📚 Molecular Structure and Geometry | 分子结构与空间构型
Molecular geometry describes the three-dimensional arrangement of atoms within a molecule. It is a central concept in chemistry because molecular shape influences polarity, reactivity, physical state, colour, and even biological function. Two molecules with the same chemical formula can behave very differently if their atoms are arranged differently in space.
分子空间构型描述分子内原子在三维空间中的排列方式。它是化学中的核心概念，因为分子形状会影响极性、反应活性、物理状态、颜色甚至生物功能。两个化学式相同的分子，如果其原子在空间中的排列方式不同，其性质可能会有很大差异。
1. VSEPR Theory | VSEPR理论（价层电子对互斥理论）
The Valence Shell Electron Pair Repulsion (VSEPR) theory is the most widely used model for predicting molecular geometry. It is based on the simple idea that electron pairs around a central atom repel one another. To minimise this repulsion, electron pairs arrange themselves as far apart as possible in three-dimensional space.
价层电子对互斥理论是预测分子空间构型最常用的模型。其基本思想是：中心原子周围的电子对会相互排斥。为了使排斥力最小化，电子对会在三维空间中尽可能远离彼此。
Each electron pair, whether it is a bonding pair or a lone pair, occupies a region of space called an electron domain. The overall arrangement of electron domains is called the electron-domain geometry, and when lone pairs are ignored, the arrangement of only the atoms is called the molecular geometry.
每对电子，无论是成键电子对还是孤对电子，都会占据一个称为“电子域”的空间区域。电子域的整体排列称为“电子域几何”；而忽略孤对电子后，仅由原子核位置确定的排列称为“分子几何”或“分子空间构型”。
2. Electron Domains and Lone Pairs | 电子域与孤对电子
In VSEPR theory, a single bond, a double bond, a triple bond, and a lone pair all count as one electron domain. The number of electron domains around the central atom determines the basic electron-domain geometry. Bonding domains are always occupied by atoms, while non-bonding domains are lone pairs of electrons.
在VSEPR理论中，单键、双键、三键和孤对电子均计为一个电子域。中心原子周围的电子域数目决定了基本的电子域几何。成键电子域由原子占据，而非成键电子域则是孤对电子。
- 2 electron domains: linear arrangement, bond angle 180°.
2个电子域：直线形排列，键角180°。 - 3 electron domains: trigonal planar arrangement, bond angle 120°.
3个电子域：平面三角形排列，键角120°。 - 4 electron domains: tetrahedral arrangement, bond angle 109.5°.
4个电子域：正四面体形排列，键角109.5°。 - 5 electron domains: trigonal bipyramidal arrangement, angles of 90° and 120°.
5个电子域：三角双锥形排列，含有90°和120°的键角。 - 6 electron domains: octahedral arrangement, bond angle 90°.
6个电子域：八面体形排列，键角90°。
Lone pairs occupy space just like bonding pairs, but they are not shared with another nucleus. Because lone pairs are held more tightly by the central atom, they exert a stronger repulsive effect than bonding pairs. This difference is crucial when determining the final molecular shape.
孤对电子与成键电子对一样占据空间，但孤对电子不与另一个原子核共享。由于孤对电子受中心原子的束缚更强，因此它们比成键电子对表现出更强的排斥效应。这种差异对确定最终分子构型至关重要。
3. Common Molecular Shapes | 常见分子空间构型
After accounting for lone pairs, the molecular geometry can be different from the electron-domain geometry. For example, methane has four bonding domains and no lone pairs, so both its electron-domain geometry and molecular geometry are tetrahedral. In contrast, ammonia has three bonding domains and one lone pair: its electron-domain geometry is tetrahedral, but its molecular geometry is trigonal pyramidal.
在考虑孤对电子之后，分子几何可能与电子域几何不同。例如，甲烷有四个成键电子域且没有孤对电子，因此其电子域几何和分子几何都是正四面体形。相比之下，氨有三个成键电子域和一个孤对电子：其电子域几何为正四面体形，但分子几何为三角锥形。
| Electron Domains 电子域数 | Electron-Domain Geometry 电子域几何 | Molecular Geometry 分子几何 | Ideal Bond Angle 理想键角 | Examples 示例 |
|---|---|---|---|---|
| 2 | Linear 直线形 | Linear 直线形 | 180° | CO₂, BeCl₂ |
| 3 | Trigonal Planar 平面三角形 | Trigonal Planar 平面三角形 / Bent 角形 | 120° / < 120° | BF₃, SO₂ |
| 4 | Tetrahedral 正四面体形 | Tetrahedral 正四面体形 / Trigonal Pyramidal 三角锥形 / Bent 角形 | 109.5° / < 109.5° / < 109.5° | CH₄, NH₃, H₂O |
| 5 | Trigonal Bipyramidal 三角双锥形 | Trigonal Bipyramidal 三角双锥形 / Seesaw 跷跷板形 / T-Shaped T字形 / Linear 直线形 | 90°, 120° | PCl₅, SF₄, ClF₃, XeF₂ |
| 6 | Octahedral 八面体形 | Octahedral 八面体形 / Square Pyramidal 四方锥形 / Square Planar 平面正方形 | 90° | SF₆, BrF₅, XeF₄ |
4. Hybridization and Orbital Types | 杂化轨道与轨道类型
Hybridization is the mixing of atomic orbitals on the central atom to form new, equivalent hybrid orbitals that are used in bonding. The type of hybridization matches the electron-domain geometry around the central atom. For example, a carbon atom with four electron domains adopts sp³ hybridization, with four equivalent orbitals pointing toward the corners of a tetrahedron.
杂化是中心原子上原子轨道混合形成新的等价杂化轨道，并用于成键的过程。杂化类型与中心原子周围的电子域几何相对应。例如，具有四个电子域的碳原子采取sp³杂化，形成四个等价的轨道，指向正四面体的四个顶点。
- sp hybridization: 2 electron domains, linear, 180°. Example: BeCl₂, CO₂.
sp杂化：2个电子域，直线形，180°。例如：BeCl₂、CO₂。 - sp² hybridization: 3 electron domains, trigonal planar, 120°. Example: BF₃, ethene C₂H₄.
sp²杂化：3个电子域，平面三角形，120°。例如：BF₃、乙烯C₂H₄。 - sp³ hybridization: 4 electron domains, tetrahedral, 109.5°. Example: CH₄, NH₃, H₂O.
sp³杂化：4个电子域，正四面体形，109.5°。例如：CH₄、NH₃、H₂O。 - sp³d hybridization: 5 electron domains, trigonal bipyramidal. Example: PCl₅.
sp³d杂化：5个电子域，三角双锥形。例如：PCl₅。 - sp³d² hybridization: 6 electron domains, octahedral. Example: SF₆.
sp³d²杂化：6个电子域，八面体形。例如：SF₆。
Lone pairs also occupy hybrid orbitals. In ammonia, the nitrogen atom is sp³ hybridised: three orbitals contain bonding pairs and one orbital contains a lone pair. This is why the observed H-N-H bond angle is slightly less than the ideal 109.5°.
孤对电子也占据杂化轨道。在氨分子中，氮原子是sp³杂化的：三个轨道含成键电子对，一个轨道含孤对电子。因此，实验测得的H-N-H键角略小于理想的109.5°。
5. Sigma and Pi Bonds | σ键与π键
Covalent bonds can be divided into sigma (σ) bonds and pi (π) bonds. A sigma bond is formed by head-on overlap of orbitals along the internuclear axis; it is the first bond formed between any two atoms. A pi bond is formed by side-by-side overlap of parallel p orbitals and only exists after a sigma bond is already present.
共价键可分为σ键和π键。σ键是原子轨道沿键轴方向“头碰头”重叠形成的，是任意两个原子之间形成的第一个键。π键是平行p轨道“肩并肩”重叠形成的，并且只有在已有σ键之后才会出现。
A single bond is one σ bond. A double bond consists of one σ bond and one π bond. A triple bond consists of one σ bond and two π bonds. For example, ethene C₂H₄ has a C=C double bond, while ethyne C₂H₂ has a C≡C triple bond.
单键由一个σ键构成。双键由一个σ键和一个π键构成。三键由一个σ键和两个π键构成。例如，乙烯C₂H₄含有C=C双键，而乙炔C₂H₂含有C≡C三键。
6. Bond Angle Trends | 键角变化规律
Bond angles are determined by the balance of repulsions among electron domains. The order of repulsion strength is:
键角由各电子域之间排斥力的平衡决定。排斥力的大小顺序为：
lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
孤对电子-孤对电子 ＞ 孤对电子-成键电子对 ＞ 成键电子对-成键电子对
Because lone pairs repel more strongly than bonding pairs, they compress adjacent bond angles. This explains the trend from methane to water:
由于孤对电子比成键电子对的排斥力更强，它们会压缩相邻的键角。这解释了从甲烷到水的键角变化趋势：
- CH₄: four bonding pairs, no lone pairs, H-C-H angle = 109.5°.
CH₄：四个成键电子对，无孤对电子，H-C-H键角为109.5°。 - NH₃: three bonding pairs, one lone pair, H-N-H angle ≈ 107°.
NH₃：三个成键电子对，一个孤对电子，H-N-H键角约为107°。 - H₂O: two bonding pairs, two lone pairs, H-O-H angle ≈ 104.5°.
H₂O：两个成键电子对，两个孤对电子，H-O-H键角约为104.5°。
7. Step-by-Step Prediction of Geometry | 预测空间构型的步骤
To predict the molecular geometry of a simple molecule or ion, a systematic procedure can be used. The central atom is usually the atom that appears once in the formula and has the lowest electronegativity.
要预测简单分子或离子的空间构型，可以采用一套系统的步骤。中心原子通常是化学式中只出现一次且电负性最低的原子。
- Draw the Lewis structure and count all valence electrons.
画出路易斯结构，并计算所有价电子总数。 - Count the total number of electron domains around the central atom, including lone pairs and multiple bonds as one domain each.
计算中心原子周围电子域的总数，孤对电子和多重键均各算作一个电子域。 - Determine the electron-domain geometry from the number of domains.
根据电子域数目确定电子域几何。 - Identify how many domains are lone pairs and how many are bonding pairs.
确认其中有多少个孤对电子域和多少个成键电子域。 - Ignore the lone pairs to name the molecular geometry.
忽略孤对电子，确定分子的几何构型名称。 - Predict the approximate bond angles and hybridization.
预测近似的键角和杂化方式。
For example, consider NH₃. Nitrogen has 5 valence electrons and each hydrogen brings 1 electron, giving 8 valence electrons. Nitrogen has three bonding pairs and one lone pair, so the electron-domain geometry is tetrahedral, but the molecular geometry is trigonal pyramidal with a bond angle near 107°.
例如，分析NH₃。氮有5个价电子，每个氢提供1个电子，共8个价电子。氮有三个成键电子对和一个孤对电子，因此其电子域几何为正四面体形，但分子几何为三角锥形，键角接近107°。
8. Molecular Polarity | 分子极性
A molecule is polar if it has a net dipole moment, meaning the bond dipoles do not cancel. Molecular geometry plays a decisive role in this cancellation. Symmetrical molecules such as CO₂, BF₃, CH₄, and SF₆ have bond dipoles that cancel, so they are non-polar even though their individual bonds are polar.
如果分子具有净偶极矩，即键偶极无法相互抵消，则该分子是极性的。分子空间构型对偶极是否抵消起决定作用。CO₂、BF₃、CH₄和SF₆等对称分子的键偶极相互抵消，因此即使其个别键具有极性，分子整体却是非极性的。
- CO₂: linear, non-polar.
CO₂：直线形，非极性。 - BF₃: trigonal planar, non-polar.
BF₃：平面三角形，非极性。 - CH₄: tetrahedral, non-polar.
CH₄：正四面体形，非极性。 - H₂O: bent, polar.
H₂O：角形，极性。 - NH₃: trigonal pyramidal, polar.
NH₃：三角锥形，极性。
Water is a classic example of a polar molecule. The two O-H bond dipoles do not cancel because the molecule is bent, and the two lone pairs on oxygen contribute to the overall dipole moment. This polarity explains water’s excellent solvent properties and its relatively high boiling point.
水是极性分子的经典例子。由于水分子为角形结构，两个O-H键偶极无法抵消，而且氧上的两对孤对电子也对整体偶极矩有贡献。这种极性解释了水优异的溶解能力和相对较高的沸点。
9. Exceptions and Expanded Octets | 例外与扩展八隅体
VSEPR theory works well for most second-period elements, but elements in the third period and beyond can have more than eight valence electrons. These species are said to have an expanded octet. Common examples include phosphorus pentachloride PCl₅, sulfur hexafluoride SF₆, and xenon tetrafluoride XeF₄.
VSEPR理论对大多数第二周期元素效果良好，但第三周期及之后的元素可以拥有超过八个价电子。这类物种被称为“扩展八隅体”。常见例子包括五氯化磷PCl₅、六氟化硫SF₆和四氟化氙XeF₄。
In PCl₅, phosphorus forms five bonding pairs, giving a trigonal bipyramidal shape. In SF₆, sulfur forms six bonding pairs, giving an octahedral shape. In XeF₄, xenon has four bonding pairs and two lone pairs: the electron-domain geometry is octahedral, but the molecular geometry is square planar.
在PCl₅中，磷形成五个成键电子对，因此呈三角双锥形。在SF₆中，硫形成六个成键电子对，因此呈八面体形。在XeF₄中，氙有四个成键电子对和两个孤对电子：其电子域几何为八面体形，但分子几何为平面正方形。
VSEPR theory also has limitations. It does not explain the relative bond energies in molecules with resonance, and it does not predict the geometry of transition metal complexes well. For those species, more advanced theories such as Valence Bond Theory with d-orbital hybridisation or Crystal Field Theory are needed.
VSEPR理论也有局限性。它无法解释具有共振结构的分子中键能的相对大小，也无法很好地预测过渡金属配合物的空间构型。对于这些物种，需要更高级的理论，如包含d轨道杂化的价键理论或晶体场理论。
10. Summary Table | 核心总结表
The table below summarises the relationships among electron domains, hybridization, molecular geometry, bond angle, and polarity for common molecules. This overview is useful for quick revision before exams.
下表总结了常见分子中电子域数、杂化方式、分子几何、键角与极性之间的关系。该概览有助于考前快速复习。
| Molecule 分子 | Electron Domains 电子域数 | Hybridisation 杂化方式 | Molecular Geometry 分子几何 | Bond Angle 键角 | Polarity 极性 |
|---|---|---|---|---|---|
| CO₂ | 2 | sp | Linear 直线形 | 180° | Non-polar 非极性 |
| BF₃ | 3 | sp² | Trigonal Planar 平面三角形 | 120° | Non-polar 非极性 |
| SO₂ | 更多咨询请联系16621398022（同微信）
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