Name: how bond polarity shapes molecular dipoles
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 48 s
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Bond Polarity, Dipole Moment, and Percent Ionic Character

El tipo de enlaces químicos, ya sean covalentes no polares, covalentes polares o iónicos, está determinado en gran medida por la diferencia en la electronegatividad entre los átomos de enlace y su longitud de enlace. En el bromo, el enlace entre los dos átomos de bromo es no polar ya que la diferencia de electronegatividad entre ellos es cero. En el bromuro de hidrógeno, el enlace es covalente polar ya que el bromo más electronegativo atrae la densidad electrónica del átomo de hidrógeno menos electronegativo.En el bromuro de potasio, el enlace es un enlace iónico debido a la mayor diferencia de electronegatividad, lo que resulta en una transferencia completa de electrones del potasio al bromuro. Cuando dos partículas con cargas iguales, pero opuestas, están separadas por una distancia, se forma un dipolo. La medida cuantitativa del dipolo se llama momento dipolar representado por la letra mu, que es el producto de la magnitud de las cargas, Q, expresada en culombios, y la distancia entre las cargas, r, expresada en metros.El momento dipolar se da en unidades Debye, donde, un debye es igual a 3.34 x 10^30 C*m. Si un compuesto es polar covalente o iónico se puede determinar calculando el porcentaje de carácter iónico, que es la relación entre el dipolo medido del momento de un enlace al momento dipolar asumiendo una transferencia completa de electrones. Los enlaces con más del 50 de carácter iónico se consideran iónicos.Considere el fluoruro de hidrógeno, donde los átomos de hidrógeno y flúor están separados por una distancia de 92 picómetros. Si el enlace es iónico, el momento del dipolo se calcula asumiendo una transferencia completa de electrones. La carga de un electrón se multiplica por la distancia entre los átomos y el valor obtenido se divide por un debye, lo que produce el momento dipolar de 4, 41 debyes.Sin embargo, el momento dipolar medido experimentalmente del fluoruro de hidrógeno es de 1, 18 debyes. Por lo tanto, para determinar el porcentaje de carácter iónico del fluoruro de hidrógeno, el valor medido de 1, 18 debyes se divide por 4, 41 debyes, lo que da un porcentaje de carácter iónico del 41%Puesto que el porcentaje de carácter iónico del fluoruro de hidrógeno es inferior al 50 es un enlace covalente polar.

polaridad de enlace, momento dipolo y porcentaje de carácter iónico

<h4>Polaridad de enlace</h4>
 
 El valor absoluto de la diferencia en electronegatividad (ΔEN) de dos átomos unidos proporciona una medida aproximada de la polaridad que se espera en el enlace y, por lo tanto, el tipo de enlace. Cuando la diferencia es muy pequeña o cero, el enlace es covalente y no polar. Cuando es grande, el enlace es covalente polar o iónico. Los valores absolutos de las diferencias de electronegatividad entre los átomos en los enlaces H–H, H–CL y Na–CL son 0 (no polar), 0,9 (covalente polar) y 2,1 (iónico), respectivamente. 
 
 El grado en que los electrones son compartidos entre los átomos varía desde completamente igual (enlace covalente puro) a nada en lo absoluto (enlace iónico). 
 
 <ul>
 <li>Por ejemplo, los átomos H y F en HF tienen una diferencia de electronegatividad de 1,9, y los átomos N y H en NH3 tienen una diferencia de 0,9, sin embargo ambos compuestos forman enlaces que se consideran covalentes polares. </li>
 <li>Asimismo, los átomos de Na y Cl en NaCl tienen una diferencia de electronegatividad de 2,1, y los átomos de Mn y I en MnI2 tienen una diferencia de 1,0, sin embargo ambas sustancias forman compuestos iónicos.</li>
 </ul>
 
 La mejor guía para el carácter covalente o iónico de un enlace es considerar los tipos de átomos involucrados y sus posiciones relativas en la tabla periódica. 
 
 <ul>
 <li>Los enlaces entre dos no metales generalmente son covalentes.</li>
 <li>El enlace entre un metal y un no metal es a menudo iónica.</li>
 </ul>
 
 Algunos compuestos contienen enlaces covalentes e iónicos. Los átomos de iones poliatómicos, como el OH–, NO3− y NH4+, están unidos por enlaces covalentes polares. Sin embargo, estos iones poliatómicos forman compuestos iónicos al combinarse con iones de carga opuesta. Por ejemplo, el nitrato de potasio, KNO3, contiene al catión K+ y al anión poliatómico NO3−. Por lo tanto, el enlace en el nitrato de potasio es iónico, resultando de la atracción electrostática entre los iones K+ y NO3−, así como covalente entre los átomos de nitrógeno y oxígeno en NO3−.
 
 <h4>Polaridad Molecular y Momento Dipolo</h4>
 
 Como se discutió anteriormente, los enlaces covalentes polares conectan dos átomos con diferentes electronegatividades, dejando un átomo con una carga positiva parcial (δ+) y el otro átomo con una carga negativa parcial (δ–), a medida que los electrones son empujados hacia el átomo más electronegativo. Esta separación de carga da lugar a un momento dipolo de enlace. La magnitud de un momento dipolo de enlace está representada por la letra griega mu (µ) y está dada por la fórmula mostrada aquí, donde Q es la magnitud de las cargas parciales (determinadas por la diferencia de electronegatividad) y r es la distancia entre las cargas:
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1062/1062_Equations-01.png" style="height: 40px;" />
 
 Este momento de enlace se puede representar como un vector, una cantidad que tiene dirección y magnitud. Los vectores dipolo se muestran como flechas apuntando junto con el enlace desde el átomo menos electronegativo hacia el átomo más electronegativo. Un signo más pequeño se dibuja en el extremo menos electronegativo para indicar el extremo parcialmente positivo del enlace. La longitud de la flecha es proporcional a la magnitud de la diferencia de electronegatividad entre los dos átomos.
 
 Una molécula entera también puede tener una separación de carga, dependiendo de su estructura molecular y de la polaridad de cada uno de sus enlaces. Si existe tal separación de carga, se dice que la molécula es una molécula polar (o dipolo); de lo contrario, se dice que la molécula es no polar. El momento dipolo mide la extensión de la separación de la carga neta en la molécula como un todo. El momento dipolo se determina añadiendo los momentos de enlace en el espacio tridimensional, teniendo en cuenta la estructura molecular.
 
 Para las moléculas diatómicas, sólo hay un enlace, por lo que su momento dipolo de enlace determina la polaridad molecular. Las moléculas diatómicas homonucleares como Br2 y N2 no tienen diferencia en la electronegatividad, por lo que su momento dipolo es cero. Para moléculas heteronucleares como EL CO, hay un pequeño momento dipolo. Para HF, hay un momento dipolo más grande porque hay una diferencia más grande en electronegatividad.
 
 Cuando una molécula contiene más de un enlace, se debe tener en cuenta la geometría. Si los enlaces en una molécula están dispuestos de tal manera que sus momentos de enlace se cancelen (la suma vectorial es igual a cero), entonces la molécula es no polar. Esta es la situación en el CO2. Cada uno de los enlaces es polar, pero la molécula en su conjunto es no polar. A partir de la estructura de Lewis, y usando la teoría de VSEPR, se determina que la molécula de CO2 es lineal con enlaces polares C=O en lados opuestos del átomo de carbono. Los momentos de enlace se cancelan porque apuntan en direcciones opuestas. En el caso de la molécula de agua, la estructura de Lewis muestra de nuevo que hay dos enlaces por un átomo central, y la diferencia de electronegatividad muestra de nuevo que cada uno de estos enlaces tiene un momento de enlace distinto de cero. En este caso, sin embargo, la estructura molecular está curvada debido a los pares solitarios en el O, y los dos momentos de enlace no se cancelan. Por lo tanto, el agua tiene un momento dipolo neto y es una molécula polar (dipolo).
 
 Este texto es adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding">Openstax, Química 2e, Capítulo 7.2: Enlace Covalente y Openstax, Química 2e, Capítulo 7.6 Estructura Molecular y Polaridad.</a>

Polaridad de Enlace, Momento Dipolo y Porcentaje de Carácter Iónico

Polaridad de enlace

El valor absoluto de la diferencia en electronegatividad (ΔEN) de dos átomos enlazados proporciona una medida aproximada de la polaridad ...

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how bond polarity shapes molecular dipoles

Bond polarity depends on electronegativity, the ability of an atom to attract shared electrons. The absolute value of the electronegativity difference, or ΔEN, gives a rough guide to bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is larger, the bond may be polar covalent or ionic.

Examples show how this works in real compounds. H–H has a ΔEN of 0, so it is nonpolar. H–Cl has a ΔEN of 0.9, so it is polar covalent. Na–Cl has a ΔEN of 2.1, so it is ionic. Even so, ΔEN is only a guide. HF has a ΔEN of 1.9 and NH3 has a ΔEN of 0.9, yet both have polar covalent bonds. NaCl has a ΔEN of 2.1 and MnI2 has a ΔEN of 1.0, yet both are ionic compounds.

The type of atoms in the periodic table is often the best clue. Bonds between two nonmetals are generally covalent. Bonds between a metal and a nonmetal are often ionic. Some compounds include both kinds of bonding. Polyatomic ions such as OH−, NO3−, and NH4+ have polar covalent bonds inside the ion. They form ionic compounds when they combine with ions of the opposite charge. Potassium nitrate, KNO3, is one example. It contains K+ and NO3−, so the attraction between those ions is ionic, while the N–O bonds inside nitrate are covalent.

Polar covalent bonds create partial charges, written as δ+ and δ−. The more electronegative atom pulls the electrons closer, so one end of the bond becomes partially negative and the other becomes partially positive. This charge separation creates a bond dipole moment, symbolized by μ. A dipole is shown as a vector, or arrow, that points from the less electronegative atom toward the more electronegative atom. A small plus sign marks the positive end. A longer arrow means a larger electronegativity difference.

A whole molecule can also have a net dipole moment. That depends on both bond polarity and molecular shape. The dipole moment of the molecule is found by adding the bond moments in three-dimensional space. For diatomic molecules, there is only one bond, so the bond dipole sets the molecular polarity. Homonuclear diatomic molecules such as Br2 and N2 have no electronegativity difference, so their dipole moment is zero. Heteronuclear molecules such as CO have a small dipole moment, and HF has a larger one because its electronegativity difference is larger.

Molecular geometry can make polar bonds cancel. CO2 is linear, so its two polar C=O bond moments point in opposite directions and cancel. The molecule is nonpolar even though its bonds are polar. Water is different. Its Lewis structure shows two bonds to oxygen, and each bond has a nonzero dipole moment. Because the molecule is bent, the bond moments do not cancel. Water therefore has a net dipole moment and is a polar molecule.

How Bond Polarity Shapes Molecular Dipoles

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How Bond Polarity Shapes Molecular Dipoles

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