Name: resonance in nitrite and carbonate ions
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 16 s
Channel: JoVE

A maioria das moléculas e íons são representados por uma única estrutura de Lewis. No entanto, em compostos específicos, alguns elétrons são deslocalizados em várias ligações ou átomos, em vez de localizados em uma ligação ou átomo específico. Esses compostos podem ser representados com precisão com várias estruturas de Lewis.
Considere a estrutura de Lewis para o trióxido de enxofre. As ligações simples entre cada oxigênio e os átomos centrais de enxofre satisfazem o octeto para os átomos de oxigênio. No entanto, para atingir um octeto completo para o enxofre, uma ligação adicional deve ser formada entre o enxofre e um dos átomos de oxigênio.
Essas múltiplas estruturas de Lewis de trióxido de enxofre são chamadas de estruturas de ressonância ou estruturas contribuintes. As posições espaciais de todos os átomos componentes permanecem as mesmas; no entanto, os elétrons de valência são distribuídos de forma diferente.
Essas setas de duas pontas podem ser consideradas vírgulas e não devem ser confundidas com estruturas de ressonância em equilíbrio. Nessas moléculas, a estrutura real é a média ponderada ou um híbrido de suas estruturas de ressonância.
As estruturas de ressonância são identificadas por 'empurrão de elétrons', ou transformando pares isolados em ligações e vice-versa, conforme denotado com setas curvas. Isso mapeia as áreas deslocalizadas.
Essa deslocalização resulta em estabilização de ressonância - isto é, uma molécula com uma energia potencial menor do que a de qualquer estrutura teórica não deslocalizada.
Se uma estrutura contribuinte é mais baixa em energia do que outra, ela se assemelha mais à estrutura molecular real. Certas preferências são usadas para estimar as energias relativas de várias estruturas contribuintes.
Em primeiro lugar, estruturas nas quais todos os átomos preencheram camadas de valência são mais estáveis. Em segundo lugar, estruturas com maior número de ligações covalentes são mais estáveis. Em terceiro lugar, as estruturas que minimizam as cobranças formais são mais estáveis.
Finalmente, estruturas que carregam cargas negativas em átomos mais eletronegativos e cargas positivas em átomos menos eletronegativos são mais estáveis.

ressonância e estruturas híbridas

Resonance and Hybrid Structures

Segundo a teoria da ressonância, se duas ou mais estruturas de Lewis com o mesmo arranjo de átomos puderem ser escritas para uma molécula, íon ou radical, a distribuição real dos elétrons será uma média daquela mostrada pelas diversas estruturas de Lewis.

Estruturas de Ressonância e Híbridos de Ressonância

A estrutura de Lewis de um ânion nitrito (NO2-) pode, na verdade, ser desenhada de duas maneiras diferentes, diferenciadas pelas localizações das ligações N – O e N = O.


<img alt="Figure1" src="/files/ftp_upload/11684/11684%20PT%20Figure%201.svg" style="text-align: center; height: 50px;">

Se os íons nitrito contiverem 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 ligação simples entre os mesmos dois átomos. No entanto, experimentos mostram que ambas as ligações N – O no NO2- têm a mesma força e comprimento e são idênticas em todas as outras propriedades. Não é possível escrever uma única estrutura de Lewis para NO2- na qual o nitrogênio tenha um octeto e ambas as ligações sejam equivalentes.

Em vez disso, é utilizado o conceito de ressonância: a distribuição real de elétrons em cada uma das ligações nitrogênio-oxigênio no NO2- é a média de uma ligação dupla e de uma ligação simples.

As estruturas de Lewis individuais são chamadas de estruturas de ressonância. A estrutura eletrônica real da molécula (a média das formas de ressonância) é chamada de híbrido de ressonância das formas de ressonância individuais. Uma seta de duas pontas entre as estruturas de Lewis indica que são formas de ressonância.


<img alt="Figure2" src="/files/ftp_upload/11684/11684%20PT%20Figure%202.svg" style="text-align: center; height: 50px;">

O ânion carbonato, CO3, fornece um segundo exemplo de ressonância.


<img alt="Figure3" src="/files/ftp_upload/11684/11684%20PT%20Figure%203.svg" style="text-align: center; height: 75px;">

<ul style="list-style-type: disc; display: flex; flex-direction: column; gap: 10px; padding: 0 0 0 40px;">
<li>Um átomo de oxigênio deve ter uma ligação dupla com o carbono para completar o octeto no átomo central.</li>
<li>Entretanto, todos os átomos de oxigênio são equivalentes, e a ligação dupla pode se formar a partir de qualquer um dos três átomos. Isso dá origem a três formas de ressonância do íon carbonato.
</li>
<li>Como três estruturas de ressonância idênticas podem ser escritas, a disposição real dos elétrons no íon carbonato é conhecida como a média das três estruturas.
</li>
<li>Novamente, os experimentos mostram que todas as três ligações C–O são exatamente iguais.
</li>
</ul>

Uma molécula descrita como híbrida de ressonância nunca possui uma estrutura eletrônica descrita por qualquer forma de ressonância. Ela não oscila entre formas de ressonância; em vez disso, a estrutura eletrônica real é sempre a média daquela mostrada por todas as formas de ressonância.


Este texto foi adaptado do <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chemistry 2e, Chapter 7.4 Formal Charges and Resonance</a>.

Ressonância e Estruturas Híbridas

De acordo com a teoria da ressonância, se duas ou mais estruturas de Lewis com o mesmo arranjo de átomos podem ser escritas para uma molécula, íon ou radical, ...

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resonance in nitrite and carbonate ions

Resonance explains why some Lewis structures do not match the measured properties of a molecule or ion. When two or more Lewis structures have the same atom arrangement, the real electron distribution is an average of the structures shown.

The nitrite anion, NO2−, is one example. It can be drawn in two ways, with the N–O and N=O bonds in different places. If nitrite had one single bond and one double bond, the two bond lengths should be different, because a double bond is shorter and stronger than a single bond.

Experiments show that both N–O bonds in NO2− have the same length and strength. They are identical in all other properties as well. A single Lewis structure cannot show this and still give nitrogen an octet, so resonance is used to describe the ion.

The separate Lewis structures are called resonance structures. The true electronic structure is called a resonance hybrid, which is the average of the resonance forms. A double-headed arrow between Lewis structures shows that they are resonance forms. Nitrite does not switch back and forth between them. Its electron distribution is always the average of the forms.

The carbonate anion, CO3 2−, gives a second example. One oxygen must form a double bond to carbon to complete the octet on the central atom, but each oxygen atom is equivalent. That means the double bond can be placed with any one of the three oxygens, creating three resonance forms.

Because the three resonance structures are identical, the electron arrangement in carbonate is the average of all three. Experiments show that all three C–O bonds are exactly the same. A resonance hybrid never has the exact electronic structure of just one resonance form.

Resonance in Nitrite and Carbonate Ions

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Explore how resonance explains equal bond lengths in nitrite and carbonate ions, and learn the difference between resonance structures and a resonance hybrid.

Resonance in Nitrite and Carbonate Ions | JoVE K12

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Resonance in Nitrite and Carbonate Ions

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