📚 The Nature of Covalent Bonds and Bond Parameters | 共价键的本质与键参数
A covalent bond is the strong electrostatic attraction between two atoms that results from the sharing of one or more pairs of electrons. This type of bond is typical between non-metal atoms, where the electronegativity difference is small enough to allow electron sharing rather than electron transfer.
共价键是两个原子之间因共享一对或多对电子而产生的强烈静电吸引作用。这类键通常出现在非金属原子之间，因为此时电负性差异较小，原子倾向于共享电子而非转移电子。
1. The Origin of Covalent Bonding | 共价键的起源
When two atoms approach each other, their atomic orbitals overlap. If the electrons are shared in a way that increases the electron density between the two nuclei, the positively charged nuclei are attracted to this negative region, and the system becomes more stable.
当两个原子相互靠近时，它们的原子轨道会发生重叠。如果电子以增加两个原子核之间电子密度的方式被共享，带正电的原子核就会被这片负电荷区域吸引，整个体系因而变得更加稳定。
The bond forms only if the overall potential energy of the system decreases. At the optimum separation, called the bond length, the attractive forces and the repulsive forces between the nuclei balance exactly.
只有当整个体系的势能降低时，化学键才能够形成。在最佳核间距处，即称为键长的平衡距离上，原子核之间的吸引力和排斥力恰好达到平衡。
2. Lewis Structures and the Octet Rule | 路易斯结构与八隅体规则
In a Lewis structure, valence electrons are represented by dots. When forming a covalent bond, two atoms share a pair of electrons, which is shown as a line between the two atomic symbols.
在路易斯结构中，价电子用点来表示。当形成共价键时，两个原子共享一对电子，这在表示中被画成两个原子符号之间的一条线。
The octet rule states that main-group atoms tend to bond so as to surround themselves with eight valence electrons, achieving a noble-gas configuration. For example, in the water molecule H₂O, oxygen shares one electron pair with each hydrogen atom, giving oxygen eight valence electrons and each hydrogen two.
八隅体规则指出，主族原子倾向于通过成键使自身周围拥有八个价电子，从而获得稀有气体的电子构型。例如，在 H₂O 水分子中，氧原子与每个氢原子共享一对电子，使氧拥有八个价电子，而每个氢拥有两个价电子。
3. Valence Bond Theory and Orbital Overlap | 价键理论与轨道重叠
Valence bond theory explains covalent bonding in terms of the overlap of atomic orbitals. The shared electrons occupy the region of space where the two half-filled orbitals overlap, and the electron density between the nuclei is maximised.
价键理论以原子轨道的重叠来解释共价键的形成。共享电子占据两个半充满轨道重叠的空间区域，从而使原子核之间的电子密度达到最大。
The strength of the bond depends on the extent of this overlap. Greater overlap generally leads to a stronger bond. That is why orbitals overlap along the axis between the nuclei whenever possible, rather than with only a sideways interaction.
键的强度取决于重叠的程度。重叠程度越大，键通常就越强。这就是为什么轨道总是尽可能地沿着两个原子核之间的轴线进行重叠，而不是仅仅发生侧向的相互作用。
4. Sigma Bonds and Pi Bonds | σ 键与 π 键
Covalent bonds can be classified by the manner in which orbitals overlap. A sigma bond (σ) forms when two atomic orbitals overlap end-to-end directly along the internuclear axis. All single bonds are sigma bonds.
共价键可以根据轨道重叠的方式进行分类。当两个原子轨道沿着核间轴线头对头直接重叠时，形成 σ 键（sigma 键）。所有单键都是 σ 键。
A pi bond (π) forms when two p orbitals overlap side-by-side, perpendicular to the internuclear axis. A double bond consists of one sigma bond and one pi bond, while a triple bond consists of one sigma bond and two pi bonds. Because the sideways overlap is less effective, pi bonds are generally weaker than sigma bonds.
当两个 p 轨道垂直于核间轴、彼此侧向重叠时，形成 π 键。双键由一个 σ 键和一个 π 键组成，而三键由一个 σ 键和两个 π 键组成。由于侧向重叠的效率较低，π 键通常比 σ 键更弱。
5. Molecular Orbital Theory in Simple Terms | 简单理解分子轨道理论
Molecular orbital theory combines atomic orbitals to form molecular orbitals that spread over the whole molecule. Two atomic orbitals combine to produce one bonding molecular orbital, lower in energy, and one antibonding molecular orbital, higher in energy.
分子轨道理论将原子轨道组合成遍布整个分子的分子轨道。两个原子轨道组合会产生一个能量较低的成键分子轨道和一个能量较高的反键分子轨道。
When electrons occupy the bonding molecular orbital, they stabilise the molecule; when they occupy the antibonding molecular orbital, they destabilise it. The bond order can be calculated as half the difference between the number of bonding electrons and antibonding electrons.
当电子填入成键分子轨道时，会使分子更稳定；而当电子填入反键分子轨道时，则会降低分子的稳定性。键级可以通过成键电子数与反键电子数之差的一半来计算。
6. Bond Length | 键长
Bond length is the equilibrium distance between the nuclei of two bonded atoms. It is determined by the size of the atoms and by the number of shared electron pairs.
键长是两个成键原子核之间的平衡距离。它取决于原子的大小以及共享电子对的数目。
- As the number of bonds between two atoms increases, the bond length decreases. A C=C double bond is shorter than a C–C single bond.
- 随着两个原子之间的键数增多，键长会变短。C=C 双键比 C–C 单键更短。
- For atoms in the same group, bond length increases down the group because atomic radius increases.
- 对于同一主族的原子，随着原子半径增大，键长向下逐渐增加。
7. Bond Energy | 键能
Bond energy is the energy required to break one mole of a specific covalent bond in the gaseous state. It is a measure of bond strength.
键能是在气态下断开一摩尔特定共价键所需的能量。它是衡量键强度的重要指标。
Bond energy increases with bond order. A triple bond has a higher bond energy than a double bond, and a double bond has a higher bond energy than a single bond. However, the relationship is not exactly proportional because a pi bond is weaker than a sigma bond.
键能随键级增大而升高。三键的键能高于双键，双键的键能高于单键。然而，这种关系并不是严格的倍数关系，因为 π 键比 σ 键弱。
For example, the C–C single bond energy is about 346 kJ mol⁻¹, while the C=C double bond energy is about 602 kJ mol⁻¹. The additional pi bond therefore contributes only about 256 kJ mol⁻¹.
例如，C–C 单键的键能约为 346 kJ mol⁻¹，而 C=C 双键的键能约为 602 kJ mol⁻¹。因此，额外增加的 π 键只贡献了大约 256 kJ mol⁻¹。
8. Bond Angle and Molecular Shape | 键角与分子形状
The bond angle is the angle between two adjacent bonds at the central atom. It is determined primarily by the number of electron domains around the central atom.
键角是中心原子上两个相邻化学键之间的夹角。它主要由中心原子周围的电子域数目决定。
According to VSEPR theory, electron domains repel each other and arrange themselves as far apart as possible. Four electron domains give a tetrahedral geometry with approximate bond angles of 109.5°, as in CH₄ and NH₃.
根据 VSEPR 理论，电子域之间相互排斥，并尽可能彼此远离。四个电子域形成四面体几何构型，键角约为 109.5°，如 CH₄ 和 NH₃。
Lone pairs occupy more space than bonding pairs, so they compress the bond angles. In H₂O, for example, the two lone pairs on oxygen reduce the bond angle from 109.5° to about 104.5°.
孤对电子比成键电子对占据更大的空间，因此会压缩键角。例如，在 H₂O 中，氧原子上的两对孤对电子使键角从 109.5° 减小到约 104.5°。
9. Bond Polarity and Electronegativity | 键的极性与电负性
When two atoms with different electronegativities form a covalent bond, the shared electron pair is attracted more strongly to the more electronegative atom. This creates a permanent dipole in the bond.
当电负性不同的两个原子形成共价键时，共享电子对会更多地被电负性更强的原子吸引，从而在键中产生永久偶极。
A pure covalent bond occurs when the electronegativity difference is zero, as in Cl₂ or H₂. A polar covalent bond occurs when the difference is small or moderate, as in H–Cl. If the difference is very large, ionic bonding becomes more favourable.
当电负性差值为零时，如 Cl₂ 或 H₂ 中，形成非极性共价键。当差值较小或中等时，如 H–Cl 中，形成极性共价键。如果差值非常大，则更倾向于形成离子键。
The bond dipole is represented by an arrow pointing towards the more electronegative atom. Molecular polarity, however, also depends on the three-dimensional arrangement of these bond dipoles.
键偶极可以用一个指向电负性更强原子的箭头来表示。然而，分子的极性还取决于这些键偶极在三维空间中的排列方式。
10. Resonance and Delocalised Bonds | 共振与离域键
In some molecules, a single Lewis structure is unable to accurately represent the electron distribution. Resonance structures are used to describe the delocalisation of electrons over multiple atoms.
在某些分子中，单一的路易斯结构无法准确描述电子分布。此时可以使用共振结构来描述电子在多个原子之间的离域。
For instance, the ozone molecule O₃ can be represented as a resonance hybrid of two equivalent structures. Neither S–O bond in SO₃ is a pure double bond or a pure single bond; all S–O bonds are identical and intermediate in length and energy.
例如，臭氧分子 O₃ 可以表示为两个等价共振结构的共振杂化体。SO₃ 中不存在纯的双键或单键，所有 S–O 键都是等价的，其键长和键能介于单键和双键之间。
In benzene, the six electrons in the π system are delocalised across the entire ring. This delocalisation lowers the energy of the molecule and makes the C–C bonds in benzene shorter and stronger than an ordinary C–C single bond.
在苯分子中，π 体系中的六个电子离域于整个环上。这种离域作用降低了分子的能量，使苯中的 C–C 键比普通 C–C 单键更短、更强。
11. Relating Bond Parameters to Molecular Properties | 键参数与分子性质的联系
Bond length, bond energy and bond angle together determine many physical and chemical properties of substances, such as reactivity, boiling point and mechanical strength.
键长、键能和键角共同决定物质的许多物理和化学性质，例如反应活性、沸点和机械强度。
- Short, strong covalent bonds make molecules more chemically stable and more resistant to decomposition.
- 短而强的共价键使分子更稳定，更不容易发生分解反应。
- Longer, weaker bonds are more easily broken, making the molecule more reactive.
- 较长且较弱的键更容易断裂，因此分子反应活性更高。
- Molecules with large bond dipoles and an unsymmetrical shape are polar and tend to have stronger intermolecular forces.
- 具有较大键偶极且形状不对称的分子是极性分子，通常分子间作用力更强。
The bond angle directly affects the shape of the molecule, which in turn determines whether individual bond dipoles cancel or add. This is why CO₂ is non-polar despite having two polar C=O bonds, whereas H₂O is polar because its bent structure prevents cancellation of the O–H bond dipoles.
键角直接决定分子形状，而分子形状又决定各键偶极是相互抵消还是相互叠加。这就是为什么 CO₂ 虽然是线性分子、包含两个极性 C=O 键，却仍然是非极性分子；而 H₂O 由于弯曲结构导致 O–H 键偶极不能抵消，因而呈现极性。
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