📚 Properties of Period 3 Elements and their Oxides | 第三周期元素及其氧化物的性质
This article covers the key trends and reactions of Period 3 elements (Na to Ar) and their oxides, focusing on AQA A-level Chemistry specification point 2.4. You will learn about melting points, electronegativity, and the acidic/basic nature of oxides, with equations you need to know for exams.
本文围绕 AQA A-level 化学考纲 2.4 节，系统讲解第三周期元素（钠到氩）及其氧化物的核心性质与反应趋势。你将掌握熔点变化、电负性规律，以及氧化物酸碱性的本质，并熟悉考试中必备的化学方程式。
1. Atomic Radius and Electronegativity | 原子半径与电负性
Across Period 3 from Na to Cl, the atomic radius decreases. This is because the number of protons increases while the electrons are added to the same principal quantum shell (n = 3). The increasing nuclear charge pulls the electron cloud closer to the nucleus.
从钠到氯，第三周期元素的原子半径逐渐减小。这是因为质子数增加，而电子却填入同一主量子层（n=3）。增大的核电荷会把电子云拉得更靠近原子核。
Electronegativity increases across the period for the same reason: the nucleus attracts bonding electrons more strongly as the nuclear charge increases and atomic radius shrinks. Ignore argon, as it has a full outer shell and does not normally form bonds.
电负性在同一周期内增大，原因相同：随着核电荷增加、原子半径缩小，原子核对成键电子的吸引更强。氩气因具有全满外层电子结构、通常不参与成键，因此不予讨论。
Electronegativity trend: Na < Mg < Al < Si < P < S < Cl
电负性趋势：Na < Mg < Al < Si < P < S < Cl
2. Melting and Boiling Points | 熔点与沸点
Period 3 elements show a general increase in melting point from Na to Si, followed by a sharp drop at P and S, and then low melting points for Cl and Ar.
第三周期元素的熔点从钠到硅总体上升，到磷和硫时骤然下降，氯和氩则保持很低的熔点。
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Sodium, magnesium, aluminium: metallic bonding. Melting point increases because the charge on the metal ion increases (Na⁺, Mg²⁺, Al³⁺) and the number of delocalised electrons available per atom increases, strengthening metallic bonds.
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Silicon: a giant covalent structure (diamond-like). Many strong covalent bonds must be broken, giving it a very high melting point.
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Phosphorus, sulfur, chlorine, argon: simple molecular structures. Weak van der Waals forces between molecules explain low melting points. Sulfur has a higher melting point than phosphorus because S₈ molecules are larger and have more electrons, giving stronger van der Waals forces.
钠、镁、铝：金属键。熔点的升高是因为金属离子的电荷增加（Na⁺, Mg²⁺, Al³⁺），并且每个原子提供的离域电子数增多，使金属键增强。
硅：巨共价结构（类似金刚石）。需要断裂大量强共价键，因此熔点极高。
磷、硫、氯、氩：简单分子结构。分子间较弱的范德华力导致低熔点。硫的熔点高于磷，因为 S₈ 分子更大、电子数更多，范德华力更强。
Melting point order: Na < Mg < Al < Si > P < S > Cl > Ar
熔点顺序：Na < Mg < Al < Si > P < S > Cl > Ar
3. Reactions with Oxygen | 与氧气的反应
All Period 3 elements except chlorine and argon react with oxygen to form oxides. The equations below are frequently tested.
除氯和氩外，第三周期元素都能与氧气反应生成氧化物。以下方程式是考试中的高频考点。
4Na + O₂ → 2Na₂O
2Mg + O₂ → 2MgO
4Al + 3O₂ → 2Al₂O₃
Si + O₂ → SiO₂
4P + 5O₂ → 2P₂O₅ (often written as P₄O₁₀)
S + O₂ → SO₂
Sodium burns with a yellow flame; magnesium with a brilliant white flame; aluminium and silicon form white solids; phosphorus burns with a white smoke; sulfur burns with a blue flame producing a pungent gas.
钠燃烧产生黄色火焰；镁燃烧发出耀眼白光；铝和硅生成白色固体；磷燃烧产生白色烟雾；硫燃烧产生蓝色火焰并生成有刺激性气味的气体。
4. Nature of Period 3 Oxides | 第三周期氧化物的酸碱性
The oxides across the period show a gradual change from basic through amphoteric to acidic.
该周期氧化物的性质呈现从碱性、两性到酸性的渐变规律。
| Oxide | Nature | Type of bonding |
| Na₂O | Basic | Ionic |
| MgO | Basic | Ionic |
| Al₂O₃ | Amphoteric | Ionic (but some covalent character) |
| SiO₂ | Acidic | Giant covalent |
| P₄O₁₀ | Acidic | Molecular covalent |
| SO₂ | Acidic | Molecular covalent |
| SO₃ | Acidic | Molecular covalent |
| 氧化物 | 性质 | 键型 |
| Na₂O | 碱性 | 离子型 |
| MgO | 碱性 | 离子型 |
| Al₂O₃ | 两性 | 离子型（有一定共价成分） |
| SiO₂ | 酸性 | 巨共价 |
| P₄O₁₀ | 酸性 | 分子共价 |
| SO₂ | 酸性 | 分子共价 |
| SO₃ | 酸性 | 分子共价 |
5. Reactions of Basic Oxides with Water and Acids | 碱性氧化物与水和酸的反应
Sodium oxide and magnesium oxide are basic oxides. They react with water to form alkaline hydroxide solutions.
氧化钠和氧化镁是碱性氧化物，它们与水反应生成碱性的氢氧化物溶液。
Na₂O(s) + H₂O(l) → 2NaOH(aq)
MgO(s) + H₂O(l) → Mg(OH)₂(aq)
The NaOH solution is strongly alkaline (pH 13-14), while Mg(OH)₂ is only slightly soluble, giving a weakly alkaline solution (pH around 9). These oxides also neutralise acids, e.g.
NaOH 溶液呈强碱性（pH 13-14），而 Mg(OH)₂ 仅微溶，形成弱碱性溶液（pH 约 9）。这些氧化物还能与酸发生中和反应，例如：
Na₂O + 2HCl → 2NaCl + H₂O
MgO + 2HNO₃ → Mg(NO₃)₂ + H₂O
6. Reactions of Amphoteric Aluminium Oxide | 两性氧化铝的反应
Aluminium oxide is amphoteric: it reacts with both acids and strong alkalis, acting as a basic oxide with acids and as an acidic oxide with bases.
氧化铝是两性氧化物：既能与酸反应，也能与强碱反应。与酸反应时表现碱性，与碱反应时表现酸性。
Reaction with acid:
与酸的反应：
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
Reaction with strong alkali (e.g. sodium hydroxide):
与强碱（如氢氧化钠）的反应：
Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄
This is why aluminium oxide is used as a refractory material and why aluminium vessels cannot be cleaned with strong alkalis.
这就是氧化铝常作耐火材料的原因，也解释了为什么不能用强碱清洗铝制容器。
7. Reactions of Acidic Oxides with Water and Bases | 酸性氧化物与水和碱的反应
Silicon dioxide is a giant covalent acidic oxide. It does not react with water at room temperature, but it does react with hot concentrated alkali and with hydrofluoric acid.
二氧化硅是巨共价酸性氧化物。常温下不与水反应，但能与热浓碱和氢氟酸反应。
SiO₂ + 2NaOH → Na₂SiO₃ + H₂OPublished by TutorHao | A-Level Chemistry Revision Series | aleveler.com
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