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

Matter and its Interactions

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Resonance

A maioria das moléculas e íons podem ser representados utilizando estruturas únicas de Lewis. No entanto, certos compostos podem ser mostrado por múltiplas estruturas Lewis, igualmente válidas. Considere a estrutura de Lewis para o trióxido de enxofre.As ligações únicas entre cada oxigênio e o enxofre central átomos satisfazem o octeto para os átomos de oxigênio. No entanto, para alcançar um octeto completo para o enxofre, deve ser formada uma ligação adicional entre o enxofre e um dos átomos de oxigênio. Uma vez que qualquer um dos três agentes oxigenados podem formar a dupla ligação com enxofre, três estruturas diferentes de Lewis podem ser desenhadas.Estas múltiplas estruturas Lewis são chamadas estruturas de ressonância, onde as estruturas esqueléticas permanecem as mesmas, mas os elétrons são distribuídos de forma diferente. Todas as três estruturas são válidas e representações equivalentes da molécula, mas todas são inexistentes na natureza. A estrutura real não oscila entre as ressonâncias mas é um híbrido ou uma média das três estruturas Lewis que podem ser medidas em comprimentos de ligação.Em sulfito, o comprimento de uma única ligação de enxofre-oxigênio é de 1, 51 angstrom, enquanto que no trióxido de enxofre, o comprimento da ligação enxofre-oxigênio é de 1, 42 angstrom. Assim, na molécula híbrida, o comprimento da ligação é um intermediário entre ligações simples e duplas. Em moléculas híbridas, os elétrons participam em ligações duplas ou pares solitários são frequentemente deslocalizadas sobre múltiplos ligações ou átomos, o que significa que não estão estacionários num átomo em particular.A deslocalização reduz a energia potencial de elétrons resultando em estabilização chamada estabilização por ressonância. A ressonância também é observada para compostos aromáticos como o benzeno. O benzeno é um anel de carbono hexagonal com um hidrogênio ligado a cada um dos átomos de carbono, e alternando ligações simples e duplas entre o carbono átomos.Com base na localização das ligações duplas de carbono-carbono, O benzeno pode ter duas estruturas de ressonância. Recorde que as ligações duplas são normalmente mais curtas do que as obrigações individuais. No entanto, todas as ligações de carbono-carbono em benzeno têm comprimentos de ligação iguais, que são intermediários entre carbono-carbono simples e duplas ligações.O benzeno, portanto, possui como um híbrido de ressonância e pode ser representado como um hexágono com um círculo no interior. O círculo indica que o benzeno é uma mistura de dois estruturas de ressonância, e as duplas ligações não podem ser localizadas em qualquer dois átomos de carbono específicos.

ressonância

A estrutura de Lewis de um anião nitrito (NO2−) pode ser na verdade desenhada de duas formas diferentes, distinguida pelas localizações das ligações N-O e N=O. 

<img alt="Figure1" src="https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_1.svg" style="height: 40px;" />

Se os iões nitrito têm realmente uma ligação simples e uma dupla, espera-se que os dois comprimentos de ligação sejam diferentes. Uma ligação dupla entre dois átomos é mais curta (e mais forte) do que uma única ligação entre os dois átomos iguais. No entanto, experiências mostram que ambas as ligações N–O em NO2− têm a mesma força e comprimento, e são idênticas em todas as outras propriedades. Não é possível escrever uma única estrutura Lewis para NO2− na qual o azoto tem um octeto e ambas as ligações são equivalentes.

Em vez disso, é utilizado o conceito de ressonância: se duas ou mais estruturas de Lewis com o mesmo arranjo de átomos podem ser escritas para uma molécula ou ião, a distribuição real de eletrões é uma média da mostrada pelas várias estruturas Lewis. A distribuição real de eletrões em cada ligação de azoto-oxigénio em NO2− é a média de uma ligação dupla e de uma ligação simples. 

As estruturas individuais de Lewis são chamadas de formas de ressonância. A estrutura eletrónica real da molécula (a média das formas de ressonância) é chamada de híbrido de ressonância das formas individuais de ressonância. Uma seta dupla entre as estruturas de Lewis indica que elas são formas de ressonância.

<img alt="Figure2" src="https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_2.svg" style="height: 40px;" />

O anião carbonato, CO32−, fornece um segundo exemplo de ressonância. 

<img alt="Figure3" src="https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_3.svg" style="height: 80px;" />

<ul>
<li>Um átomo de oxigénio deve ter uma ligação dupla com um carbono para completar o octeto no átomo central. </li>
<li>No entanto, todos os átomos de oxigénio são equivalentes e a ligação dupla pode formar-se a partir de qualquer um dos três átomos. Isto dá origem a três formas de ressonância do ião do carbonato. </li>
<li>Como três estruturas de ressonância idênticas podem ser escritas, sabe-se que o arranjo real dos eletrões no ião do carbonato é a média das três estruturas. </li>
<li>Mais uma vez, experiências mostram que as três ligações C–O são exactamente iguais.</li>
</ul>

Lembre-se sempre que uma molécula descrita como um híbrido de ressonância nunca possui uma estrutura eletrónica descrita por qualquer uma das formas de ressonância. Não flutua entre as formas de ressonância; em vez disso, a estrutura eletrónica real é sempre a média daquela mostrada por todas as formas de ressonância. 

George Wheland, um dos pioneiros da teoria da ressonância, usou uma analogia histórica para descrever a relação entre as formas de ressonância e os híbridos de ressonância. Um viajante medieval, nunca antes tendo visto um rinoceronte, descreveu-o como um híbrido de um dragão e um unicórnio porque tinha muitas propriedades em comum com ambos. Assim como um rinoceronte não é nem um dragão algumas vezes, nem um unicórnio de outras vezes, um híbrido de ressonância não é nenhuma das suas formas de ressonância em um dado momento.

Como um rinoceronte, é uma entidade real que a evidência experimental mostrou existir. Tem algumas características em comum com as suas formas de ressonância, mas as formas de ressonância por si são imagens convenientes e imaginárias (como o unicórnio e o dragão).

Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chemistry 2e, Section 7.4: Formal Charges and Resonance.</a>

Ressonância

A estrutura de Lewis de um ânion nitrito (NO2−) pode realmente ser desenhada de duas maneiras diferentes, distinguidas pelas localizações do N-O e N=O ...

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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

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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...

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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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Resonance Forms in Lewis Structures

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