📚 Covalent Bonding for Cambridge A-Level Chemistry | 剑桥A-Level化学：共价键
In Cambridge A-Level Chemistry, covalent bonding is a core topic that explains how non-metal atoms share electrons to form molecules and giant structures. This article covers the definitions, Lewis diagrams, bond polarity, molecular shapes, sigma and pi bonds, and giant covalent lattices you need for Papers 1, 2 and 4.
在剑桥A-Level化学中，共价键是解释非金属原子如何共享电子形成分子和巨型结构的核心主题。本文涵盖定义、路易斯图、键的极性、分子形状、σ键与π键以及巨型共价晶格等考试重点，适用于Paper 1、2和4。
1. What Is Covalent Bonding? | 什么是共价键？
A covalent bond is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei of the two bonded atoms. It forms mainly between non-metal atoms that have high ionisation energies, so transferring electrons is not energetically favourable.
共价键是共享电子对与两个成键原子带正电的原子核之间的静电吸引力。它主要在高电离能的非金属原子之间形成，因为转移电子在能量上并不有利。
Instead of losing or gaining electrons completely, the atoms overlap atomic orbitals and share one or more electron pairs. This sharing allows each atom to achieve a more stable outer-shell configuration.
原子并不完全失去或获得电子，而是重叠原子轨道并共享一对或多对电子。这种共享使每个原子都能获得更稳定的最外层电子构型。
Covalent substances can be simple molecules such as H₂O, CO₂ and CH₄, or giant covalent structures such as diamond, graphite and silicon dioxide.
共价物质可以是简单分子，如H₂O、CO₂和CH₄，也可以是巨型共价结构，如金刚石、石墨和二氧化硅。
2. The Octet Rule and Electron Sharing | 八隅体规则与电子共享
The octet rule states that atoms tend to share electrons until they have eight electrons in their outer shell, giving the electron configuration of a noble gas. Hydrogen is an exception because it only needs two electrons to fill its 1s orbital.
八隅体规则指出，原子倾向于共享电子，直到其最外层拥有8个电子，达到稀有气体的电子构型。氢是一个例外，因为它只需要2个电子即可填满1s轨道。
For example, each hydrogen atom in H₂ shares one electron to gain the 1s² configuration of helium. Carbon in CH₄ shares four pairs to achieve eight outer electrons, while each fluorine in F₂ shares one pair to reach the neon configuration.
例如，H₂中每个氢原子共享1个电子以获得氦的1s²构型。CH₄中的碳共享4对电子以达到8个最外层电子，而F₂中每个氟共享1对电子以达到氖的构型。
There are important exceptions. BeCl₂ and BF₃ have incomplete octets around the central atom, while PCl₅ and SF₆ expand the octet by using low-lying d orbitals.
也存在重要例外。BeCl₂和BF₃的中心原子为缺电子八隅体，而PCl₅和SF₆通过使用较低能级的d轨道扩展八隅体。
3. Lewis (Dot-and-Cross) Diagrams | 路易斯点叉图
Dot-and-cross diagrams show outer-shell electrons with dots for one atom and crosses for the other. Bonding pairs are drawn between the two atoms, while lone pairs remain on individual atoms.
点叉图用点表示一种原子的最外层电子，用叉表示另一种原子的最外层电子。成键电子对画在两原子之间，孤对电子则留在单个原子旁。
For CO₂, draw two double bonds: O=C=O. Each oxygen has two lone pairs and carbon has no lone pair after bonding. For N₂, draw a triple bond N≡N with one lone pair on each nitrogen.
对于CO₂，画出两个双键：O=C=O。每个氧原子有两对孤对电子，碳原子成键后没有孤对电子。对于N₂，画出三键N≡N，每个氮原子上各有一对孤对电子。
When drawing Lewis structures, first count all valence electrons, connect atoms with single bonds, then place remaining electrons as lone pairs on terminal atoms to satisfy the octet. Multiple bonds are introduced only when necessary.
画路易斯结构时，应先计算所有价电子，用单键连接原子，然后将剩余电子作为孤对电子放在末端原子上以满足八隅体规则。仅在必要时才引入多重键。
4. Single, Double and Triple Bonds | 单键、双键与三键
A single bond contains one shared electron pair, for example Cl−Cl. A double bond contains two shared pairs, for example O=O. A triple bond contains three shared pairs, for example N≡N.
单键含有1个共享电子对，例如Cl−Cl。双键含有2个共享电子对，例如O=O。三键含有3个共享电子对，例如N≡N。
Multiple bonds are shorter and stronger than single bonds between the same atoms. Bond length decreases and bond energy increases as the number of shared pairs rises.
同一对原子之间，多重键比单键更短、更强。随着共享电子对数目增加，键长缩短，键能增大。
| Bond type | Shared pairs | Example | Relative length | Relative strength |
|---|---|---|---|---|
| Single | 1 | C−C | Longest | Weakest |
| Double | 2 | C=C | Intermediate | Intermediate |
| Triple | 3 | C≡C | Shortest | Strongest |
5. Dative Covalent (Coordinate) Bonds | 配位共价键
A dative covalent bond is a covalent bond in which both electrons of the shared pair come from the same atom. Once formed, a dative bond is indistinguishable from an ordinary covalent bond.
配位共价键是一种共价键，其中共享电子对的两个电子都来自同一个原子。一旦形成，配位键与普通共价键无法区分。
In NH₄⁺, the nitrogen atom in NH₃ donates its lone pair to a proton, H⁺, forming a fourth N−H bond. In H₃O⁺, the oxygen atom in H₂O donates a lone pair to H⁺.
在NH₄⁺中，NH₃中的氮原子将其孤对电子提供给质子H⁺，形成第四个N−H键。在H₃O⁺中，H₂O中的氧原子将孤对电子提供给H⁺。
Al₂Cl₆ is another common example. At high temperature AlCl₃ exists as a dimer in which each aluminium atom accepts a lone pair from a chlorine atom on the other AlCl₃ unit.
Al₂Cl₆是另一个常见例子。在较高温度下，AlCl₃以二聚体形式存在，其中每个铝原子接受来自另一个AlCl₃单元中氯原子的孤对电子。
When drawing a dative bond, use an arrow from the donor atom to the acceptor atom instead of a line, but remember that the final bond has the same properties as other covalent bonds.
画配位键时，从供体原子向受体原子画一个箭头而不是直线，但请记住，最终形成的键与其他共价键具有相同的性质。
6. Bond Polarity and Electronegativity | 键的极性与电负性
Electronegativity is the power of an atom to attract the bonding electrons in a covalent bond. Pauling values increase across a period and decrease down a group in the periodic table.
电负性是原子在共价键中吸引成键电子的能力。鲍林标度值在同周期从左到右增大，同族从上到下减小。
When two atoms have different electronegativities, the bonding pair is unequally shared, producing a polar covalent bond with partial charges δ⁺ and δ⁻.
当两个原子的电负性不同，成键电子对会被不均匀共享，产生极性共价键，带有部分电荷δ⁺和δ⁻。
For example, in HCl, chlorine is more electronegative than hydrogen
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