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

La teoria VSEPR aiuta a determinare le geometrie delle coppie di elettroni e le geometrie molecolari.
Una serie di passaggi viene utilizzata per prevedere la geometria e gli angoli di legame delle molecole, come il tricloruro di fosforo.
Il primo passo è disegnare la struttura di Lewis della molecola.
Quindi, conta il numero totale di gruppi di elettroni sull'atomo centrale. Intorno al fosforo ci sono quattro gruppi di elettroni: tre coppie di legame e una coppia solitaria.
Ora, determina la geometria della coppia di elettroni. La geometria della coppia di elettroni è tetraedrica. Tuttavia, a causa della coppia solitaria, la geometria molecolare è piramidale trigonale. La coppia solitaria riduce l'angolo di legame a meno di 109,5°.
Le coppie di elettroni di legame non sono sempre condivise equamente tra i due atomi di legame.
In un legame covalente come quello dell'acido fluoridrico, gli elettroni vengono attratti verso l'atomo più elettronegativo, indicato da una carica parziale. Tali legami sono chiamati legami polari.
La separazione di carica crea un vettore chiamato momento di dipolo di legame, che è indicato dalla lettera greca μ. Il suo valore è il prodotto dell'entità delle cariche parziali e della distanza tra di esse.
I momenti di dipolo sono comunemente espressi in debye. Un debye equivale a 3,336 × 10−30 coulomb-metri.
Il vettore punta dall'atomo meno a quello più elettronegativo e indica il momento di dipolo del legame. La sua lunghezza è proporzionale all'entità della differenza di elettronegatività tra i due atomi. La maggior parte delle molecole biatomiche contenenti atomi di elementi diversi hanno momenti di dipolo e quindi sono molecole polari.
Nei composti poliatomici, il momento di dipolo netto è determinato dai singoli momenti di dipolo di legame e dalla geometria del composto.
Consideriamo una molecola d'acqua con due legami polari. Ha una forma piegata ed è una molecola polare.
Al contrario, una molecola di anidride carbonica è lineare. I due legami carbonio-ossigeno sono polari ma sono orientati in direzioni opposte, annullando il momento di dipolo l'uno dell'altro e rendendo la molecola complessiva non polare.

geometria molecolare e momenti dipolari

La teoria VSEPR può essere utilizzata per determinare le geometrie delle coppie di elettroni e le strutture molecolari come segue:

<ol>
	<li>Scrivi la struttura di Lewis della molecola o dello ione poliatomico.</li>
	<li>Conta il numero di gruppi di elettroni (coppie solitarie e legami) attorno all'atomo centrale. Un legame singolo, doppio o triplo conta come una regione di densità elettronica.</li>
	<li>Identifica la geometria della coppia di elettroni in base al numero di gruppi elettronici.</li>
	<li>Utilizza il numero di coppie solitarie per determinare la struttura molecolare. Se più di una disposizione di coppie solitarie e legami chimici è possibile, scegliere quella che minimizza le repulsioni.</li>
</ol>

Momento dipolare di una molecola

Quando atomi con diversa elettronegatività formano un legame, gli elettroni vengono attratti verso l’atomo più elettronegativo, lasciando un atomo con una carica parziale positiva (δ+) e l’altro atomo con una carica parziale negativa (δ–). Tali legami sono chiamati legami covalenti polari e la separazione della carica dà origine ad un momento dipolare di legame. L'entità del momento di dipolo di legame è rappresentata dalla lettera greca µ ed è data da:

μ = Qr

dove Q è l'entità delle cariche parziali (determinata dalla differenza di elettronegatività) e r è la distanza tra loro. I momenti di dipolo sono comunemente espressi in debye, che è pari a 3.336 × 10-30C·m.

Il momento di dipolo di legame è un vettore rappresentato da una freccia che punta lungo il legame dall'atomo meno elettronegativo verso l'atomo più elettronegativo, con un piccolo segno più sull'estremità meno elettronegativa.

Un'intera molecola può anche avere una separazione di carica, a seconda della sua struttura molecolare e della polarità di ciascuno dei suoi legami. Tali molecole sono dette polari. Il momento dipolare misura l'entità della separazione di carica netta nella molecola nel suo insieme. Nelle molecole biatomiche, il momento di dipolo di legame determina la polarità molecolare.

Quando una molecola contiene più di un legame, è necessario tener conto della geometria. Se i legami in una molecola sono disposti in modo tale che la somma vettoriale dei loro momenti di legame è uguale a zero, allora la molecola è non polare (ad esempio, CO2). La molecola dell’acqua ha una struttura molecolare piegata e i due momenti di legame non si annullano. Pertanto, l'acqua è una molecola polare con un momento dipolare netto.

Questo testo è stato adattato da <a href="https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity">Openstax, Chemistry 2e, Section 7.6 Molecular Structure and Polarity</a>.

Geometria molecolare e momenti dipolari

La teoria VSEPR può essere utilizzata per determinare le geometrie delle coppie di elettroni e le strutture molecolari come segue:

Scrivi la struttura di Lewis della molecola o ...

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JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Covalent Bonding and Structure

Legame covalente e struttura

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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polar covalent bonds

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molecular geometry and dipole moments

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...

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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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