Name: resonance forms in lewis structures
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Resonance

La mayoría de moléculas y iones se pueden representar utilizando estructuras de Lewis únicas. Sin embargo, ciertos compuestos pueden mostrarse con estructuras de Lewis múltiples igualmente válidas. Considere la estructura de Lewis para el trióxido de azufre.Los enlaces simples entre cada átomo de oxígeno y los del azufre central satisfacen el octeto de átomos de oxígeno. Sin embargo, para alcanzar un octeto completo para el azufre, debe formarse un enlace adicional entre el azufre y uno de los átomos de oxígeno. Dado que cualquiera de los tres oxígenos puede formar el enlace doble con el azufre, se pueden dibujar tres estructuras de Lewis diferentes.Estas estructuras de Lewis múltiples son llamadas estructuras de resonancia, donde las estructuras esqueléticas siguen siendo las mismas, pero los electrones se distribuyen de manera diferente. Las tres estructuras son representaciones válidas y equivalentes de la molécula. Sin embargo, todas son inexistentes en la naturaleza.La estructura real no oscila entre las estructuras de resonancia, pero es un híbrido o un promedio de las tres estructuras de Lewis, que se pueden medir en longitudes de enlace. En el sulfito, la longitud de un enlace sencillo azufre-oxígeno es de 1, 51 angstrom, mientras que en el trióxido de azufre, la longitud del enlace azufre-oxígeno es de 1, 42 angstrom. Así, en la molécula híbrida, la longitud del enlace es un intermedio entre enlaces simples y dobles.En las moléculas híbridas, los electrones que participan en enlaces dobles o pares solitarios son a menudo deslocalizados en enlaces o átomos múltiples, lo que significa que no están estacionarios en un átomo en particular. La deslocalización reduce la energía potencial de los electrones resultando en una estabilización llamada estabilización por resonancia. También se observa resonancia para compuestos aromáticos, como el benceno.El benceno es un anillo de carbono hexagonal con un hidrógeno unido a cada uno de los átomos de carbono, y con alternancia de enlaces simples y dobles entre los átomos de carbono. Según la ubicación de los enlaces dobles de carbono-carbono, El benceno puede tener dos estructuras de resonancia. Recuerde que los enlaces dobles suelen ser más cortos que los enlaces simples.Sin embargo, todos los enlaces carbono-carbono del benceno tienen longitudes de enlace iguales, que son intermedias entre los enlaces de carbono-carbono simples y dobles. El benceno, por lo tanto, existe como un híbrido de resonancia y se puede representar como un hexágono con un círculo dentro. El círculo indica que el benceno es una mezcla de dos estructuras de resonancia, y los enlaces dobles no pueden ser localizados en dos átomos de carbono específicos.

resonancia

La estructura de Lewis de un anión de nitrito (NO2−) puede ser dibujada de dos maneras diferentes, distinguidas por la ubicación de los enlaces N-O y N=O. 
 
 <img alt="Figure1" src="https://www.jovecore.com/CDNSource/lessons/1064/1064Image_1.svg" style="height: 40px;" />
 
 Si los iones de nitrito efectivamente contienen un enlace sencillo y uno doble, se espera que las dos longitudes de enlace sean diferentes. Un enlace doble entre dos átomos es más corto (y más fuerte) que un enlace sencillo entre los mismos dos átomos. Sin embargo, los experimentos muestran que ambos enlaces N–O en NO2– tienen la misma fuerza y longitud, y son idénticos en todas las demás propiedades. No es posible escribir una sola estructura de Lewis para el NO2− en la cual el nitrógeno tenga un octeto y ambos enlaces son equivalentes.
 
 En cambio, se utiliza el concepto de resonancia: Si dos o más estructuras de Lewis con el mismo arreglo de átomos pueden escribirse para una molécula o ión, la distribución real de los electrones es un promedio de lo que muestran las diversas estructuras de Lewis. La distribución real de electrones en cada uno de los enlaces nitrógeno-oxígeno en el NO2– es el promedio de un enlace doble y un enlace sencillo. 
 
 Las estructuras de Lewis individuales se llaman formas de resonancia. La estructura electrónica real de la molécula (el promedio de las formas de resonancia) se llama un híbrido de resonancia de las formas de resonancia individuales. Una flecha de dos puntas entre las estructuras de Lewis indica que son formas de resonancia.
 
 <img alt="Figure2" src="https://www.jovecore.com/CDNSource/lessons/1064/1064Image_2.svg" style="height: 40px;" />
 
 El anión carbonato, CO32−, constituye un segundo ejemplo de resonancia. 
 
 <img alt="Figure3" src="https://www.jovecore.com/CDNSource/lessons/1064/1064Image_3.svg" style="height: 80px;" />
 
 <ul>
 <li>Un átomo de oxígeno debe tener un enlace doble con un carbono para completar el octeto en el átomo central. </li>
 <li>Sin embargo, todos los átomos de oxígeno son equivalentes y el enlace doble podría formarse a partir de cualquiera de los tres átomos. Esto da lugar a tres formas de resonancia del ión carbonato. </li>
 <li>Dado que se pueden escribir tres estructuras de resonancia idénticas, se sabe que la disposición real de los electrones en el ión carbonato es el promedio de las tres estructuras. </li>
 <li>Una vez más, los experimentos muestran que los tres enlaces C–O son exactamente los mismos.</li>
 </ul>
 
 Recuerde siempre que una molécula descrita como un híbrido de resonancia nunca posee una estructura electrónica descrita por ninguna de las formas de resonancia. No fluctúa entre las formas de resonancia; más bien, la estructura electrónica real es siempre el promedio de lo que muestran todas las formas de resonancia. 
 
 George Wheland, uno de los pioneros de la teoría de la resonancia, utilizó una analogía histórica para describir la relación entre las formas de resonancia y los híbridos de resonancia. Un viajero medieval, que nunca antes había visto un rinoceronte, lo describió como un híbrido de un dragón y un unicornio porque tenía muchas propiedades en común con ambos. Así como un rinoceronte no es ni un dragón a veces ni un unicornio en otras ocasiones, un híbrido de resonancia no es ninguna de sus formas de resonancia en un momento dado.
 
 Al igual que un rinoceronte, es una entidad real que la evidencia experimental ha demostrado existir. Tiene algunas características en común con sus formas de resonancia, pero las formas de resonancia son imágenes convenientes e imaginarias (como el unicornio y el dragón).
 
 Este texto es adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Química 2e, Sección 7.4: Cargas Formales y Resonancia.</a>

Resonancia

La estructura de Lewis de un anión nitrito (NO2-) puede dibujarse de dos maneras diferentes, distinguidas por las ubicaciones de N-O y 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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