HKDSE Chemistry Core
Section 12: Patterns in the Chemical World | 化學世界的模式
Billy Sir’s Smart Notes: Master Periodic Trends, Bonding Variations, Oxides, and Transition Metals.
由資深導師 Billy Sir 編寫，助你極速掌握週期趨勢、鍵結變化、氧化物及過渡金屬。
1. Periodic Law & Atomic Radii | 週期律與原子半徑
The Periodic Law states that the properties of elements are periodic functions of their atomic numbers. 週期律指出，元素的性質是其原子序的週期函數。
📉 Variation of Atomic Radii across a Period | 橫越週期的原子半徑變化
Across Period 2 and Period 3, the atomic radius decreases from left to right. 橫越第2及第3週期，原子半徑由左至右遞減。
- Reason: The number of protons increases, meaning the nuclear charge increases. (原因：質子數目增加，核電荷增加。)
- The shielding effect of inner shell electrons remains roughly the same. (內層電子的屏蔽效應大致相同。)
- Therefore, the effective nuclear charge increases, pulling the outermost electrons closer to the nucleus. (因此，有效核電荷增加，將最外層電子拉得更近原子核。)
2. Structure, Bonding & Melting Point (Period 3) | 結構、化學鍵與熔點 (第三週期)
The variation in melting points across a period is directly related to the change in structure and bonding. 橫越週期的熔點變化與結構及化學鍵的改變有直接關係。
| Elements (元素) | \( \text{Na, Mg, Al} \) | \( \text{Si} \) | \( \text{P}_4, \text{S}_8, \text{Cl}_2 \) | \( \text{Ar} \) |
|---|---|---|---|---|
| Structure 結構 |
Giant Metallic 巨型金屬結構 |
Giant Covalent 巨型共價結構 |
Simple Molecular 簡單分子結構 |
Monatomic 單原子 |
| Bonding Broken 被打破的鍵 |
Metallic Bonds 金屬鍵 |
Covalent Bonds 共價鍵 |
Van der Waals’ forces 范德華力 |
Van der Waals’ forces 范德華力 |
| Melting Point Trend 熔點趨勢 |
Increases: More delocalized electrons and higher ionic charge. 遞增：離域電子增多，離子電荷變大。 |
Highest: Extensive network of strong covalent bonds. 最高：龐大的強共價鍵網絡。 |
Low: Weak intermolecular forces. (\( \text{S}_8 > \text{P}_4 > \text{Cl}_2 \)) 低：微弱的分子間引力。 |
Lowest: Very weak forces between single atoms. 最低：單原子間引力極弱。 |
3. Electronegativity & Electrical Conductivity | 電負性與導電性
⚠️ Exam Trap: Electronegativity | 考試陷阱：電負性
Electronegativity is the relative tendency of an atom to attract a bonding pair of electrons. It increases across a period because the effective nuclear charge increases, making the nucleus attract the bonding electrons more strongly. Noble gases (e.g., Ar) are typically excluded from this trend as they do not normally form bonds! 電負性是原子吸引共用電子對的相對傾向。橫越週期，電負性遞增，因為有效核電荷增加。貴氣體（如氬）通常不計算在內，因為它們極少形成化學鍵！
Electrical Conductivity (導電性):
- Na, Mg, Al: Good conductors due to a “sea” of delocalized mobile electrons. (良導體：含有流動的離域電子海。)
- Si: Semi-conductor (metalloid). (半導體：類金屬。)
- P, S, Cl, Ar: Non-conductors as they lack mobile electrons or ions. (不導電：缺乏流動的電子或離子。)
4. Periodic Variation of Oxides (Period 3) | 氧化物的週期變化 (第三週期)
The nature of oxides changes from basic to amphoteric, and finally to acidic across Period 3. 橫越第三週期，氧化物的性質由鹼性變為兩性，最後變為酸性。
| Oxide (氧化物) | \( \text{Na}_2\text{O}, \text{MgO} \) | \( \text{Al}_2\text{O}_3 \) | \( \text{SiO}_2 \) | \( \text{P}_4\text{O}_{10}, \text{SO}_2, \text{Cl}_2\text{O} \) |
|---|---|---|---|---|
| Acid-Base Nature 酸鹼性質 |
Basic 鹼性 |
Amphoteric 兩性 |
Acidic 酸性 |
Acidic 酸性 |
| Structure 結構 |
Giant Ionic 巨型離子 |
Giant Ionic (with covalent character) 巨型離子(具共價特徵) |
Giant Covalent 巨型共價 |
Simple Molecular 簡單分子 |
| Reaction with Water 與水反應 |
Forms alkaline solutions (NaOH, Mg(OH)₂). 形成鹼性溶液。 |
Insoluble in water. 不溶於水。 |
Insoluble in water. 不溶於水。 |
Forms acidic solutions (e.g., H₂SO₃). 形成酸性溶液。 |
5. Transition Metals | 過渡金屬
Transition metals are d-block elements that form at least one stable ion with a partially filled d-subshell. 過渡金屬是 d 區元素，能形成至少一種具有未全滿 d 亞層的穩定離子。
⭐ Key Properties of Transition Metals | 過渡金屬的關鍵特性
- Variable Oxidation States: They can lose different numbers of electrons (e.g., \( \text{Fe}^{2+} \) and \( \text{Fe}^{3+} \)). (可變氧化態：能失去不同數量的電子。)
- Coloured Ions: Most of their aqueous ions and compounds are coloured (e.g., \( \text{Cu}^{2+}(\text{aq}) \) is blue). (有色離子：其水溶液及化合物多帶有顏色。)
- Catalytic Properties: They and their compounds act as excellent catalysts (e.g., Fe in Haber Process, \( \text{MnO}_2 \) in decomposition of \( \text{H}_2\text{O}_2 \)). (催化特性：它們及其化合物是優良的催化劑。)
- Complex Ion Formation: They can form complex ions with ligands (e.g., \( [\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+} \)). (形成絡離子：能與配位體形成絡離子。)
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