Name: lewis structures, bonds, and formal charge
Uploaded: 2025-05-22T09:05:32.000Z
Duration: 2 min 19 s
Channel: JoVE

K12 Standards

Tennessee Academic Standards

High School

Chemistry

Matter and Its Interactions

CHEM1.PS1.12

Lewis Structures and Formal Charges

Las estructuras de Lewis son representaciones simplificadas de enlaces químicos entre átomos. Consideremos el etileno, donde dos átomos de carbono con cuatro electrones de valencia cada uno están rodeados por cuatro átomos de hidrógeno con un electrón de valencia cada uno. Por lo tanto, el eteno tiene un total de doce electrones de valencia.
Se necesitan veinticuatro electrones para satisfacer los octetos de dos átomos de carbono y los dúos de cuatro átomos de hidrógeno. Por lo tanto, doce electrones deben ser compartidos entre los átomos. Con solo doce electrones disponibles, todos deben estar enlazando electrones para permitir que todos los átomos alcancen una configuración electrónica estable.
A veces, los átomos en estructuras poliatómicas no exhiben el número estándar de electrones de valencia. En tales casos, una carga formal distinta de cero, F, se asocia con el átomo anómalo. La carga formal es igual al número de electrones de valencia en el átomo neutro menos el número de enlaces y electrones no compartidos en ese átomo.
Por ejemplo, en el ion amonio, cada átomo de hidrógeno tiene un electrón de valencia en su forma neutra y está unido por un solo enlace, lo que produce una carga formal de cero.
Esto deja al átomo de nitrógeno con cuatro electrones de valencia en lugar de sus cinco electrones de valencia habituales. Como le falta un electrón, tiene una carga formal de uno más. Por lo tanto, la carga total del ion amonio es de uno más.
Las configuraciones estables generalmente minimizan los cargos formales. Para algunos elementos más grandes, el octeto puede expandirse para compartir más de ocho electrones para alcanzar una configuración con menos cargas formales.
Por ejemplo, el átomo de azufre en el trióxido de azufre puede formar enlaces simples y dobles con los átomos de oxígeno circundantes. Sin embargo, se prefiere la disposición de doble enlace, ya que reduce la carga formal general en el átomo.
Otras excepciones a la regla del octeto incluyen compuestos que contienen boro, que tienen un objetivo de seis electrones compartidos, en lugar de ocho, porque lograr un octeto requiere una carga formal.

Adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/7-3-lewis-symbols-and-structures">Openstax, Chemistry 2e, Section 7.3: Lewis Symbols and Structures</a> y <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chemistry 2e, Section 7.4: Formales Charges and Resonance</a>.

estructuras de lewis y cargas formales

Los símbolos de Lewis se pueden utilizar para indicar la formación de enlaces covalentes, que se muestran en las estructuras de Lewis: dibujos que describen los enlaces en moléculas e iones poliatómicos. La tabla periódica se puede utilizar para predecir la cantidad de electrones de valencia en un átomo y la cantidad de enlaces que se formarán para alcanzar un octeto. Los elementos del grupo 18, como el argón y el helio, tienen configuraciones electrónicas llenas y, por lo tanto, rara vez participan en enlaces químicos. Sin embargo, los átomos del grupo 17, como el bromo o el yodo, sólo necesitan un electrón para alcanzar el octeto. Por tanto, los átomos que pertenecen al grupo 17 pueden formar un enlace covalente único. Los átomos del grupo 16 necesitan dos electrones para alcanzar un octeto; por tanto, pueden formar dos enlaces covalentes. De manera similar, el carbono, que pertenece al grupo 14, necesita cuatro electrones más para alcanzar un octeto; por tanto, el carbono puede formar cuatro enlaces covalentes.


 Considere la estructura de Lewis de la molécula de cloro:


 <img alt="Figure1" src="/files/ftp_upload/11681/PTfigure_1.svg" style="text-align: center; height: 30px;">


 La estructura de Lewis indica que cada átomo de Cl tiene tres pares de electrones que no se utilizan en el enlace (llamados pares libres) y un par de electrones compartido (escrito entre los átomos). A veces se utiliza un guión (o línea) para indicar un par de electrones compartido:  Cl–Cl


 Un único par de electrones compartido se llama enlace simple. Cada átomo de Cl interactúa con ocho electrones de valencia: los seis en los pares libres y los dos en el enlace simple. Sin embargo, es posible que un par de átomos necesite compartir más de un par de electrones para lograr el octeto requerido.


 Un doble enlace se forma cuando se comparten dos pares de electrones entre un par de átomos, como entre los átomos de carbono y oxígeno en el CH2O (formaldehído).


 <img alt="Figure2" src="/files/ftp_upload/11681/PTfigure_2.svg" style="text-align: center; height: 60px;">


 Un triple enlace se forma cuando un par de átomos comparte tres pares de electrones, como en el monóxido de carbono (CO).


 <img alt="Figure3" src="/files/ftp_upload/11681/PTfigure_3.svg" style="text-align: center; height: 20px;">


 La carga formal de un átomo en una molécula es la carga hipotética que tendría el átomo si los electrones en los enlaces estuvieran distribuidos uniformemente entre los átomos. La carga formal se puede calcular restando la suma del número de electrones no enlazantes y el número de enlaces de un átomo (o la mitad del número de electrones enlazantes) del número de electrones de valencia del átomo neutro:


 Carga formal = # electrones de la capa de valencia (átomo libre) − # pares de electrones libres − # enlaces


 Los cálculos de los cargos formales se pueden verificar determinando la suma de los cargos formales para toda la estructura. La suma de las cargas formales de todos los átomos de una molécula neutra debe ser cero; la suma de las cargas formales de un ion debe ser igual a la carga del ion. Recuerde que la carga formal calculada para un átomo no es la carga real del átomo en la molécula. El cargo formal es sólo un procedimiento contable útil; no indica la presencia de cargos reales.

Estructuras de Lewis y cargas formales

Los símbolos de Lewis se pueden usar para indicar la formación de enlaces covalentes, que se muestran en las estructuras de Lewis, dibujos que describen el enlace en ...

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

lewis structures, bonds, and formal charge

Lewis structures show how atoms share electrons in covalent bonds and in polyatomic ions. Lewis symbols help draw these structures, and the periodic table helps predict how many valence electrons an atom has and how many bonds it can form to reach an octet.

Group 18 elements, such as argon and helium, already have filled electron configurations. They rarely take part in chemical bonding. Atoms in group 17, such as bromine and iodine, need one more electron to reach an octet, so they can form one covalent bond. Atoms in group 16 need two electrons, so they can form two covalent bonds. Carbon is in group 14 and needs four more electrons, so it can form four covalent bonds.

The chlorine molecule is a simple example of a Lewis structure. Each Cl atom has three lone pairs, which are electron pairs not used in bonding, and one shared pair between the atoms. A dash or line can show this shared pair, so the bond can be written as Cl–Cl. A single shared pair is a single bond, and each chlorine atom reaches eight valence electrons when its lone pairs and bond are counted.

Some atoms must share more than one pair of electrons to complete an octet. A double bond forms when two pairs of electrons are shared, as in formaldehyde, CH2O, between carbon and oxygen. A triple bond forms when three electron pairs are shared, as in carbon monoxide, CO. These bond types help show how atoms connect in a molecule.

Formal charge is a bookkeeping tool for Lewis structures. It is the hypothetical charge an atom would have if bonding electrons were shared evenly. To calculate it, subtract the number of lone pair electrons and the number of bonds from the number of valence shell electrons in the free atom. The formal charges in a neutral molecule must add to zero, and the formal charges in an ion must add to the ion’s charge. Even so, formal charge is not the same as actual charge on an atom.

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CHEM1.PS1.1

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CHEM1.PS1.2

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Dialysis

Dialysis separates desired analyte ions or molecules from unwanted molecules, or &#39;interferents&#39;, by allowing the sample solution to diffuse ...

Analyze solutions to identify solutes and solvents, quantitatively analyze concentrations (molarity, percent composition, and ppm), and perform separation methods such as evaporation, distillation, and/or chromatography and show conceptual understanding of distillation. Construct argument to justify the use of certain separation methods under different conditions. EXPLANATION: In seventh grade, students developed an understanding that chemical and physical properties of a sample of matter could be used to identify a substance. Building on this understanding, students now explore the use of these physical and chemical properties to separate mixtures. Analysis of the substances is performed to aid in further discussion and facilitate further quantitative analysis.

CHEM1.PS1.7

Lewis Structures, Bonds, and Formal Charge

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