📚 Covalent Bonding: CIE A-Level Chemistry Focus | 共价键考点精讲
A covalent bond is one of the cornerstones of A-Level Chemistry. Mastering how atoms share electrons, how this leads to a variety of molecular structures, and how to interpret bonding in terms of orbital overlap and electronegativity is essential for success in the CIE examination. This article highlights the key concepts and common exam pitfalls.
共价键是 A-Level 化学的基石之一。掌握原子如何共享电子、如何由此产生多样的分子结构，以及如何从轨道重叠和电负性角度理解键合，对 CIE 考试至关重要。本文聚焦核心概念和常见考试误区。
1. What is a Covalent Bond? | 什么是共价键？
A covalent bond is a chemical link formed when two atoms, typically non-metals, share one or more pairs of electrons. This sharing allows each atom to achieve a noble gas electron configuration, resulting in a stable molecule.
共价键是两个原子（通常是非金属）共享一对或多对电子而形成的化学键。这种共享使每个原子达到惰性气体电子构型，从而产生稳定的分子。
The shared electron pair is attracted to the nuclei of both atoms, creating a strong, directional bond. Covalent bonds can be single, double, or triple depending on the number of shared electron pairs.
共享电子对同时受到两个原子核的吸引，形成强而具有方向性的键。根据共享电子对的数量，共价键可以是单键、双键或三键。
For instance, in a hydrogen molecule (H₂), the two hydrogen atoms share one pair of electrons, indicated as H–H. In oxygen (O₂), two electron pairs are shared, giving a double bond O=O. Nitrogen (N₂) has a triple bond N≡N.
例如，氢分子 (H₂) 中，两个氢原子共享一对电子，表示为 H–H。氧分子 (O₂) 共享两对电子，形成双键 O=O。氮分子 (N₂) 则含有三键 N≡N。
2. Dot-and-Cross Diagrams | 点叉图（路易斯结构）
Dot-and-cross diagrams illustrate how outer shell electrons are arranged in covalent compounds. Different symbols (dots and crosses) are used to distinguish electrons from different atoms.
点叉图用来显示共价化合物中外层电子的排列方式。用不同的符号（点和叉）区分来自不同原子的电子。
When drawing these diagrams, show the central atom surrounded by other atoms. Pair up unpaired electrons from different atoms as shared pairs, and ensure each atom (except hydrogen) achieves an octet whenever possible.
绘制这些图时，将中心原子置于中间，其它原子围绕它。将不同原子的未成对电子配对成共享电子对，并尽可能使每个原子（氢除外）达到八隅体。
For water (H₂O), oxygen shares one electron with each of two hydrogen atoms, leaving two lone pairs on the oxygen. For methane (CH₄), carbon shares its four valence electrons with four hydrogen atoms, forming four single covalent bonds with no lone pairs on carbon.
对于水 (H₂O)，氧与两个氢各共享一个电子，氧上留有两对孤对电子。对于甲烷 (CH₄)，碳与四个氢原子共享它的四个价电子，形成四个单共价键，碳上没有孤对电子。
The CIE exam may ask you to draw dot-and-cross diagrams for simple molecules or ions containing elements like C, N, O, Cl. Always check that all electrons are accounted for and that no atom exceeds its maximum bonding capacity.
CIE 考试可能要求你画出含 C、N、O、Cl 等元素的简单分子或离子的点叉图。务必核验所有电子都已计入，且没有任何原子超过其最大成键能力。
3. Sigma and Pi Bonds | σ 键和 π 键
When atomic orbitals overlap, two types of covalent bonds can form: sigma (σ) and pi (π) bonds. A sigma bond is formed by end-to-end (head-on) overlap of orbitals directly between the two atomic nuclei. It is the first bond to form between any two atoms.
当原子轨道重叠时，可形成两种共价键：σ 键和 π 键。σ 键由原子轨道沿两核连线“头对头”重叠而形成，它是两个原子间最先形成的键。
A pi bond results from sideways (parallel) overlap of adjacent p orbitals above and below the plane of the nuclei. Pi bonds are weaker than sigma bonds and exist only if a sigma bond is already present (double or triple bonds).
π 键是由相邻 p 轨道在核平面上下方“肩并肩”平行重叠形成的。π 键比 σ 键弱，且只存在于已有一个 σ 键的情况下（双键或三键）。
In ethene (C₂H₄), the carbon–carbon double bond consists of one σ bond and one π bond. In the nitrogen molecule (N≡N), there is one strong σ bond and two π bonds. All single bonds (like in ethane, C₂H₆) are σ bonds.
在乙烯 (C₂H₄) 中，碳碳双键由一个 σ 键和一个 π 键组成。在氮分子 (N≡N) 中，有一个坚固的 σ 键和两个 π 键。所有单键（如乙烷 C₂H₆）都是 σ 键。
Be prepared to identify the number of σ and π bonds in a given molecule. For example, in benzene, the delocalised π system involves six electrons in π bonds above and below the ring, but each carbon–carbon connection still has one σ bond.
要能指出给定分子中 σ 键和 π 键的数目。例如，在苯中，离域 π 体系涉及环平面上下的六个电子，但每个碳-碳连接仍有一个 σ 键。
4. Bond Length and Bond Energy | 键长与键能
Bond length is the average distance between the nuclei of two bonded atoms. Bond energy (bond dissociation energy) is the energy required to break one mole of a particular covalent bond in a gaseous molecule.
键长是两个成键原子核间的平均距离。键能（键离解能）是断裂气态分子中 1 摩尔特定共价键所需的能量。
As the number of shared pairs increases, bond length decreases and bond energy increases. Multiple bonds are shorter and stronger than single bonds between the same elements.
随着共享电子对数增加，键长变短，键能增大。相同元素之间，多重键比单键更短更强。
| Bond | Bond length / pm | Bond energy / kJ mol⁻¹ |
| C–C | 154 | 347 |
| C=C | 134 | 614 |
| C≡C | 120 | 839 |
Note that bond energy values are averages and can be used to estimate overall enthalpy changes during reactions (ΔH ≈ sum of bonds broken – sum of bonds formed).
注意，键能数值是平均值，可用于估算反应过程中的焓变 (ΔH ≈ 断裂键的总键能 – 形成键的总键能)。
5. Dative Covalent (Coordinate) Bonding | 配位共价键
A dative covalent bond (or coordinate bond) is a covalent bond in which both electrons of the shared pair come from the same atom. The atom donating the electron pair must have a lone pair, and the acceptor must be electron-deficient.
配位共价键（又称配位键）是共享电子对的两个电子都来自同一原子的共价键。提供电子对的原子必须有孤对电子，而接受体必须缺电子。
Once formed, a dative bond is identical to any other covalent bond in terms of strength and length. It is often represented by an arrow (→) from the donor to the acceptor.
配位键一旦形成，在强度和键长上与其他共价键完全相同。它常用从供体指向受体的箭头 (→) 表示。
Classic examples include the ammonium ion (NH₄⁺), where a lone pair on nitrogen in NH₃ donates to a proton (H⁺): NH₃ + H⁺ → NH₄⁺. Another is the adduct H₃N→BF₃, formed between ammonia and boron trifluoride.
经典例子包括铵离子 (NH₄⁺)，其中 NH₃ 中氮上的孤对电子向 H⁺ 提供：NH₃ + H⁺ → NH₄⁺。另一个例子是氨与三氟化硼形成的加合物 H₃N→BF₃。
In graphite and metallic complexes, dative bonding also plays a role. The CIE exam may ask you to recognise dative bonds in diagrams or to predict which species can act as a lone pair donor.
在石墨和金属配合物中，配位键也起作用。CIE 考试可能要求你识别图中的配位键，或预测哪种微粒可作为孤对电子供体。
6. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity is the power of an atom to attract the shared electron pair in a covalent bond. The Pauling scale is commonly used, with fluorine having the highest value (4.0).
电负性是原子在共价键中吸引共享电子对的能力。常用鲍林标度，氟的电负性最高 (4.0)。
If the two bonded atoms have equal electronegativity (e.g., H–H, Cl–Cl), the bonding electron pair is shared equally, resulting in a non‑polar covalent bond.
如果成键的两个原子电负性相同（如 H–H、Cl–Cl），成键电子对平均共享，形成非极性共价键。
When there is a significant difference in electronegativity (typically 0.5–1.7), the electron pair is pulled closer to the more electronegative atom, creating a polar covalent bond with partial charges δ⁺ and δ⁻.
当电负性差异显著（一般在 0.5–1.7 之间）时，电子对会被拉向电负性更强的原子，形成极性共价键，产生部分电荷 δ⁺ 和 δ⁻。
In hydrogen chloride (H–Cl), chlorine is more electronegative, so we indicate the bond polarity as Hδ⁺–Clδ⁻. This separation of charge gives rise to a bond dipole moment.
在氯化氢 (H–Cl) 中，氯电负性更强，因此键的极性表示为 Hδ⁺–Clδ⁻。这种电荷分离产生了键偶极矩。
7. Shapes of Molecules: VSEPR Theory | 分子形状：VSEPR 理论
Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom repel each other and therefore arrange themselves as far apart as possible. The shape is determined by the number of bonding pairs and lone pairs.
价层电子对互斥理论 (VSEPR) 指出，中心原子周围的电子对相互排斥，因此尽可能远离。分子的形状由成键电子对和孤对电子的数目决定。
Lone pairs repel more strongly than bonding pairs, so they compress the bond angles. The order of repulsion is: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.
孤对电子的斥力大于成键电子对，因此它们会压缩键角。排斥力顺序为：孤对-孤对 > 孤对-成键对 > 成键对-成键对。
| Molecule | Bonding pairs | Lone pairs | Shape (English/中文) | Bond angle |
| CH₄ | 4 | 0 | 更多咨询请联系16621398022（同微信）
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