Name: how bond polarity shapes molecular dipoles
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Bond Polarity, Dipole Moment, and Percent Ionic Character

Le type de liaisons chimiques, qu'il soit covalent non polaire, covalent polaire ou ionique, est largement déterminé par la différence d'électronégativité entre les atomes de liaison et leur longueur de liaison. Dans le brome, la liaison entre les deux atomes de brome est non polaire puisque la différence d'électronégativité entre eux est nulle. Dans le bromure d'hydrogène, la liaison est covalente polaire puisque le brome plus électronégatif attire la densité électronique loin de l'atome d'hydrogène moins électronégatif.Dans le bromure de potassium, la liaison est une liaison ionique en raison de la plus grande différence d'électronégativité, entraînant un transfert complet d'électrons du potassium au bromure. Lorsque deux particules chargées de manière égale mais opposée sont séparées par une distance, un dipôle est formé. La mesure quantitative du dipôle est appelée le moment dipolaire représenté par la lettre mu, qui est le produit de l'amplitude des charges, Q, rapportée en coulombs, et de la distance entre les charges, r, rapportée en mètres.Le moment dipolaire est donné en unités Debye, où un debye est égal à 3, 34 x 10^30 C*m. On peut déterminer si un composé est covalent polaire ou ionique en calculant le pourcentage de caractère ionique, qui est le rapport entre le moment dipolaire mesuré d'une liaison et le moment dipolaire en supposant un transfert d'électrons complet. Les liaisons avec plus de 50%de caractère ionique sont considérées comme ioniques.Considérons le fluorure d'hydrogène, où les atomes d'hydrogène et de fluor sont séparés par une distance de 92 pm. Si la liaison est ionique, le moment dipolaire est calculé en supposant un transfert d'électrons complet. La charge d'un électron est multipliée par la distance entre les atomes et la valeur obtenue est divisée par un debye, ce qui donne le moment dipolaire de 4, 41 D.Cependant, le moment dipolaire mesuré expérimentalement du fluorure d'hydrogène est de 1, 18 D.Par conséquent, pour déterminer le pourcentage de caractère ionique du fluorure d'hydrogène, la valeur mesurée de 1, 18 D est divisée par 4, 41 D, ce qui donne un pourcentage de caractère ionique de 41%Puisque le pourcentage de caractère ionique du fluorure d'hydrogène est inférieur à 50%Il s'agit d'une liaison covalente polaire.

polarité de liaison, moment dipolaire et nature de la liaison

<h4>Polarisation d'une liaison </h4>
 
 La valeur absolue de la différence d'électronégativité (ΔEn) de deux atomes liés fournit une mesure approximative de la polarisation à laquelle il faut s'attendre dans la liaison et, par conséquent, du type de liaison. Lorsque la différence est très faible ou nulle, la liaison est covalente et non polarisée. Lorsqu'elle est grande, la liaison est covalente et polarisée, ou ionique. Les valeurs absolues des différences d'électronégativité entre les atomes dans les liaisons H–H, H–Cl et Na–Cl sont respectivement 0 (non polarisée), 0,9 (covalente polarisée) et 2,1 (ionique).  
 
 Le point auquel les électrons sont partagés entre les atomes varie de complètement égal (liaison covalente pure) à pas du tout (liaison ionique).  
 
 <ul>
 <li>Par exemple, les atomes H et F dans HF ont une différence d'électronégativité de 1,9, et les atomes N et H dans NH3 une différence de 0,9, et pourtant ces deux composés forment des liaisons considérées comme covalentes polarisées.  </li>
 <li>De même, les atomes Na et Cl dans NaCl ont une différence d'électronégativité de 2,1 et les atomes Mn et I dans MnI2 ont une différence de 1,0 ; mais ces deux substances forment des composés ioniques. </li>
 </ul>
 
 Le meilleur guide du caractère covalent ou ionique d'une liaison est de considérer les types d'atomes impliqués et leurs positions relatives dans le tableau périodique.  
 
 <ul>
 <li>Les liaisons entre deux non-métaux sont généralement covalentes. </li>
 <li>La liaison entre un métal et un non-métal est souvent ionique. </li>
 </ul>
 
 Certains composés contiennent à la fois des liaisons covalentes et ioniques. Les atomes des ions polyatomiques, tels que OH–, NO3−  et NH4+, sont maintenus ensemble par des liaisons covalentes polarisées. Cependant, ces ions polyatomiques forment des composés ioniques en se combinant avec des ions de la charge opposée. Par exemple, le nitrate de potassium, KNO3, contient le cation K+ et l'anion polyatomique NO3−. Ainsi, la liaison dans le nitrate de potassium est ionique, résultant de l'attraction électrostatique entre les ions K+ et NO3−, ainsi que covalente entre les atomes d'azote et d'oxygène dans NO3−. 
 
 <h4>Polarité moléculaire et moment dipolaire </h4>
 
 Comme nous l'avons vu précédemment, les liaisons covalentes polarisées relient deux atomes avec des électronégativités différentes, laissant un atome avec une charge partielle positive (δ+) et l'autre atome avec une charge partielle négative (δ–), car les électrons sont tirés vers l'atome le plus électronégatif. Cette séparation des charges donne lieu au moment dipolaire d'une liaison. L'intensité du moment dipolaire d'une liaison est représentée par la lettre grecque mu (µ) et elle est donnée par la formule illustrée ici, où Q est l'intensité des charges partielles (déterminée par la différence d'électronégativité) et r est la distance entre les charges : 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1062/1062_Equations-01.png" style="height: 40px;" />
 
 Ce moment de la liaison peut être représenté comme un vecteur, une grandeur ayant à la fois une direction et une norme. Les vecteurs de dipôles sont représentés par des flèches pointant le long de la liaison, de l'atome le moins électronégatif vers l'atome le plus électronégatif. Un petit signe plus est dessiné sur l'extrémité la moins électronégative pour indiquer l'extrémité partiellement positive de la liaison. La longueur de la flèche est proportionnelle à la valeur absolue de la différence d'électronégativité entre les deux atomes. 
 
 Une molécule entière peut aussi avoir une séparation des charges, selon sa structure moléculaire et la polarisation de chacune de ses liaisons. Si une telle séparation des charges existe, on dit que la molécule est une molécule polaire (ou dipôle) ; sinon, on dit que la molécule est non polaire. Le moment dipolaire mesure l'étendue de la séparation nette des charges dans l'ensemble de la molécule. Le moment dipolaire est déterminé en ajoutant les moments des liaisons dans l'espace tridimensionnel, en tenant compte de la structure moléculaire. 
 
 Pour les molécules diatomiques, il n'y a qu'une seule liaison, donc le moment dipolaire de sa liaison détermine la polarité moléculaire. Les molécules diatomiques homonucléaires telles que Br2 et N2 n'ont aucune différence au niveau de l'électronégativité, de sorte que leur moment dipolaire est zéro. Pour les molécules hétéronucléaires telles que CO, il existe un petit moment dipolaire. Pour HF, il y a un moment dipolaire plus grand car il y a une plus grande différence d'électronégativité. 
 
 Lorsqu'une molécule contient plus d'une liaison, la géométrie doit être prise en compte. Si les liaisons d'une molécule sont disposées de manière à ce que les moments des liaisons s'annulent (somme vectorielle égale à zéro), alors la molécule est non polaire. C'est ce qu'il se passe dans CO2. Chacune des liaisons est polaire, mais la molécule dans son ensemble est non polaire. À partir de la structure de Lewis, et en utilisant la théorie VSEPR, on a déterminé que la molécule de CO2 est linéaire avec des liaisons C=O polarisées sur les côtés opposés de l'atome de carbone. Les moments des liaisons s'annulent car ils pointent dans des directions opposées. Dans le cas de la molécule d'eau, la structure de Lewis montre à nouveau qu'il y a deux liaisons sur un atome central, et la différence d'électronégativité montre à nouveau que chacune de ces liaisons a un moment de liaison non nul. Dans ce cas, cependant, la structure moléculaire est courbée en raison des doublets libres sur O, et les moments des deux liaisons ne s'annulent pas. Par conséquent, l'eau a un moment dipolaire net et c'est une molécule polaire (dipôle). 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding">Openstax, Chimie 2e, Chapitre 7.2 : Liaison covalente et Openstax, Chimie 2e, Chapitre 7.6 Structure moléculaire et polarité.</a>

Polarité de liaison, moment dipolaire et nature de la liaison

Polarité des liaisons

La valeur absolue de la différence d’électronégativité (ΔEN) de deux atomes liés fournit une mesure approximative de la polarité à ...

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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.

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How Bond Polarity Shapes Molecular Dipoles

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