Name: molecular shapes from electron pairs
Uploaded: 2025-05-22T09:05:32.000Z
Duration: 2 min 37 s
Channel: JoVE

价层电子对互斥理论（VSEPR）认为，参与单键、双键或孤对电子的电子基团之间相互排斥，并倾向于彼此保持尽可能远的距离。
分子的几何构型由中心原子周围各种电子基团的排列方式决定。
有五种基本的分子形状：两个成键电子对对应线形，三个对应平面三角形，四个对应四面体形，五个对应三角双锥形，六个对应八面体形。
在预测分子几何构型时，需记住孤对电子与孤对电子之间的排斥作用强于孤对电子与成键电子对之间的排斥作用，而后者又强于成键电子对与成键电子对之间的排斥作用。
考虑甲烷、氨和水的路易斯结构。在每种分子中，中心原子周围都有四个电子基团。
在甲烷中，四个成键电子对呈四面体排列，理想的 H–C–H 键角为 109.5°。
在氨分子中，氮原子有三个成键电子对和一个孤对电子。
孤对电子占据的空间比成键电子对更大。这是因为孤对电子仅受一个原子核的束缚，而成键电子组则由两个原子核共享。
H–N–H 键角小于甲烷中观察到的预期四面体角 109.5°。这种键角的压缩归因于孤对电子对相邻成键电子基团施加的排斥力。
电子对的排列方式称为电子对几何构型。分子几何构型描述的是原子的排列方式，与电子对几何构型不同。氨分子的电子对几何构型为四面体，而其分子形状为三角锥形。
一个水分子在中心原子周围也有四个电子基团。其电子对几何构型同样为四面体，包含两个成键电子基团和两个孤对电子。
两对孤对电子产生的更大排斥力进一步压缩了水分子中的H–O–H键角。该键角远小于理想的四面体键角，因此分子几何构型呈弯曲形。

vsepr理论

价层电子对排斥理论（VSEPR 理论）使我们能够通过检查路易斯结构中的键对和孤电子对的数量来预测中心原子周围的分子结构。 VSEPR 模型假设中心原子价电子层中的电子对将通过尽可能增大它们之间的距离来最小化排斥力，从而选择一种排列方式。中心原子价层中的电子或形成主要位于键合原子之间的键对，或形成孤对。


 在分子中，两个电子密度区域位于中心原子的相对两侧，呈线性排列以最大限度地减少排斥力。 类似地，三个电子基团排列成三角平面几何形状，四个电子基团形成四面体，五个电子基团优选三角双锥几何形状，而六个电子基团呈现八面体取向。


 值得注意的是，中心原子周围的电子对几何形状并不总是与其分子结构相同。 电子对几何结构描述了分子中所有电子区域的位置，无论是键对还是孤对电子。 而分子结构描述的是分子中原子的位置，而不是电子。 因此，只有当中心原子周围没有孤电子对时，电子对几何形状才与分子结构相同。


 孤对电子占据空间比键合电子更大，这是因为孤对电子只与一个原子核结合，而键合电子基团由两个原子核共享。 因此，孤对-孤对之间的排斥力大于孤对-键合对之间的排斥力，后者又大于键合对-键合对之间的排斥力。


 根据VSEPR理论，线性、三角平面、四面体和八面体电子对几何形状中的末端原子位置是等效的，因此任何位置都可以由单个孤对占据。 然而，在三角双锥几何形状中，两个轴向位置不同于三个赤道位置。 此处，由于 120° 键角，赤道位置具有更多可用空间，并且是较大孤对电子的首选位置。 类似地，当两个孤电子对和四个键对围绕中心原子以八面体排列时，两个孤电子对相距 180°，从而形成方形平面分子结构。


 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity">Openstax, Chemistry 2e, Section 7.6 Molecular Structure and Polarity</a>

VSEPR理论

价层电子对互斥理论（VSEPR 理论）使我们能够通过分析中心原子周围的电子对排布来预测其分子结构。

Education

教育

JoVE Core

Organic Chemistry

有机化学

Unit 1

第一单元

Covalent Bonding and Structure

共价键与结构

KEY TERMS AND CONCEPTS

关键术语与概念

VSEPR Theory

molecular shapes from electron pairs

Molecular shape can be predicted from the way electron pairs arrange around a central atom. Valence shell electron-pair repulsion theory, or VSEPR theory, uses a Lewis structure to count bonding pairs and lone electron pairs. The main idea is simple: electron pairs spread out as far as possible to reduce repulsion.

In a molecule, the valence electrons around the central atom can be bonding pairs or lone pairs. Bonding pairs are found mainly between bonded atoms. Lone pairs stay on one atom and take up more space than bonding pairs because they are held by only one nucleus.

The number of electron groups helps determine the electron-pair geometry. Two regions of electron density arrange in a linear shape. Three electron groups form a trigonal planar geometry, four form a tetrahedron, five prefer a trigonal bipyramidal geometry, and six arrange in an octahedral pattern.

Electron-pair geometry is not always the same as molecular structure. Electron-pair geometry includes both bonds and lone pairs. Molecular structure shows only the positions of the atoms, so the two match only when the central atom has no lone pairs.

Lone pair repulsions are not equal to bonding pair repulsions. Lone pair–lone pair repulsions are strongest. Lone pair–bonding pair repulsions are next, and bonding pair–bonding pair repulsions are weakest.

In some geometries, the terminal positions are equivalent. This is true for linear, trigonal planar, tetrahedral, and octahedral electron-pair geometries, so a lone pair can occupy any one of those positions. In a trigonal bipyramidal geometry, the two axial positions are different from the three equatorial positions. The equatorial position has more space because of the 120° bond angles, so larger lone pairs prefer it.

A square planar molecular structure can also form in an octahedral arrangement. When two lone pairs and four bonding pairs are placed octahedrally around a central atom, the lone pairs sit 180° apart. That arrangement leaves the atoms in a square planar shape.

Molecular Shapes from Electron Pairs

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chemical bonds

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polar covalent bonds

Electronegativity in Polar Covalent Bonds

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vsepr theory

Molecular shape can be predicted from the way electron pairs arrange around a central atom. Valence shell electron-pair repulsion theory, or VSEPR theory, uses a Lewis structure to count bonding pairs and lone electron pairs. The main idea is simple: electron pairs spread out as far as possible to reduce repulsion. In a molecule, the valence electrons around the central atom can be bonding pairs or lone pairs. Bonding pairs are found mainly between bonded atoms. Lone pairs stay on one atom and...

molecular geometry and dipole moments

Predicting Shape and Molecular Polarity

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introduction to functional groups

Functional Groups and Organic Molecules

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key terms and concepts

Learn how VSEPR theory uses bonding pairs and lone pairs to predict molecular shape, electron-pair geometry, and common arrangements like linear and octahedral.

Molecular Shapes from Electron Pairs | JoVE K12

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Molecular Shapes from Electron Pairs

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