Name: resonance forms in lewis structures
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 52 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Resonance

大多数分子和离子可以 使用独特的路易斯结构表示。但是，某些化合物可以通过 多个同等有效的路易斯结构表示。比如说三氧化硫的路易斯结构。每个氧原子和中心硫原子之间的单键 满足氧原子的八隅体要求。但是，为了使硫达成完整的八隅体状态，硫原子和一个氧原子之间必须形成 一个额外的键。由于三个氧中的任何一个均可与硫形成 双键，因此可绘制出三种不同的路易斯结构。这些路易斯结构 被称为共振结构，其中骨架结构 保持相同，但电子分布不同。这三种结构都是分子的有效和等价表示形式，但它们在自然界中并不存在。实际结构不会在这些共振结构之间振荡，而是三个路易斯结构的混合或平均，它可以用键长来测量。在亚硫酸盐中，硫-氧单键的长度 为 1.51 埃，而在三氧化硫中，硫-氧单键的长度为 1.42 埃。因此，在混杂分子中，键长 介于单键键长和双键键长之间。在混杂分子中，参与 双键或孤对的电子通常 离域在多个键或原子上，这意味着电子不是固定在一个特定原子上。离域化降低了电子的潜在能量 从而导致结构更稳定，被称为共振稳定。芳香族化合物也可以观察到共振现象 例如苯。苯是一个六边形碳环，每个碳原子键合有一个氢，并且碳原子之间交替存在 单键和双键。根据碳-碳双键的位置，苯可有两种共振结构。大家回想一下，双键通常 比单键短。然而，苯中所有的碳-碳键 具有相等的键长，它 介于碳-碳单键键长和双键键长 之间。因此，苯以共振杂化的形式存在 并且可以表示为内部带有一个圆圈的六边形。圆圈表示苯是两个共振结构 的混合物，双键不能 定域于任何两个特定的碳原子上。

共振

亚硝酸盐− (NO2 μ m) 的路易斯结构实际上可以通过两种不同的方式绘制，区别于 N-O 和 N=O 键的位置。  
 
 <img alt="Figure1" src="https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_1.svg" style="height: 40px;" />
 
 如果亚硝酸盐确实含有一个和一个双键，则两个键长度预期会有所不同。 两个原子之间的双键比同一两个原子之间的单个键更短 (更强)。 但是，实验表明， NO2−中的两个 N – O 键的强度和长度都相同，所有其他性质中的强度和长度都相同。不能为NO2− 编写一个路易斯结构，其中氮气具有八偶体，两个键都是等效的。 
 
 相反，使用了共振概念：如果可以为分子或离子编写两个或多个原子排列相同的路易斯结构，那么电子的实际分布就是各种路易斯结构所示的平均值。 NO2−中每个氮氧键中的电子的实际分布是双键和单键的平均值。  
 
 单个路易斯结构称为共振形式。 分子的实际电子结构 (共振形式的平均值) 被称为单个共振形式的共振混合。 路易斯结构之间的双头箭头表示它们是共振形式。 
 
 <img alt="Figure2" src="https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_2.svg" style="height: 40px;" />
 
 碳酸盐阴离子 CO32− 提供了第二个共振示例。  
 
 <img alt="Figure3" src="https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_3.svg" style="height: 80px;" />
 
 <ul>
 <li>一个氧原子必须有一个双键到一个碳，才能在中央原子上完成八偶体。  </li>
 <li>但是，所有氧原子都是等效的，双键可能来自三个原子中的任何一个。 这会产生三种碳酸盐离子共振形式。  </li>
 <li>由于可以写入三个相同的共振结构，因此已知碳酸盐离子中电子的实际排列是三个结构的平均值。  </li>
 <li>同样，实验表明所有三个 C – O 键完全相同。 </li>
 </ul>
 
 切记，被称为共振混合物的分子永远不会拥有任何一种共振形式描述的电子结构。 它不会在共振形式之间波动；相反，实际电子结构始终是所有共振形式所示的平均值。  
 
 共振理论的先驱之一 乔治·惠兰德（George Wheland） 用一个历史类比来描述共振形态和共振混合物之间的关系。 一个中世纪的旅行者，从未见过犀牛，把它描述为龙和独角兽的混合，因为它有许多性质，两者都有共通之处。 正如犀牛有时既不是龙，也不是独角兽，在任何特定时间，共振混合物都不是共振形式。 
 
 与犀牛一样，实验证据证明存在的是一个真实实体。 它有一些与共振形式相同的特征，但共振形式本身是方便的假想图像 (如独角兽和龙)。 
 
 本文改编自 <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, 化学2e, 第7.4节：形式电荷与共振。</a>

亚硝酸根阴离子（NO₂⁻）的路易斯结构实际上可以绘制为两种不同的形式，其区别在于 N-O 和 N=O 键的位置……

Education

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JoVE Core

化学

Chemical Bonding: Basic Concepts

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KEY TERMS AND CONCEPTS

关键术语与概念

resonance forms in lewis structures

Resonance forms in Lewis structures help explain molecules and ions that cannot be shown correctly with just one drawing. In nitrite ion, NO2−, the Lewis structure can be drawn in two ways. The N-O and N=O bonds switch positions in the two drawings, but the atoms stay in the same arrangement.

If nitrite really had one single bond and one double bond, the two bond lengths should be different. A double bond is shorter and stronger than a single bond between the same atoms. Experiments show that both N-O bonds in NO2− have the same length, strength, and other properties. That means the real structure is not one Lewis form alone.

Resonance is the idea used when two or more Lewis structures with the same atom arrangement can be written for one species. The actual electron distribution is an average of the electrons shown in the different forms. In NO2−, each N-O bond has an average of a single bond and a double bond. The separate Lewis structures are called resonance forms, and the real structure is called a resonance hybrid.

A double-headed arrow is written between resonance forms to show this relationship. Carbonate ion, CO3 2−, is another clear example. One oxygen must form a double bond to carbon to complete the carbon octet, but any of the three oxygen atoms can make that bond. This gives three resonance forms, and experiments show that all three C-O bonds are exactly the same.

A resonance hybrid never has the electronic structure of just one resonance form. It does not switch back and forth between forms. Instead, it always exists as the average of all the resonance forms. George Wheland used a historical comparison to explain this idea. A medieval traveler who had never seen a rhinoceros described it as a hybrid of a dragon and a unicorn, but a rhino is neither one at different times. In the same way, resonance forms are useful imaginary pictures, while the resonance hybrid is the real entity shown by evidence.

Resonance Forms in Lewis Structures

types of chemical bonds

Chemical Bonding and Bond Properties

Chemical bonding explains how atoms and ions join to form molecules and compounds. American chemist Gilbert N. Lewis pioneered chemical bonding theory with the Lewis model. This model helps explain why bonds form, why some are stronger than others, and why compounds have fixed formulas like H2O instead of H4O. Ionic bonding involves ions, which are atoms or molecules with an electrical charge. A cation is a positive ion formed when a neutral atom loses one or more electrons from its valence...

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covalent bonding and lewis structures

Shared Electrons in Covalent Molecules

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resonance

Resonance forms in Lewis structures help explain molecules and ions that cannot be shown correctly with just one drawing. In nitrite ion, NO2−, the Lewis structure can be drawn in two ways. The N-O and N=O bonds switch positions in the two drawings, but the atoms stay in the same arrangement. If nitrite really had one single bond and one double bond, the two bond lengths should be different. A double bond is shorter and stronger than a single bond between the same atoms. Experiments show that...

formal charges

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Learn how resonance forms explain equal bond lengths in nitrite and carbonate ions, and how a resonance hybrid represents the real electron distribution.

Resonance Forms in Lewis Structures | JoVE K12

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JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

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Ohio’s Learning Standards and Model Curriculum

Physical Science

Study Of Matter

Bonding and compounds

Structure And Properties Of Matter

Chemical bonding

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.d

Physics

PH.9

PH.9.e

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New York State Science Learning Standards

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HS. Structure and Properties of Matter

HS-PS1-3.

HS. Chemical Reactions

HS-PS1-2.

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	...

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Resonance Forms in Lewis Structures

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