Name: predicting shape and molecular polarity
Uploaded: 2023-04-30T18:28:31.000Z
Duration: 2 min 36 s
Channel: JoVE

价层电子对互斥理论有助于确定电子对几何构型和分子几何构型。
可以使用一系列步骤来预测分子（如三氯化磷）的几何构型和键角。
第一步是绘制该分子的路易斯结构。
接下来，计算中心原子上的电子基团总数。在磷原子周围共有四个电子基团：三个成键电子对和一个孤对电子。
现在确定电子对几何构型。电子对几何构型为四面体。然而，由于存在孤对电子，分子几何构型为三角锥形。孤对电子使键角减小至小于109.5°。
成键电子对在两个成键原子之间并不总是均等共享的。
在像氟化氢这样的共价键中，电子会被吸引到电负性更强的原子上，这表现为部分电荷。这类键称为极性键。
电荷分离产生一个称为键偶极矩的矢量，用希腊字母 µ 表示。其数值是部分电荷量与它们之间距离的乘积。
偶极矩通常以德拜（debye）为单位表示。1 德拜等于 3.336 × 10−30 库仑·米。
矢量从电负性较小的原子指向电负性较大的原子，表示键的偶极矩。其长度与两个原子间电负性差异的大小成正比。大多数由不同元素原子组成的双原子分子都具有偶极矩，因此属于极性分子。
在多原子化合物中，净偶极矩由各个键的偶极矩以及化合物的几何构型决定。
考虑一个具有两个极性键的水分子。它呈弯曲形状，是一种极性分子。
相比之下，二氧化碳分子呈线性结构。两个碳-氧键虽为极性键，但方向相反，彼此抵消了各自的偶极矩，从而使整个分子为非极性分子。

分子几何与偶极矩

VSEPR 理论可用于确定电子对几何形状和分子结构，步骤如下:

 <ol>
 <li>写出分子或多原子离子的路易斯电子式。</li>
 <li>计算中心原子周围的电子基团数量（包括孤对电子和键）。 单键、双键或三键算作一个电子密度区域。</li>
 <li>根据电子基团的数量确定电子对的几何形状。</li>
 <li>利用孤对电子的数量来确定分子结构。 如果有多种可能的孤对电子和化学键的排列方式，选择能够最大程度减少排斥力的排列。</li>
 </ol>
 <h4>分子偶极矩</h4>

 当具有不同电负性的原子形成化学键时，电子会被拉向电负性较大的原子，使一个原子带有部分正电荷(δ+)，另一个原子带有部分负电荷(δ–)。 这种键称为极性共价键，电荷分离会产生键偶极矩。 键偶极矩的大小由希腊字母 µ 表示，计算公式为:


 μ = Qr


 其中 Q 是净电荷的大小（由电负性差决定），r 是它们之间的距离。 偶极矩通常以德拜表示，其中 1 德拜等于 3.336 × 10−30 C·m.。


 键偶极矩是一个矢量，用箭头表示，箭头从电负性较小的原子指向电负性较大的原子，箭头的起点带有一个小加号。


 整个分子也可能具有电荷分离，具体取决于其分子结构和每个键的极性。 此类分子被认为是极性的。 偶极矩衡量整个分子中净电荷分离的程度。 在双原子分子中，键偶极矩决定分子极性。


 当分子包含多个键时，必须考虑几何形状。 如果分子中键的排列使得键矩矢量和等于零，则该分子是非极性的（例如，CO2）。 水分子具有弯曲的分子结构，两个键矩不抵消。 因此，水是具有净偶极矩的极性分子。


 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

价层电子对互斥理论（VSEPR理论）可用于确定电子对几何构型和分子结构，步骤如下：

写出该分子或离子的路易斯结构……

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

Molecular Geometry and Dipole Moments

predicting shape and molecular polarity

Molecular shape and molecular polarity can be predicted with VSEPR theory and dipole moments. Start by writing the Lewis structure of the molecule or polyatomic ion. Then count the electron groups around the central atom. Lone pairs and bonds both count as electron groups, and a single, double, or triple bond each counts as one region of electron density.

Use that count to identify the electron-pair geometry. Next, use the number of lone pairs to determine the molecular structure. If more than one arrangement of lone pairs and bonds is possible, choose the one with the fewest repulsions. This step helps show how atoms arrange themselves in space.

Bond polarity depends on electronegativity, which is how strongly an atom pulls shared electrons. When atoms with different electronegativities form a bond, the electrons are pulled toward the more electronegative atom. That leaves one atom with a partial positive charge, written as δ+, and the other with a partial negative charge, written as δ−. These are polar covalent bonds.

The charge separation in a polar bond creates a bond dipole moment. Its magnitude is written with the Greek letter μ and follows the relation μ = Qr, where Q is the size of the partial charges and r is the distance between them. Dipole moments are usually measured in debyes. One debye equals 3.336 × 10^−30 C·m. A bond dipole is shown as an arrow pointing from the less electronegative atom toward the more electronegative atom, with a small plus sign on the less electronegative end.

A whole molecule can also have a dipole moment. That depends on both the polarity of its bonds and its molecular shape. In diatomic molecules, the bond dipole determines the molecular polarity. In molecules with more than one bond, the bond dipoles must be added as vectors. If the vector sum is zero, the molecule is nonpolar, as in CO2. Water has a bent molecular structure, so its bond moments do not cancel. As a result, water is a polar molecule with a net dipole moment.

Predicting Shape and Molecular Polarity

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Learn how VSEPR theory predicts molecular shape, how bond polarity forms, and why some molecules like CO2 are nonpolar while water is polar.

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Predicting Shape and Molecular Polarity

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