acidic |
10. Summary of Periodicity Trends | 10. 周期性趋势总结
For AQA A-Level Chemistry, you should be able to reproduce these trends quickly and use them to explain unfamiliar examples. The key ideas are nuclear charge, shielding, electron-electron repulsion, and the type of structure.
对于 AQA A-Level 化学，你应该能够快速复述这些趋势，并用它们解释不熟悉的例子。关键概念包括核电荷、屏蔽效应、电子-电子排斥和结构类型。
Atomic radius decreases across Period 3.
原子半径沿第三周期减小。
First ionisation energy generally increases, with drops between Mg/Al and P/S.
第一电离能总体增大，但在 Mg/Al 和 P/S 之间出现下降。
Melting points reflect metallic bonding, giant covalent structure, and molecular size.
熔点反映金属键、巨型共价结构和分子大小。
Electronegativity increases from Na to Cl.
电负性从 Na 到 Cl 增大。
Oxides change from basic to amphoteric to acidic.
氧化物从碱性变为两性，再变为酸性。
Chlorides change from ionic to covalent, and from neutral to acidic in water.
氯化物从离子型变为共价型，在水中从中性变为酸性。
Mastering Periodicity gives you a powerful framework for predicting the chemistry of other elements. Always start by writing the electron configuration and then reason from nuclear charge, shielding, and structure.
掌握周期性为你预测其他元素的化学性质提供了强大框架。做题时，先从电子排布出发，再基于核电荷、屏蔽效应和结构进行推理。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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