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

A teoria VSEPR ajuda a determinar geometrias de pares de elétrons e geometrias moleculares.
Uma série de etapas é usada para prever a geometria e os ângulos de ligação das moléculas, como o tricloreto de fósforo.
O primeiro passo é desenhar a estrutura de Lewis da molécula.
Em seguida, conte o número total de grupos de elétrons no átomo central. Em torno do fósforo, existem quatro grupos de elétrons: três pares de ligação e um par isolado.
Agora, determine a geometria do par de elétrons. A geometria do par de elétrons é tetraédrica. No entanto, por causa do par solitário, a geometria molecular é piramidal trigonal. O par solitário reduz o ângulo de ligação para menos de 109,5°.
Os pares de elétrons de ligação nem sempre são compartilhados igualmente entre os dois átomos de ligação.
Em uma ligação covalente como a do ácido fluorídrico, os elétrons são puxados em direção ao átomo mais eletronegativo, indicado por uma carga parcial. Essas ligações são chamadas de ligações polares.
A separação de carga cria um vetor chamado momento de dipolo de ligação, que é indicado pela letra grega μ. Seu valor é o produto da magnitude das cargas parciais e da distância entre elas.
Os momentos de dipolo são comumente expressos em debyes. Um debye é igual a 3,336 × 10−30 coulomb-metros.
O vetor aponta do átomo menor para o mais eletronegativo e indica o momento de dipolo da ligação. Seu comprimento é proporcional à magnitude da diferença de eletronegatividade entre os dois átomos. A maioria das moléculas diatômicas contendo átomos de diferentes elementos tem momentos de dipolo e, portanto, são moléculas polares.
Em compostos poliatômicos, o momento de dipolo líquido é determinado pelos momentos de dipolo de ligação individuais e pela geometria do composto.
Considere uma molécula de água com duas ligações polares. Tem uma forma dobrada e é uma molécula polar.
Em contraste, uma molécula de dióxido de carbono é linear. As duas ligações carbono-oxigênio são polares, mas são orientadas em direções opostas, cancelando o momento de dipolo uma da outra e tornando a molécula geral apolar.

geometria molecular e momentos dipolares

A teoria RPECV pode ser usada para determinar as geometrias dos pares de elétrons e estruturas moleculares da seguinte forma:

<ol style="display: flex; flex-direction: column; gap: 10px;">
	<li>Escreva a estrutura de Lewis da molécula ou íon poliatômico.</li>
	<li>Conte o número de grupos de elétrons (pares solitários e ligações) ao redor do átomo central. Uma ligação simples, dupla ou tripla conta como uma região de densidade eletrônica.</li>
	<li>Identifique a geometria do par de elétrons com base no número de grupos de elétrons.</li>
	<li>Use o número de pares solitários para determinar a estrutura molecular. Se for possível mais de um arranjo de pares solitários e ligações químicas, escolha aquele que minimize as repulsões.</li>
</ol>

Momento dipolar de uma molécula

Quando átomos com eletronegatividades diferentes formam uma ligação, os elétrons são atraídos em direção ao átomo mais eletronegativo, deixando um átomo com carga parcial positiva (δ+) e o outro átomo com carga parcial negativa (δ–). Tais ligações são chamadas de ligações covalentes polares, e a separação de carga dá origem a um momento dipolar da ligação. A magnitude de um momento dipolar de ligação é representada pela letra grega µ e é dada por:

µ = Qr

onde Q é a magnitude das cargas parciais (determinada pela diferença de eletronegatividade) e r é a distância entre elas. Momentos dipolares são comumente expressos em debyes, onde um debye é igual a 3,336 × 10-30C·m.

O momento dipolar da ligação é um vetor representado por uma seta apontando ao longo da ligação do átomo menos eletronegativo para o átomo mais eletronegativo, com um pequeno sinal de mais na extremidade menos eletronegativa.

Uma molécula inteira também pode ter separação de cargas, dependendo de sua estrutura molecular e da polaridade de cada uma de suas ligações. Dizemos que tais moléculas são polares. O momento dipolar mede a extensão da separação da carga líquida na molécula como um todo. Em moléculas diatômicas, o momento dipolar de ligação determina a polaridade molecular.

Quando uma molécula contém mais de uma ligação, a geometria deve ser levada em consideração. Se as ligações em uma molécula estiverem dispostas de modo que a soma vetorial de seus momentos de ligação seja igual a zero, então a molécula é apolar (por exemplo, CO2). A molécula de água tem uma estrutura molecular curvada e os dois momentos de ligação não se cancelam. Portanto, a água é uma molécula polar com um momento dipolar líquido.

“Este texto foi adaptado dohref="https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity">Openstax, Chemistry 2e, Section 7.6 Molecular Structure and Polarity.

Geometria molecular e momentos dipolares

A teoria VSEPR pode ser usada para determinar as geometrias dos pares de elétrons e as estruturas moleculares da seguinte forma:

Escreva a estrutura de Lewis da molécula ou ...

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JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Covalent Bonding and Structure

Ligação Covalente e Estrutura

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

organic chemistry

JoVE Core - Química Orgânica explica conceitos básicos de estereoquímica, reações e mecanismos usando vídeos de animação concisos e fáceis de entender. Além disso, os vídeos de Cientistas em ação demonstram a aplicação de conceitos relacionados em experimentos de pesquisa clássicos e originais realizados nos laboratórios atuais em todo o mundo.

Predicting Shape and Molecular Polarity

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