📚 Covalent Bonding | 共价键
Covalent bonding is the chemical link that holds atoms together through the sharing of electron pairs. It is the dominant bonding mode in organic compounds, biological molecules and many inorganic species such as water, carbon dioxide and the halogens. Understanding covalent bonding is fundamental to explaining molecular shape, polarity, reactivity and physical properties, all of which feature prominently in the Cambridge A‑Level Chemistry syllabus.
共价键是通过共用电子对将原子连接在一起的化学键。它是有机化合物、生物分子以及水、二氧化碳、卤素等许多无机物中最主要的键合方式。理解共价键是解释分子形状、极性、反应活性和物理性质的基础，而这些内容在剑桥 A‑Level 化学大纲中占据重要地位。
1. What is Covalent Bonding? | 什么是共价键？
A covalent bond forms when two atomic nuclei simultaneously attract the same shared pair of electrons. The electrostatic attraction between the positively charged nuclei and the negatively charged shared electrons holds the atoms together. This type of bonding typically occurs between non‑metal atoms with similar electronegativities, allowing both atoms to achieve a more stable electronic configuration, often that of the nearest noble gas.
当两个原子核同时吸引同一对共用电子时，就形成了共价键。带正电的原子核与带负电的共用电子之间的静电引力将原子维系在一起。这类键通常出现在电负性相近的非金属原子之间，使双方都能达到更稳定的电子构型，通常是邻近稀有气体的构型。
A single covalent bond consists of one shared pair of electrons and is represented by a single line between atoms (e.g. H–H, Cl–Cl). The shared electrons count towards the outer‑shell electron total of each bonded atom, effectively completing their octets or, in the case of hydrogen, the duplet.
单键由一对共用电子构成，用原子间的一条短线表示（例如 H–H、Cl–Cl）。共用电子计入每个成键原子的最外层电子总数，从而有效填满八隅体（对于氢则是双电子层）。
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 gain, lose or share electrons in order to obtain eight electrons in their outermost shell. In covalent bonding, sharing allows non‑metals to reach this stable octet without forming ions. For example, in a fluorine molecule (F₂), each fluorine atom brings seven valence electrons; by sharing one pair, both achieve the configuration of neon.
八隅体规则指出，原子倾向于通过得到、失去或共享电子来使最外层达到八个电子。在共价键中，共享电子使非金属原子能够在不形成离子的情况下达到稳定的八隅体。例如，在氟分子（F₂）中，每个氟原子有七个价电子；通过共用一对电子，双方都达到了氖的构型。
Lewis dot‑and‑cross diagrams are the standard tool for representing covalent bonding. Outer electrons are shown as dots for one atom and crosses for the other, clearly illustrating which electrons are shared and which remain as lone pairs. For water (H₂O), the oxygen shares two electrons with two hydrogen atoms whilst retaining two lone pairs, giving it a total of eight valence electrons.
路易斯点叉图是表示共价键的标准工具。其中一个原子的最外层电子用点表示，另一个用叉表示，清晰地展示出哪些电子被共用，哪些以孤对电子形式存在。对于水（H₂O），氧原子与两个氢原子共用两个电子，同时保留两对孤对电子，使氧的价电子总数达到八个。
3. Exceptions: Incomplete and Expanded Octets | 例外：不足八隅体与扩展八隅体
The octet rule is a useful guideline but has notable exceptions. Some molecules have an incomplete octet around the central atom. Boron trifluoride (BF₃) is a classic example: boron has only six valence electrons in its bonding shell, making it electron‑deficient and a potent Lewis acid.
八隅体规则是一条有用的指导原则，但也存在显著例外。有些分子的中心原子具有不完全的八隅体。三氟化硼（BF₃）是典型例子：硼的成键层只有六个价电子，使其成为缺电子化合物和强路易斯酸。
Conversely, elements from Period 3 onwards can expand their octets by using energetically accessible low‑lying d‑orbitals. Phosphorus pentachloride (PCl₅) has ten electrons around the phosphorus atom, and sulfur hexafluoride (SF₆) has twelve electrons around sulfur. These are referred to as expanded octet molecules.
相反，第三周期及之后的元素可以利用能量上可及的低层 d 轨道来扩展八隅体。五氯化磷（PCl₅）中磷原子周围有十个电子，六氟化硫（SF₆）中硫原子周围有十二个电子。这些被称为扩展八隅体分子。
4. Coordinate (Dative) Covalent Bonds | 配位共价键
A coordinate bond (also called a dative bond) is a covalent bond in which both of the shared electrons originate from the same atom. Once formed, it is indistinguishable from any other covalent bond. This type of bond is common in complex ions and in molecules such as the ammonium ion (NH₄⁺) and the hydronium ion (H₃O⁺).
配位键（又称配价键）是一种特殊的共价键，其中共用电子对完全来自同一个原子。一旦形成，它与普通的共价键毫无区别。这类键常见于配离子以及铵离子（NH₄⁺）和水合氢离子（H₃O⁺）等分子中。
In the formation of the ammonium ion, an ammonia molecule (NH₃) uses its lone pair on nitrogen to form a bond with a hydrogen ion (H⁺). The nitrogen atom provides both electrons for the new N–H bond, and the ion has a tetrahedral shape with a formal positive charge on the nitrogen.
在铵离子形成过程中，氨分子（NH₃）利用氮上的孤对电子与氢离子（H⁺）成键。氮原子为新 N–H 键提供了一对电子，该离子呈四面体形状，氮上带有形式正电荷。
5. Electronegativity and Bond Polarity | 电负性与键的极性
Electronegativity is the power of an atom to attract the bonding pair of electrons towards itself. When two atoms with different electronegativities form a covalent bond, the electron density is pulled towards the more electronegative atom, creating a polar bond. This leads to partial charges, denoted by δ⁺ and δ⁻.
电负性是原子吸引成键电子对的能力。当两个电负性不同的原子形成共价键时，电子密度会被拉向电负性较大的原子，从而形成极性键，并产生部分电荷，用 δ⁺ 和 δ⁻ 表示。
The following table relates the electronegativity difference (ΔEN, using the Pauling scale) to bond type:
下表表示了电负性差值（ΔEN，使用鲍林标度）与键型的关系：
| ΔEN (Pauling) | Bond Type | Example |
|---|---|---|
| 0 – 0.4 | Non‑polar covalent | Cl–Cl, C–H |
| 0.5 – 1.7 | Polar covalent | H–Cl, C–O |
| > 1.7 | Ionic | Na⁺Cl⁻ |
6. Polar and Non‑polar Molecules | 极性分子与非极性分子
A molecule is polar if it possesses an overall dipole moment, which depends on both the polarity of individual bonds and the molecular geometry. In carbon dioxide (CO₂), the C=O bonds are polar, but the linear shape causes the dipoles to cancel exactly, making CO₂ a non‑polar molecule.
如果一个分子具有整体的偶极矩，它就是极性分子，这既取决于个别键的极性，也取决于分子的几何形状。在二氧化碳（CO₂）中，C=O 键是极性的，但直线形使得键偶极完全抵消，因此 CO₂ 是非极性分子。
Water (H₂O), on the other hand, has a bent shape with two polar O–H bonds that do not cancel. The oxygen atom carries a partial negative charge and the hydrogen atoms carry partial positive charges, giving water a large net dipole moment and making it an excellent solvent for ionic and polar substances.
相反，水（H₂O）具有弯曲形状，两个极性的 O–H 键不能相互抵消。氧原子带部分负电荷，氢原子带部分正电荷，使水分子具有较大的净偶极矩，因而成为离子化合物和极性物质优良的溶剂。
7. Sigma (σ) and Pi (π) Bonds | σ键与π键
A sigma (σ) bond is formed by the end‑on overlap of atomic orbitals, giving a region of high electron density directly between the two nuclei. All single covalent bonds are σ bonds. A pi (π) bond arises from the sideways overlap of parallel p‑orbitals and has electron density above and below the plane of the nuclei.
σ 键由原子轨道“头对头”重叠形成，在两个原子核之间直接产生高电子密度区域。所有单键都是 σ 键。π 键则由平行的 p 轨道侧面重叠产生，电子密度分布在核平面的上方和下方。
In a double bond, one bond is σ and the second is π (e.g. C=C in ethene). A triple bond comprises one σ and two π bonds (e.g. N≡N). Because the π overlap is weaker than the σ overlap, π bonds are more reactive and can be broken relatively easily in addition reactions.
在双键中，一个键是 σ，第二个键是 π（如乙烯中的 C=C）。三键由一根 σ 键和两根 π 键组成（如 N≡N）。由于 π 重叠比 σ 重叠弱，π 键更具反应活性，在加成反应中较容易断裂。
8. Hybridisation in Carbon and Other Atoms | 碳原子及其他原子的杂化
Hybridisation is the mixing of atomic orbitals to form new, degenerate hybrid orbitals that point in specific directions. It explains the observed bond angles and molecular shapes that simple s and p orbitals cannot account for. Carbon exhibits three main hybridisation states: sp, sp² and sp³.
杂化是原子轨道重新组合形成新的简并杂化轨道的过程，这些杂化轨道指向特定的方向。它解释了简单 s 和 p 轨道无法解释的实验键角和分子形状。碳表现出三种主要的杂化状态：sp、sp² 和 sp³。
- sp³ hybridisation – one s and three p orbitals mix to give four tetrahedrally arranged orbitals (bond angle ~109.5°), as in CH₄.
- sp³ 杂化――一个 s 和三个 p 轨道混合，形成四个四面体排列的轨道（键角约 109.5°），如 CH₄。
- sp² hybridisation – one s and two p orbitals mix, leaving one unhybridised p orbital that forms a π bond; bond angle ~120°, as in ethene (C₂H₄).
- sp² 杂化――一个 s 和两个 p 轨道混合，留下一个未杂化的 p 轨道形成 π 键；键角约 120°，如乙烯（C₂H₄）。
- sp hybridisation – one s and one p orbital mix, giving two linear orbitals and two unhybridised p orbitals for two π bonds; bond angle 180°, as in ethyne (C₂H₂).
- sp 杂化――一个 s 和一个 p 轨道混合，产生两个直线型轨道和两个未杂化 p 轨道形成两个 π 键；键角 180°，如乙炔（C₂H₂）。
9. VSEPR Theory: Predicting Molecular Shapes | VSEPR理论：预测分子形状
Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom will arrange themselves to minimise repulsion. Both bonding pairs and lone pairs must be considered, but lone pairs repel more strongly than bonding pairs, reducing bond angles slightly.
价层电子对互斥（VSEPR）理论指出，中心原子周围的电子对会通过尽可能远离来使排斥力最小。必须同时考虑成键电子对和孤对电子，但孤对电子的排斥力强于成键电子对，因此会略微减小键角。
Key shapes required for Cambridge A‑Level include:
剑桥 A‑Level 要求掌握的关键形状包括：
| Electron pairs (bonding / lone) | Shape | Bond angle (°) | Example |
|---|---|---|---|
| 2 / 0 | Linear | 180 | CO₂, BeCl₂ |
| 3 / 0 | Trigonal planar | 120 | BF₃ |
| 4 / 0 | Tetrahedral | 109.5 | CH₄ |
| 3 / 1 | Trigonal pyramidal | ~107 | NH₃ |
| 2 / 2 | Bent (V‑shaped) | ~104.5 | H₂O |
| 5 / 0 | Trigonal bipyramidal | 90, 120 | PCl₅ |
| 6 / 0 | Octahedral | 90 | SF₆ |
The reduction in bond angle from the ideal tetrahedral angle in NH₃ and H₂O is a direct consequence of increasing lone‑pair repulsion.
NH₃ 和 H₂O 中键角相对理想四面体角的减小，正是孤对电子排斥力增加的直接结果。
10. Bond Length, Bond Energy, and Bond Order | 键长、键能与键级
Bond length is the equilibrium distance between the nuclei of two bonded atoms. Bond energy (bond enthalpy) is the energy required to break one mole of a given covalent bond in the gaseous state. Bond order refers to the number of bonding electron pairs shared between two atoms.
键长是两个成键原子核之间的平衡距离。键能（键焓）是指断裂气态中一摩尔某特定共价键所需的能量。键级是指两个原子之间共用的成键电子对数目。
As a general trend, multiple bonds are shorter and stronger than single bonds between the same atoms. For carbon–carbon bonds: C–C (single, bond order 1) has a bond length of about 154 pm and a bond energy around 350 kJ mol⁻¹, whereas C=C (double, bond order 2) is about 134 pm and 610 kJ mol⁻¹, and C≡C (triple, bond order 3) is about 120 pm and 840 kJ mol⁻¹.
一般而言，同种原子之间的多重键比单键更短、更强。就碳–碳键而言：C–C（单键，键级 1）键长约为 154 pm，键能约 350 kJ mol⁻¹；C=C（双键，键级 2）键长约 134 pm，键能约 610 kJ mol⁻¹；C≡C（三键，键级 3）键长约 120 pm，键能约 840 kJ mol⁻¹。
11. Giant Covalent Structures | 巨型共价结构
Some substances consist of a vast continuous network of covalent bonds extending throughout the entire structure. These macromolecular, or giant covalent, substances have very high melting and boiling points because a great deal of energy is needed to break the strong covalent bonds. Diamond, graphite, graphene and silicon dioxide are key examples.
某些物质由贯穿整个结构的庞大共价键网络组成。这些大分子或称巨型共价物质具有极高的熔点和沸点，因为断裂强大的共价键需要大量能量。金刚石、石墨、石墨烯和二氧化硅是关键实例。
In diamond, each carbon atom is sp³ hybridised and bonded tetrahedrally to four other carbon atoms. This rigid three‑dimensional network makes diamond the hardest known natural material and an electrical insulator, as all electrons are localised in σ bonds.
在金刚石中，每个碳原子为 sp³ 杂化，以四面体方式与另外四个碳原子成键。这种刚性的三维网络使金刚石成为已知最硬的天然材料，同时由于所有电子都定域在 σ 键中，它是电绝缘体。
Graphite consists of layers of sp² hybridised carbon atoms arranged in hexagonal rings. Within each layer, delocalised π electrons above and below the plane allow electrical conductivity parallel to the layers. The weak van der Waals forces between layers permit them to slide over one another, giving graphite its lubricating properties.
石墨由 sp² 杂化碳原子的六角形环层组成。在每一层内，层平面上下离域的 π 电子使得石墨可以平行于层间导电。层与层之间微弱的范德华力允许它们相互滑动，赋予了石墨润滑性。
12. Comparing Covalent Bonding with Other Bond Types | 共价键与其他键型的比较
It is important to distinguish covalent bonding from ionic and metallic bonding. Covalent bonds involve the localised sharing of electron pairs between non‑metals, whereas ionic bonding arises from the electrostatic attraction between oppositely charged ions, typically between a metal and a non‑metal. Metallic bonding, found in pure metals and alloys, consists of a lattice of positive metal ions immersed in a sea of delocalised electrons.
区分共价键与离子键、金属键至关重要。共价键涉及非金属之间定域的电子对共享，而离子键源于带相反电荷的离子之间的静电引力，通常出现在金属与非金属之间。金属键存在于纯金属和合金中，由沉浸在离域电子“海洋”中的正金属离子晶格构成。
A summary of properties helps to differentiate the three bonding types: covalent compounds often have low melting points and poor electrical conductivity (except for giant covalent structures), ionic compounds are hard, brittle and conduct electricity when molten or dissolved, and metals are malleable, ductile and excellent conductors due to their mobile electrons.
性质总结有助于区分三种键型：共价化合物通常熔点较低、导电性差（巨型共价结构除外）；离子化合物坚硬、易碎，在熔融或溶解时导电；金属则因可移动电子而具有延展性、可锻性和优良的导电性。
These comparisons underline why a thorough grasp of covalent bonding theory – from simple Lewis structures to hybridisation and giant networks – is essential for success in Cambridge A‑Level Chemistry.
这些对比强调了为什么透彻掌握共价键理论——从简单的路易斯结构到杂化和巨型网络——对于在剑桥 A‑Level 化学考试中取得成功至关重要。
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