Name: electronegativity and polar bonds
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 54 s
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Chemistry

Matter and its Interactions

PS.Chm.3.1

Electronegativity

Les non-métaux forment des liaisons covalentes en partageant des électrons. Mais ces électrons sont-ils partagés également entre les deux atomes, ou un atome attire-t-il les électrons plus que l'autre? Le modèle de Lewis dépeint toutes les liaisons covalentes comme des électrons également partagés;cependant, ce n'est pas toujours le cas.Par exemple, si l'azote gazeux est placé dans un champ électrique, il s'orientera également entre les pôles. Mais lorsque le chlorure d'hydrogène gazeux, une molécule neutre, est placé dans un champ électrique, l'hydrogène s'oriente vers la cathode et le chlore vers l'anode, ce qui indique que l'hydrogène a une charge positive partielle et que le chlore a une charge négative partielle. La capacité d'un atome à attirer des électrons vers lui-même est appelée électronégativité.On dit donc que le chlore est plus électronégatif que l'hydrogène, attirant les électrons partagés vers lui-même, tout en résistant à l'élimination de ses propres électrons. Ceci, cependant, ne rend pas la liaison ionique. Dans une liaison ionique, les électrons sont transférés des métaux aux non-métaux, tandis que dans HCl, les électrons sont inégalement partagés.La densité électronique est plus élevé sur le chlore que sur l'atome d'hydrogène formant une liaison covalente polaire. Plus la différence d'électronégativité entre deux atomes est grande, plus la liaison sera polaire. Ainsi, en plus des liaisons covalentes ou ioniques non polaires, des liaisons covalentes polaires sont trouvées à travers une grande variété de composés.Le chimiste américain Linus Pauling a étudié les énergies nécessaires pour briser les liaisons dans des molécules telles que le chlore diatomique ou l'hydrogène. Il a établi une échelle d'électronégativité basée sur des données thermochimiques, ce qui aide à prévoir les types de liaisons. L'électronégativité est associée à l'énergie d'ionisation et à l'affinité électronique des atomes.Dans le tableau périodique, les valeurs d'électronégativité augmentent de gauche à droite les métaux sont moins électronégatifs que les non-métaux, à l'exception des métaux de transition. De plus, les valeurs d'électronégativité diminuent dans la colonne et avec l'augmentation de la taille atomique, car les atomes sont moins capables d'attirer les électrons vers eux-mêmes. Le fluor, l'élément le plus électronégatif, a la valeur d'électronégativité arbitrairement attribuée de 3, 98.Le Francium, d'autre part, est l'élément le moins électronégatif avec la valeur d'électronégativité de 0, 7. L'électronégativité n'a pas d'unité;elle ne peut pas être déterminée expérimentalement.

Électronégativité

La question de savoir si une liaison covalente est non polarisée ou polarisée est déterminée par une propriété des atomes de la liaison appelée électronégativité.  
 
 Les valeurs d'électronégativité des éléments ont été proposées par l'un des chimistes les plus célèbres du XXe siècle : Linus Pauling. Pauling a étudié les énergies requises pour rompre les liaisons dans des molécules hétéronucléaires comme l'hydrogène et le fluor. Sur la base des valeurs, il a proposé que l'énergie nécessaire pour rompre une liaison soit la moyenne des énergies de liaison de H2 (436 kJ/mol) et F2 (155 kJ/mol), soit 296 kJ/mol. Cependant, l'énergie de liaison obtenue expérimentalement de HF est de 565 kJ/mol, ce qui est beaucoup plus élevé que la valeur prévue. Pour tenir compte de cette différence, Pauling a suggéré que la liaison doit avoir un caractère ionique, qui est déterminé par le concept d'électronégativité.  
 
 L'électronégativité est une mesure de la tendance d'un atome à attirer des électrons (ou densité électronique) vers lui-même.  
 
 L'électronégativité détermine comment les électrons en commun sont distribués entre les deux atomes dans une liaison. Plus un atome attire fortement les électrons dans ses liaisons, plus son électronégativité est importante. Les électrons dans une liaison covalente polarisée sont déplacés vers l'atome le plus électronégatif ; ainsi, l'atome le plus électronégatif est celui avec la charge partielle négative. Plus la différence d'électronégativité est grande, plus la distribution des électrons est polarisée et plus les charges partielles des atomes sont importantes. 
 
 <h4>L'électronégativité et le tableau périodique </h4>
 
 <ul>
 <li>L'électronégativité augmente de gauche à droite sur une période dans le tableau périodique et diminue en descendant dans un groupe.  </li>
 <li>Les valeurs d'électronégativité dérivées de Pauling suivent des tendances périodiques prévisibles, avec des électronégativités plus élevées vers le haut à droite du tableau périodique. </li>
 <li>Ainsi, les non-métaux, qui se trouvent en haut à droite, ont tendance à avoir les électronégativités les plus élevées, avec le fluor qui est l'élément le plus électronégatif de tous (En = 4,0).  </li>
 <li>Les métaux ont tendance à être des éléments moins électronégatifs, et les métaux du groupe 1 ont les électronégativités les plus faibles.  </li>
 <li>Les gaz nobles sont exclus de la liste de l'électronégativité parce que ces atomes n'ont généralement pas d'électrons en commun avec d'autres atomes, puisqu'ils ont une couche de valence pleine. (Bien que des composés de gaz nobles tels que XeO2 existent, ils ne peuvent être formés que dans des conditions extrêmes, et donc ils ne s'intègrent pas parfaitement dans le modèle général de l'électronégativité.) </li>
 </ul>
 
 <h4>Électronégativité versus affinité électronique </h4>
 
 Veillez à ne pas confondre électronégativité et affinité électronique. L'affinité électronique d'un élément est une grandeur physique mesurable, à savoir l'énergie libérée ou absorbée lorsqu'un atome isolé en phase gazeuse acquiert un électron, mesurée en kJ/mol. L'électronégativité, d'autre part, décrit à quel point un atome attire les électrons dans une liaison. Il s'agit d'une grandeur sans dimension qui est calculée et non mesurée. Pauling a déduit les premières valeurs d'électronégativité en comparant les quantités d'énergie nécessaires pour rompre différents types de liaisons. Il a choisi une échelle relative arbitraire allant de 0 à 4. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding">Openstax, Chimie 2e, Section: 7.2 Liaison covalente.</a>

Le fait qu’une liaison soit non polaire ou polaire covalente est déterminé par une propriété des atomes de liaison appelée électronégativité. 

Valeurs d’électronégativité de ...

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electronegativity and polar bonds

Electronegativity helps explain why some covalent bonds are nonpolar and others are polar covalent. It is a measure of how strongly an atom attracts shared electrons toward itself. That pull controls how electron density is distributed in a bond.

Linus Pauling proposed the first electronegativity values by studying bond energies in heteronuclear molecules such as hydrogen fluoride. He expected the bond energy of HF to be about the average of the bond energies of H2 and F2, or 296 kJ/mol. The experimental bond energy of HF was much higher, at 565 kJ/mol, which suggested the bond had some ionic character.

In a polar covalent bond, the shared electrons shift toward the more electronegative atom. That atom carries the partial negative charge. The larger the electronegativity difference between two atoms, the more the electrons are pulled to one side, and the larger the partial charges become.

Electronegativity follows clear periodic trends. It increases from left to right across a period and decreases down a group. As a result, the highest values are found toward the upper right of the periodic table, especially among nonmetals. Fluorine is the most electronegative element, with a value of 4.0, while group 1 metals have the lowest values. Noble gases are usually left off electronegativity lists because they already have full valence shells and usually do not share electrons. Some noble gas compounds, such as XeO2, can form only under extreme conditions.

Do not confuse electronegativity with electron affinity. Electron affinity is the measurable energy released or absorbed when an isolated gas-phase atom gains an electron. Electronegativity is different. It describes how strongly an atom attracts electrons in a bond, so it is calculated rather than measured. Pauling placed these values on an arbitrary relative scale from 0 to 4.

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electronegativity

Electronegativity helps explain why some covalent bonds are nonpolar and others are polar covalent. It is a measure of how strongly an atom attracts shared electrons toward itself. That pull controls how electron density is distributed in a bond. Linus Pauling proposed the first electronegativity values by studying bond energies in heteronuclear molecules such as hydrogen fluoride. He expected the bond energy of HF to be about the average of the bond energies of H2 and F2, or 296 kJ/mol. The...

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Learn how electronegativity explains polar covalent bonds, Pauling’s bond-energy idea, periodic trends, and the difference from electron affinity.

Electronegativity and Polar Bonds | JoVE K12

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Electronegativity and Polar Bonds

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