HKDSE Chemistry Core
Section 6: Microscopic World II | 微觀世界 II
Billy Sir’s Smart Notes: Master Molecular Shapes, Polarity, and Intermolecular Forces.
由資深導師 Billy Sir 編寫，助你極速掌握分子形狀、極性及分子間引力。
1. Non-Octet Structures & Dative Covalent Bonds | 非八隅體結構與配位共價鍵
While most atoms follow the octet rule, there are exceptions in some covalent molecules. 雖然大多數原子遵循八隅體規則，但在某些共價分子中存在例外。
- Incomplete Octet (不完全八隅體): The central atom has fewer than 8 electrons in its outermost shell. Example: \( \text{BF}_3 \) (Boron has 6 electrons). (中心原子的最外層電子少於8個。例子：三氟化硼)
- Expanded Octet (擴展八隅體): The central atom (Period 3 or beyond) has more than 8 electrons. Examples: \( \text{PCl}_5 \) (10 electrons), \( \text{SF}_6 \) (12 electrons). (中心原子擁有超過8個電子。例子：五氯化磷、六氟化硫)
🤝 Dative Covalent Bond | 配位共價鍵
A covalent bond in which both shared electrons are provided by one atom. Once formed, it is identical to a normal covalent bond. 一種共價鍵，其中兩個共用電子均由同一個原子提供。一旦形成，它與普通的共價鍵無異。
Example: Ammonium ion (\( \text{NH}_4^+ \)). The nitrogen atom in \( \text{NH}_3 \) donates its lone pair to an \( \text{H}^+ \) ion. (例子：銨離子。氨分子中的氮原子將其孤對電子提供給氫離子。)
2. Shapes of Molecules (VSEPR Theory) | 分子形狀 (價層電子對互斥理論)
The shape of a molecule is determined by the repulsion between electron pairs in the outermost shell of the central atom. They arrange themselves to be as far apart as possible to minimize repulsion. 分子的形狀由中心原子最外層電子對之間的排斥力決定。它們會盡量遠離彼此以減少排斥。
⚠️ Order of Repulsion | 排斥力大小順序
Lone Pair – Lone Pair > Lone Pair – Bond Pair > Bond Pair – Bond Pair
孤對電子-孤對電子 > 孤對電子-鍵合電子 > 鍵合電子-鍵合電子
| Electron Pairs (Bond + Lone) | Shape (形狀) | Bond Angle (鍵角) | Examples (例子) |
|---|---|---|---|
| 2 Bond, 0 Lone | Linear (直線形) | 180° | \( \text{BeCl}_2 \), \( \text{CO}_2 \) |
| 3 Bond, 0 Lone | Trigonal Planar (平面三角形) | 120° | \( \text{BF}_3 \) |
| 4 Bond, 0 Lone | Tetrahedral (四面體形) | 109.5° | \( \text{CH}_4 \) |
| 3 Bond, 1 Lone | Trigonal Pyramidal (三角錐體形) | 107° | \( \text{NH}_3 \) |
| 2 Bond, 2 Lone | V-shaped / Bent (V形 / 角形) | 104.5° | \( \text{H}_2\text{O} \) |
3. Electronegativity & Bond Polarity | 電負性與鍵極性
Electronegativity (電負性): The ability of an atom to attract shared electrons in a covalent bond. Fluorine (F) is the most electronegative element. 原子在共價鍵中吸引共用電子的能力。氟 (F) 的電負性最高。
- Polar Bond (極性鍵): Formed between atoms with different electronegativities. The more electronegative atom gains a partial negative charge (\( \delta- \)). (在電負性不同的原子間形成。電負性較高的原子帶微負電荷。)
- Polar Molecule (極性分子): A molecule is polar if it has polar bonds AND its shape is asymmetrical (the dipole moments do not cancel out). Example: \( \text{H}_2\text{O} \), \( \text{NH}_3 \). (如果分子具有極性鍵且形狀不對稱，偶極矩無法抵消，則為極性分子。例子：水、氨。)
- Non-polar Molecule (非極性分子): Even if polar bonds exist, if the molecule is highly symmetrical (e.g., \( \text{CO}_2 \), \( \text{CCl}_4 \)), the dipole moments cancel out completely. (即使存在極性鍵，如果分子高度對稱，偶極矩會完全抵消。例子：二氧化碳、四氯化碳。)
4. Intermolecular Forces | 分子間引力
These are the forces of attraction between molecules. They are much weaker than covalent bonds. 這些是分子之間的引力，比共價鍵弱得多。
| Type of Force (引力類型) | Description (描述) | Strength (強度) |
|---|---|---|
| Van der Waals’ Forces 范德華力 |
Exists between ALL molecules. Strength increases with molecular size (more electrons). 存在於所有分子之間。分子越大（電子越多），引力越強。 |
Weak 弱 |
| Hydrogen Bond 氫鍵 |
A strong dipole-dipole attraction between an H atom bonded to a highly electronegative atom (F, O, or N) and a lone pair on another F, O, or N atom. 與高電負性原子 (F, O, N) 鍵合的 H 原子，與另一個 F, O, N 原子的孤對電子之間的強烈偶極引力。 |
Stronger (but still weaker than covalent bonds) 較強 (但仍弱於共價鍵) |
5. Water and Hydrogen Bonding | 水與氫鍵的特殊性質
Because of extensive hydrogen bonding, water has unique anomalous properties: 由於廣泛的氫鍵，水具有獨特的反常性質：
- High Boiling Point (高沸點): Extra energy is required to break the strong hydrogen bonds between water molecules compared to other molecules of similar size (e.g., \( \text{H}_2\text{S} \)). (與大小相若的分子相比，需要額外能量來打破水分子間強大的氫鍵。)
- Density of Ice < Water (冰的密度低於水): In ice, water molecules are held in a rigid, open tetrahedral structure by hydrogen bonds, creating empty spaces. When ice melts, the structure collapses, and molecules move closer together. (在冰中，水分子被氫鍵固定在一個開放的四面體結構中，產生空隙。冰融化時結構坍塌，分子靠得更近。)
- High Surface Tension (高表面張力): Strong hydrogen bonding pulls water molecules at the surface inward. (強大的氫鍵將表面的水分子向內拉。)
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