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

La tipologia di legami chimici, sia covalente non polare, che covalente polare o ionico, è in gran parte determinato dalla differenza nell'elettronegatività fra gli atomi di legame e la loro lunghezza di legame. Nel bromo, il legame fra due atomi, è non polare, poiché la differenza di elettronegatività fra loro è zero. Nel bromuro di idrogeno, il legame è covalente polare poiché il bromo, più elettronegativo, attrae la densità elettronica lontano dall'atomo di idrogeno, meno elettronegativo.

Nel bromuro di potassio, il legame è un legame ionico a causa della maggiore differenza di elettronegatività, con conseguente trasferimento completo di elettroni dal potassio al bromuro. Quando due particelle ugualmente cariche ma opposte, sono separate da una distanza, si forma un dipolo. La misura quantitativa del dipolo è detta momento di dipolo, rappresentato dalle lettere mu, che é il prodotto della grandezza delle cariche, Q, riportata in coulomb, e della distanza fra le cariche, r, riportata in metri.

Il momento di dipolo è espresso in unità di Debye, dove un debye è uguale a 3, 34 x 10^30 C m. Se un composto è polare covalente o ionico, può essere determinato calcolando il carattere ionico percentuale, che è il rapporto fra il momento di dipolo misurato in un legame e il momento di dipolo assumendo un trasferimento di elettroni completo. I legami con più del 50 percento di carattere ionico sono considerati ionici.

Considerate l'acido fluoridrico, dove gli atomi di idrogeno e fluoro sono separati da una distanza di 92 pm. Se il legame è ionico, il momento di dipolo viene calcolato assumendo un trasferimento di elettroni completo. La carica di un elettrone viene moltiplicata per la distanza tra gli atomi e il valore ottenuto viene diviso per un Debye, che restituisce il momento di dipolo di 4, 41 D.Tuttavia, il momento di dipolo misurato sperimentalmente del fluoruro di idrogeno è 1, 18 D.Pertanto, per determinare il carattere ionico percentuale del fluoruro di idrogeno, il valore misurato di 1, 18 D viene diviso per 4, 41 D, che fornisce un carattere ionico percentuale del 41 pecento.

Poiché il carattere ionico percentuale del fluoruro di idrogeno è inferiore al 50 percento, si tratta di un legame covalente polare.

polarità di legame, momento di dipolo e carattere ionico percentuale

Polarità del legame

Il valore assoluto della differenza di elettronegatività (ΔEN) di due atomi legati fornisce una misura approssimativa della polarità prevista nel legame e, quindi, del tipo di legame. Quando la differenza è molto piccola o nulla, il legame è covalente e non polare. Quando è grande, il legame è covalente polare o ionico. I valori assoluti delle differenze di elettronegatività tra gli atomi nei legami H–H, H–Cl e Na–Cl sono rispettivamente 0 (non polare), 0,9 (covalente polare) e 2,1 (ionico).

Il grado di condivisione degli elettroni tra gli atomi varia da completamente uguale (legame covalente puro) a per niente (legame ionico).

<ol style="list-style-type: disc;">
	<li>Ad esempio, gli atomi di H e F in HF hanno una differenza di elettronegatività di 1,9, e gli atomi di N e H in NH_3 hanno una differenza di 0,9, ma entrambi questi composti formano legami che sono considerati covalenti polari.</li>
	<li>Allo stesso modo, gli atomi di Na e Cl in NaCl hanno una differenza di elettronegatività di 2,1, e gli atomi di Mn e I in MnI_2 hanno una differenza di 1,0, eppure entrambe queste sostanze formano composti ionici.</li>
</ol>

La migliore guida al carattere covalente o ionico di un legame è considerare i tipi di atomi coinvolti e le loro posizioni relative nella tavola periodica.

<ol style="list-style-type: disc;">
	<li>I legami tra due non metalli sono generalmente covalenti.</li>
	<li>Il legame tra un metallo e un non metallo è spesso ionico.</li>
</ol>

Alcuni composti contengono sia legami covalenti che ionici. Gli atomi degli ioni poliatomici, come OH^–, NO_3^− e NH_4^+, sono tenuti insieme da legami covalenti polari. Tuttavia, questi ioni poliatomici formano composti ionici combinandosi con ioni di carica opposta. Ad esempio, il nitrato di potassio, KNO_3, contiene il catione K^+ e l'anione poliatomico NO_3^−. Pertanto, il legame nel nitrato di potassio è ionico, risultante dall'attrazione elettrostatica tra gli ioni K^+ e NO_3^−, nonché covalente tra gli atomi di azoto e ossigeno in NO_3^−.

Polarità molecolare e momento dipolare

Come discusso in precedenza, i legami covalenti polari collegano due atomi con diversa elettronegatività, lasciando un atomo con una carica parzialmente positiva (δ+) e l’altro atomo con una carica parziale negativa (δ–), poiché gli elettroni vengono attratti verso l’atomo più elettronegativo. Questa separazione di carica dà origine a un momento di dipolo di legame. L'entità del momento di dipolo di legame è rappresentata dalla lettera greca mu (μ) ed è data dalla formula qui riportata, dove Q è l'entità delle cariche parziali (determinata dalla differenza di elettronegatività) e r è la distanza tra le cariche :

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1062/1062_Equations-01.png" style="height: 40px;" />

Questo momento di legame può essere rappresentato come un vettore, una quantità avente sia direzione che grandezza. I vettori dipolo sono mostrati come frecce che puntano insieme al legame dall'atomo meno elettronegativo verso l'atomo più elettronegativo. Viene disegnato un piccolo segno più sull'estremità meno elettronegativa per indicare l'estremità parzialmente positiva del legame. La lunghezza della freccia è proporzionale all'entità della differenza di elettronegatività tra i due atomi.

Un'intera molecola può anche avere una separazione di carica, a seconda della sua struttura molecolare e della polarità di ciascuno dei suoi legami. Se esiste una tale separazione di carica, si dice che la molecola è una molecola polare (o dipolo); altrimenti la molecola si dice non polare. Il momento dipolare misura l'entità della separazione netta di carica nella molecola nel suo insieme. Il momento di dipolo viene determinato sommando i momenti di legame nello spazio tridimensionale, tenendo conto della struttura molecolare.

Per le molecole biatomiche, esiste un solo legame, quindi il suo momento dipolare di legame determina la polarità molecolare. Le molecole biatomiche omonucleari come Br_2 e N_2 non hanno differenze di elettronegatività, quindi il loro momento dipolare è zero. Per le molecole eteronucleari come la CO, c'è un piccolo momento dipolare. Per HF, c'è un momento di dipolo maggiore perché c'è una maggiore differenza di elettronegatività.

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 i loro momenti di legame si annullino (la somma vettoriale è uguale a zero), allora la molecola è non polare. Questa è la situazione della CO_2. Ciascuno dei legami è polare, ma la molecola nel suo insieme è non polare. Dalla struttura di Lewis e utilizzando la teoria VSEPR, si determina che la molecola di CO2 è lineare con legami polari C=O sui lati opposti dell'atomo di carbonio. I momenti di legame si annullano perché sono rivolti in direzioni opposte. Nel caso della molecola d'acqua, la struttura di Lewis mostra ancora una volta che ci sono due legami con un atomo centrale, e la differenza di elettronegatività mostra ancora una volta che ciascuno di questi legami ha un momento di legame diverso da zero. In questo caso, però, la struttura molecolare è piegata a causa delle coppie solitarie sull’O, e i due momenti di legame non si annullano. Pertanto, l'acqua ha un momento dipolare netto ed è una molecola polare (dipolo).

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding">Openstax, Chemistry 2e, Chapter 7.2: Covalent Bonding and  Openstax, Chemistry 2e, Chapter 7.6 Molecular Structure and Polarity.</a>

Polarità di legame, momento di dipolo e carattere ionico percentuale

Polarità del legame

Il valore assoluto della differenza di elettronegatività (ΔEN) di due atomi legati fornisce una misura approssimativa della polarità da ...

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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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bond polarity, dipole moment, and percent ionic character

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...

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How Bond Polarity Shapes Molecular Dipoles

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