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

La teoría VSEPR ayuda a determinar geometrías de pares de electrones y geometrías moleculares.
Se utiliza una serie de pasos para predecir la geometría y los ángulos de enlace de las moléculas, como el tricloruro de fósforo.
El primer paso es dibujar la estructura de Lewis de la molécula.
A continuación, cuenta el número total de grupos de electrones en el átomo central. Alrededor del fósforo, hay cuatro grupos de electrones: tres pares de enlace y un par solitario.
Ahora, determine la geometría del par de electrones. La geometría del par de electrones es tetraédrica. Sin embargo, debido al par solitario, la geometría molecular es piramidal trigonal. El par solitario reduce el ángulo de enlace a menos de 109,5°.
Los pares de electrones de enlace no siempre se comparten por igual entre los dos átomos de enlace.
En un enlace covalente como el del ácido fluorhídrico, los electrones son atraídos hacia el átomo más electronegativo, indicado por una carga parcial. Tales enlaces se llaman enlaces polares.
La separación de carga crea un vector llamado momento dipolar de enlace, que se indica con la letra griega μ. Su valor es el producto de la magnitud de las cargas parciales y la distancia entre ellas.
Los momentos dipolares se expresan comúnmente en debyes. Un debye equivale a 3.336 × 10-30 metros de culombio.
El vector apunta desde el átomo menor al más electronegativo e indica el momento dipolar de enlace. Su longitud es proporcional a la magnitud de la diferencia de electronegatividad entre los dos átomos. La mayoría de las moléculas diatómicas que contienen átomos de diferentes elementos tienen momentos dipolares y, por lo tanto, son moléculas polares.
En los compuestos poliatómicos, el momento dipolar neto está determinado por los momentos dipolares de enlace individuales y la geometría del compuesto.
Consideremos una molécula de agua con dos enlaces polares. Tiene una forma doblada y es una molécula polar.
Por el contrario, una molécula de dióxido de carbono es lineal. Los dos enlaces carbono-oxígeno son polares, pero están orientados en direcciones opuestas, lo que cancela el momento dipolar del otro y hace que la molécula en general sea no polar.

geometría molecular y momentos dipolares

La teoría VSEPR se puede utilizar para determinar las geometrías de los pares de electrones y las estructuras moleculares de la siguiente manera:


 <ol>
 <li>Escribe la estructura de Lewis de la molécula o ion poliatómico.</li>
 <li>Cuenta el número de grupos de electrones (pares libres y enlaces) alrededor del átomo central. Un enlace simple, doble o triple cuenta como una región de densidad electrónica.</li>
 <li>Identifica la geometría del par de electrones según el número de grupos de electrones.</li>
 <li>Utiliza el número de pares libres para determinar la estructura molecular. Si es posible más de una disposición de pares libres y enlaces químicos, elige aquella que minimice las repulsiones.</li>
 </ol>
 

 Momento dipolar de una molécula


 Cuando átomos con diferentes electronegatividades forman un enlace, los electrones son atraídos hacia el átomo más electronegativo, dejando un átomo con una carga parcial positiva (δ+) y el otro átomo con una carga parcial negativa (δ–). Estos enlaces se denominan enlaces covalentes polares y la separación de cargas da lugar a un momento dipolar del enlace. La magnitud de un momento dipolar de enlace está representada por la letra griega µ y viene dada por:


 µ = Qr


 donde Q es la magnitud de las cargas parciales (determinada por la diferencia de electronegatividad) y r es la distancia entre ellas. Los momentos dipolares se expresan comúnmente en debyes, donde un debye es igual a 3,336 × 10^−30 C·m.


 El momento dipolar del enlace es un vector representado por una flecha que apunta a lo largo del enlace desde el átomo menos electronegativo hacia el más electronegativo, con un pequeño signo más en el extremo menos electronegativo.


 Una molécula completa también puede tener una separación de carga, dependiendo de su estructura molecular y de la polaridad de cada uno de sus enlaces. Se dice que estas moléculas son polares. El momento dipolar mide el grado de separación de carga neta en la molécula en su conjunto. En las moléculas diatómicas, el momento dipolar del enlace determina la polaridad molecular.


 Cuando una molécula contiene más de un enlace, se debe tener en cuenta la geometría. Si los enlaces en una molécula están dispuestos de manera que la suma vectorial de sus momentos de enlace sea igual a cero, entonces la molécula es apolar (por ejemplo, CO_2). La molécula de agua tiene una estructura molecular doblada y los dos momentos de enlace no se cancelan. Por tanto, el agua es una molécula polar con un momento dipolar neto.

Geometría molecular y momentos dipolares

La teoría VSEPR se puede utilizar para determinar las geometrías de los pares de electrones y las estructuras moleculares de la siguiente manera:

Escriba la estructura de Lewis de la molécula o ...

Education

Educación

JoVE Core

Organic Chemistry

Química Orgánica

Unit 1

Unidad 1

Covalent Bonding and Structure

Enlace covalente y estructura

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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.

Predicting Shape and Molecular Polarity | JoVE K12

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Predicting Shape and Molecular Polarity

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