📚 Covalent Bonding | 共价键
Covalent bonding is the fundamental type of chemical bond in which two atoms share one or more pairs of valence electrons. This mutual attraction between the nuclei and the shared electron cloud holds atoms together in molecules or network solids. Covalent bonds typically form between non-metal atoms with similar electronegativities, enabling each atom to attain a stable noble gas electron configuration.
共价键是最基本的化学键类型之一，两个原子通过共享一对或多对价电子相互结合。原子核对共享电子云的共同吸引将原子维系在分子或网络固体中。共价键通常形成于电负性相近的非金属原子之间，使每个原子都能获得稳定的稀有气体电子构型。
1. What is Covalent Bonding? | 什么是共价键？
A covalent bond arises when atomic orbitals overlap, allowing electrons to be shared between two nuclei. The shared pair of electrons is often represented as a line between the two atomic symbols, such as H–H for a hydrogen molecule. The overlapping orbitals create a region of increased electron density between the nuclei, which balances the repulsive forces between the positively charged nuclei.
当原子轨道重叠，使得电子在两个核之间共享时，就形成了共价键。共享电子对通常用两个原子符号之间的线段表示，例如氢气分子可表示为 H–H。轨道重叠在核间形成了一个电子密度增大的区域，从而平衡了两个带正电荷原子核之间的排斥力。
In a covalent bond, the shared electrons belong to the overall molecule rather than to individual atoms. This delocalisation of electrons lowers the potential energy of the system, making the bonded state more stable than the isolated atoms. The strength and length of the bond depend on the degree of orbital overlap and the nature of the bonded atoms.
在共价键中，共享电子属于整个分子而非单个原子。电子的这种离域降低了体系的势能，使成键状态比孤立原子更稳定。键的强度和长度取决于轨道重叠的程度以及成键原子的性质。
2. Lewis Dot Structures | 路易斯点式
Lewis dot structures provide a simple way to visualise covalent bonding. In these diagrams, the valence electrons of an atom are represented as dots placed around the atomic symbol. A shared pair of electrons, or bonding pair, is shown as a line or a pair of dots between the two symbols. Lone pairs, which are not involved in bonding, remain as dots on individual atoms.
路易斯点式提供了一种直观表示共价键的简单方法。在这些图中，原子的价电子用围绕原子符号的点来表示。共享电子对（即成键电子对）用两个符号之间的一条线段或一对点表示。未参与成键的孤对电子则保留为单个原子周围的点。
To construct a Lewis structure, one must calculate the total number of valence electrons available from all atoms, connect atoms with single bonds, and then distribute the remaining electrons to satisfy the octet rule or other stable configurations. The central atom is typically the least electronegative element (excluding hydrogen). Multiple bonds can be introduced if the octets are not complete.
构建路易斯结构时，需要先计算所有原子提供的价电子总数，用单键连接原子，然后分配剩余电子以满足八隅体规则或其他稳定构型。中心原子通常是电负性最低的元素（氢除外）。如果八隅体不完整，可以引入多重键。
3. The Octet Rule and Exceptions | 八隅体规则与例外
The octet rule states that atoms tend to form covalent bonds until they are surrounded by eight valence electrons, achieving a noble gas electron configuration. This rule explains the bonding in a vast number of molecules, such as methane (CH₄), water (H₂O) and carbon dioxide (CO₂). The driving force is the attainment of a filled s and p subshell.
八隅体规则指出，原子倾向于形成共价键，直到周围拥有八个价电子，达到稀有气体的电子构型。这条规则解释了大量分子的成键情况，如甲烷 (CH₄)、水 (H₂O) 和二氧化碳 (CO₂)。驱动力是填满 s 和 p 亚层。
However, there are notable exceptions. Hydrogen obeys the duet rule and is stable with only two electrons. Incomplete octets occur in electron-deficient molecules such as boron trifluoride (BF₃), where boron has only six electrons. Expanded octets are found in elements of period 3 and beyond, such as phosphorus in PCl₅ (10 electrons) or sulfur in SF₆ (12 electrons), owing to accessible d orbitals.
然而，也存在明显的例外。氢遵循二电子规则，仅有两个电子即达到稳定。在缺电子分子（如三氟化硼 BF₃）中，硼原子仅拥有六个电子，属于不完全八隅体。第三周期及之后的元素可以出现扩展八隅体，例如 PCl₅ 中的磷有 10 个电子，SF₆ 中的硫有 12 个电子，这是因为有可用的 d 轨道。
4. Multiple Bonds | 多重键
When two electron pairs are shared between two atoms, a double bond is formed, as in ethene (C₂H₄) or carbon dioxide (CO₂). Sharing three electron pairs produces a triple bond, found in nitrogen gas (N₂) and ethyne (C₂H₂). Multiple bonds are shorter and stronger than single bonds between the same pair of atoms because of the increased electron density between the nuclei.
当两个原子共享两对电子时，形成双键，例如乙烯 (C₂H₄) 或二氧化碳 (CO₂)。共享三对电子产生三键，存在于氮气 (N₂) 和乙炔 (C₂H₂) 中。同一对原子之间的多重键比单键更短、更强，因为核间电子密度更大。
A double bond consists of one σ bond and one π bond, while a triple bond consists of one σ bond and two π bonds. The presence of π bonds restricts rotation around the bond axis, leading to geometric isomerism in alkenes. Multiple bonds also affect bond angles and molecular shape as predicted by VSEPR theory.
双键由一个 σ 键和一个 π 键组成，三键则由一个 σ 键和两个 π 键组成。π 键的存在限制了绕键轴的旋转，导致烯烃中出现几何异构。多重键也会影响键角和分子形状，这可以通过 VSEPR 理论预测。
5. Bond Polarity and Electronegativity | 键的极性与电负性
In a covalent bond between two identical atoms, electrons are shared equally, producing a non-polar bond. When atoms of different electronegativities bond, the more electronegative atom attracts the bonding pair more strongly, creating a polar covalent bond. The shift in electron density is indicated by partial charges δ⁺ and δ⁻.
在两个相同原子之间的共价键中，电子均等地共享，产生非极性键。当电负性不同的原子成键时，电负性较大的原子会更强烈地吸引成键电子对，形成极性共价键。电子密度的偏移用部分电荷 δ⁺ 和 δ⁻ 表示。
Electronegativity differences (ΔEN) can be used to predict bond type. A ΔEN below 0.5 typically indicates a non-polar covalent bond, 0.5–1.7 suggests a polar covalent bond, and a value above 1.7 often signifies ionic bonding, though there is a continuum between ionic and covalent. The polarity of individual bonds contributes to the overall dipole moment of a molecule.
电负性差 (ΔEN) 可用于预测键型。ΔEN 小于 0.5 通常为非极性共价键，0.5–1.7 属于极性共价键，大于 1.7 往往意味着离子键，但离子与共价之间是连续过渡的。单个键的极性决定了分子的整体偶极矩。
6. Bond Energy and Bond Length | 键能与键长
Bond energy (bond enthalpy) is the energy required to break one mole of a specific covalent bond in the gaseous state. It is a measure of bond strength and is always positive, as energy must be supplied to overcome the attractive forces. Bond length is the equilibrium distance between the nuclei of two bonded atoms.
键能（键焓）是指断裂气态下 1 摩尔某种特定共价键所需要的能量。它是键强度的量度，且总为正值，因为必须提供能量来克服吸引力。键长是两个成键原子核之间的平衡距离。
bit higher bond energies and shorter bond lengths compared to single bonds. For example, C– y 347 kJ mol⁻¹ with a length of 154 pm, while C=C is around 614 kJ mol⁻¹ and 134 pm, and C≡ se trends are influenced by the number of shared electron pairs and the size of the atoms.
多重键与单键相比具有更高的键能和更短的键长。例如，C–C 单键键能约为 347 kJ mol⁻¹，键长 154 pm；C=C 约为 614 kJ mol⁻¹，134 pm；C≡C 约为 839 kJ mol⁻¹，120 pm。这些趋势受共享电子对数目和原子大小的影响。
| Bond Type | Bond Energy (kJ mol⁻¹) | Bond Length (pm) |
|---|---|---|
| C–C | 347 | 154 |
| C=C | 614 | 134 |
| C≡C | 839 | 120 |
7. Resonance Structures | 共振结构
Resonance occurs when two or more valid Lewis structures, differing only in the distribution of electrons, can be drawn for a molecule or ion. The actual structure is a resonance hybrid — a weighted average of the contributing forms. Classic examples include the carbonate ion (CO₃²⁻) and ozone (O₃).
当可以为一个分子或离子画出两个或更多仅在电子分布上不同的合理路易斯结构时，就存在共振。实际结构是共振杂化体——即各个贡献形式的加权平均。典型实例包括碳酸根离子 (CO₃²⁻) 和臭氧 (O₃)。
In the resonance hybrid, bond lengths are equalised and intermediate between those of single and double bonds. Delocalised electrons are spread over several atoms, which increases stability. Benzene (C₆H₆) is a prominent example where the six carbon–carbon bonds are all equivalent, with a bond order of 1.5.
在共振杂化体中，键长均一化，介于单键和双键之间。离域电子分散在多个原子上，从而增加稳定性。苯 (C₆H₆) 是一个显著例子，六个碳碳键完全相等，键级为 1.5。
8. Formal Charge | 形式电荷
ure. It is calculated for each atom using the formula: Formal charge = (valence electrons) – (non-bonding electrons) – ero as possible, and any negative formal charges reside on the most electronegative atoms.
形式电荷有助于评估最合理的路易斯结构。每个原子的形式电荷按公式计算：形式电荷 = 价电子数 – 非键电子数 – ½(成键电子数)。最稳定的结构通常使形式电荷尽可能接近零，任何负的形式电荷应位于电负性最强的原子上。
For example, in the cyanate ion (OCN⁻), the arrangement with a triple bond between C and N and a single bond between O and C gives the lowest formal charge distribution, making it the preferred structure. Formal charge is a bookkeeping tool and does not represent the real charge distribution.
例如，在氰酸根离子 (OCN⁻) 中，C 与 N 之间为三键、O 与 C 之间为单键的排列具有最低的形式电荷分布，因而是优先结构。形式电荷是一种记账工具，并不代表真实的电荷分布。
9. Valence Shell Electron Pair Repulsion (VSEPR) Theory | 价层电子对互斥理论
VSEPR theory predicts molecular geometry by assuming that electron pairs around a central atom arrange themselves to minimise repulsions. Both bonding pairs and lone pairs are treated as regions of electron density. The basic electron-pair geometries include linear (2 pairs), trigonal planar (3 pairs), tetrahedral (4 pairs), trigonal bipyramidal (5 pairs) and octahedral (6 pairs).
VSEPR 理论通过假设中心原子周围的电子对排列成使排斥力最小的构型来预测分子几何形状。成键电子对和孤对电子均被视为电子密度区域。基本的电子对几何构型包括直线形（2 对）、三角形平面（3 对）、四面体（4 对）、三角双锥（5 对）和八面体（6 对）。
Lone pairs exert greater repulsion than bonding pairs, so they compress bond angles. For instance, in ammonia (NH₃), the H–N–H angle is reduced from the tetrahedral 109.5° to about 107° due to one lone pair. In water (H₂O), two lone pairs compress the H–O–H angle further to approximately 104.5°.
孤对电子的排斥力大于成键电子对，因此会压缩键角。例如，在氨 (NH₃) 中，由于一对孤对电子的存在，H–N–H 键角从四面体的 109.5° 减小到约 107°。在水 (H₂O) 中，两对孤对电子将 H–O–H 键角进一步压缩至约 104.5°。
The table below summarises common molecular shapes and bond angles predicted by VSEPR.
下表总结了 VSEPR 预测的常见分子形状和键角。
| Electron Pairs | Lone Pairs | Shape | Approximate Bond Angle | Example |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | BeCl₂ |
| 3 | 0 | Trigonal planar | 120° | BF₃ |
| 3 | 1 | Bent | <120° | SO₂ |
| 4 | 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 1 | Trigonal pyramidal | ~107° | NH₃ |
| 4 | 2 | Bent | ~104.5° | H₂O |
| 5 | 0 | Trigonal bipyramidal | 90°, 120° | PCl₅ |
| 6 | 0 | Octahedral | 90° | SF₆ |
10. Sigma and Pi Bonds | σ键与π键
Covalent bonds can be classified according to the type of orbital overlap. A sigma (σ) bond results from head-on overlap of atomic orbitals along the internuclear axis. It is the strongest type of covalent bond and is always present as the first bond in any multiple bond. A pi (π) bond is formed by sideways overlap of p orbitals above and below the internuclear axis.
共价键可以根据轨道重叠的类型分类。σ 键由原子轨道沿核间轴以头对头的方式重叠形成。这是最强的共价键类型，在任何多重键中总是作为第一个键出现。π 键则是由 p 轨道在核间轴的上方和下方侧向重叠形成。
σ bonds allow free rotation around the bond axis, while π bonds restrict rotation, leading to cis-trans isomerism. In a double bond, one σ and one π bond combine; in a triple bond, one σ and two π bonds are present. The electron density in a π bond is concentrated in two lobes above and below the plane of the atoms, making it more reactive than a σ bond.
σ 键允许绕键轴自由旋转，而 π 键限制旋转，导致顺反异构现象。双键由一个 σ 键和一个 π 键组成；三键则有一个 σ 键和两个 π 键。π 键的电子密度集中在原子平面上下两个叶瓣中，因此比 σ 键更活泼。
11. Dative Covalent Bonds | 配位共价键
A dative covalent bond, or coordinate bond, is a special type of covalent bond in which both electrons in the shared pair originate from the same atom. Once formed, a dative bond is indistinguishable from an ordinary covalent bond in terms of length and strength. The donor atom must have a lone pair, and the acceptor atom must have an available low-energy orbital.
配位共价键（或称配价键）是一种特殊的共价键，其中共享的电子对完全来自同一个原子。一旦形成，配位键在键长和强度上与普通共价键无法区分。供体原子必须具有孤对电子，受体原子必须有可用的低能轨道。
Common examples include the formation of the ammonium ion (NH₄⁺) by the reaction of ammonia with a hydrogen ion, and the hydronium ion (H₃O⁺) from water and H⁺. Transition metal complexes extensively involve coordinate bonds from ligands to the central metal ion. Lewis acid-base reactions are often dative bond formations.
常见例子包括氨与氢离子反应形成铵离子 (NH₄⁺)，以及水与 H⁺ 形成水合氢离子 (H₃O⁺)。过渡金属配合物中配体与中心金属离子之间大量存在配位键。路易斯酸碱反应通常就是配位键的形成。
12. Giant Covalent Structures | 巨型共价结构
Some elements and compounds form giant covalent lattices, also called network solids, where atoms are held together by covalent bonds extending throughout the entire crystal. Unlike simple molecular substances, these materials have very high melting and boiling points because strong covalent bonds must be broken to change state.
一些元素和化合物形成巨型共价晶格，也称为网络固体，其中原子通过贯穿整个晶体的共价键相互连接。与简单分子物质不同，这些材料具有极高的熔点和沸点，因为改变状态必须断裂强共价键。
Diamond is an allotrope of carbon where each carbon atom is tetrahedrally bonded to four others, forming a rigid three-dimensional network. This makes diamond extremely hard, an electrical insulator, and an excellent thermal conductor. Graphite, another carbon allotrope, has layers of hexagonal rings held together by weak van der Waals forces, allowing the layers to slide and conduct electricity along the planes due to delocalised electrons.
金刚石是碳的一种同素异形体，每个碳原子与另外四个碳原子形成四面体键，构成刚硬的三维网络。这使得金刚石极为坚硬，是电绝缘体，但却是优良的热导体。石墨是碳的另一种同素异形体，具有由弱范德华力维系的六边形环状层，层与层之间可以滑动，并由于离域电子可以在层内导电。
Other important giant covalent structures include silicon dioxide (SiO₂), which forms the basis of quartz and sand, and silicon carbide (SiC). These substances have tetrahedral units similar to diamond but with alternating atoms. They are widely used in high-temperature and abrasive applications.
其他重要的巨型共价结构包括二氧化硅 (SiO₂)，它是石英和沙子的主要成分，以及碳化硅 (SiC)。这些物质具有类似于金刚石的四面体单元，但原子交替排列。它们广泛用于高温和磨料应用领域。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图