Name: lewis structures with formal charge
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Formal Charges

Certaines molécules ou ions polyatomiques peuvent être représentés par plusieurs structures de Lewis, mais comment décider laquelle est la structure dominante? En calculant les charges formelles des atomes, la structure de Lewis la plus proche de la structure réelle de la molécule peut être déterminée. On attribue à chaque atome une charge hypothétique appelée charge formelle, qui serait la charge sur l'atome si tous les autres atomes de la molécule avaient la même électronégativité.Il suppose que chaque électron de liaison est également partagé par les deux atomes. Considérez le chlorure d'hydrogène. Pour déterminer la charge formelle sur chaque atome, ajoutez d'abord le nombre d'électrons non liés à la moitié du nombre d'électrons de liaison, puis soustrayez la valeur obtenue du nombre d'électrons de valence.Un agrégat de toutes les charges formelles dans une molécule ou un ion est égal à la charge nette de la molécule ou de l'ion. Par exemple, l'oxyde nitreux peut être représenté par trois structures de Lewis possibles-une avec deux doubles liaisons, une avec une triple liaison entre les atomes d'azote, et une avec une triple liaison entre l'azote et l'oxygène tout ce qui satisfait l'octet. La meilleure structure de Lewis est identifiée par des calculs de charge formels.L'azote a cinq électrons de valence, et l'oxygène a six électrons de valence. Le calcul basé sur le nombre d'électrons non liés et la moitié du nombre d'électrons de liaison donne la charge formelle pour chaque structure. Parce que l'oxyde nitreux est une molécule neutre, la somme de toutes les charges formelles doit être nulle.Généralement, les charges formelles sur les atomes individuels dans une structure de Lewis dominante sont les plus proches de zéro. Par conséquent, la troisième structure avec une charge formelle plus élevée peut être ignorée. De plus, une charge formelle négative, si elle est présente, devrait être portée par l'atome le plus électronégatif.Puisque l'oxygène est plus électronégatif que l'azote, la deuxième structure, avec la charge formelle négative sur l'oxygène, est identifiée comme la principale structure de l'oxyde nitreux. Les charges formelles ne sont pas les charges réelles des molécules ou des atomes, mais une convention de comptabilité. La charge réelle de la molécule dépend de plusieurs facteurs, y compris la différence d'électronégativité entre les atomes constitutifs.

charge formelle

Dans certains cas, il semble y avoir plus d'une structure de Lewis valide pour les molécules et les ions polyatomiques. Le concept des charges formelles peut servir à prévoir la structure de Lewis la plus appropriée lorsqu'il existe plus d'une structure raisonnable. 
 
 <h4>Calcul des charges formelles </h4>
 
 La charge formelle d'un atome dans une molécule est la charge hypothétique que l'atome aura si les électrons dans les liaisons sont répartis uniformément entre les atomes. Alternativement, la charge formelle résulte quand à partir du nombre d'électrons de valence d'un atome neutre, les électrons non-liants sont d'abord réduits, ce qui est suivi par la soustraction du nombre de liaisons reliées à cet atome dans la structure Lewis. 
 
 Ainsi, la charge formelle est calculée comme suit :  
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1065/1065_Equations-01.png" style="height: 60px;" />
 
 Les calculs de la charge formelle peuvent être vérifiés en déterminant la somme des charges formelles pour l'ensemble de la structure. La somme des charges formelles de tous les atomes dans une molécule doit être égale à zéro ; la somme des charges formelles d'un ion doit être égale à la charge de l'ion. Rappelez-vous que la charge formelle calculée pour un atome n'est pas la charge réelle de l'atome dans la molécule. La charge formelle n'est qu'une procédure utile pour tenir les comptes ; elle n'indique pas la présence de charges réelles. 
 
 <h4>Calcul de la charge formelle à partir de structures de Lewis </h4>
 
 Les étapes suivantes sont suivies afin d’attribuer des charges formelles à chaque atome dans l'ion inter-halogéné ICl4−. 
 
 Étape 1. Diviser les doublets d'électrons liants de manière égale pour toutes les liaisons I–Cl : 
 
 <img alt="Figure1" src="https://www.jovecore.com/CDNSource/lessons/1065/1065image_1.svg" style="height: 120px;" />
 
 Étape 2. Attribuez des doublets libres d'électrons à leurs atomes. Chaque atome de chlore a maintenant sept électrons qui lui sont attribués, et l'atome d'iode en a huit. 
 
 Étape 3. Soustraire ce nombre du nombre d'électrons de valence pour l'atome neutre : 
 Iode : 7 – 8 = –1 
 Chlore : 7 – 7 = 0 
 La somme des charges formelles de tous les atomes est égale à –1, ce qui est identique à la charge de l'ion (–1). 
 
 <h4>Utiliser la charge formelle pour prévoir la structure moléculaire </h4>
 
 La disposition des atomes dans une molécule ou un ion est appelée sa structure moléculaire. Dans de nombreux cas, le fait de suivre les étapes lorsqu'on écrit des structures de Lewis peut conduire à plus d'une structure moléculaire possible — plusieurs liaisons et positionnements d'électrons de doublets libres différents ou des dispositions différentes d'atomes, par exemple. Quelques directives concernant la charge formelle peuvent être utiles pour décider lesquelles des structures possibles sont les plus probables pour une molécule ou un ion particulier : 
 
 <ol>
 <li>Une structure moléculaire dans laquelle toutes les charges formelles sont nulles est préférable à une structure dans laquelle certaines charges formelles ne sont pas nulles. </li>
 <li>Si la structure de Lewis doit avoir des charges formelles non nulles, la disposition ayant les charges formelles non nulles les plus faibles est favorisée. </li>
 <li>Des structures de Lewis sont favorisées lorsque les charges formelles adjacentes sont nulles ou du signe opposé. </li>
 <li>Lorsqu'on choisit parmi plusieurs structures de Lewis ayant des distributions similaires de charges formelles, la structure ayant les charges formelles négatives sur les atomes les plus électronégatifs est favorisée. </li>
 </ol>
 
 Pour voir comment ces lignes directrices s'appliquent, considérez quelques structures possibles pour le dioxyde de carbone, le CO2. On sait que l'atome le moins électronégatif occupe généralement la position centrale, mais les charges formelles aident à comprendre pourquoi cela se produit. Trois possibilités pour la structure peuvent être dessinées : le carbone au centre avec deux doubles liaisons, le carbone au centre avec une liaison simple et triple, et l'oxygène au centre avec des doubles liaisons.  
 
 <img alt="Figure2" src="https://www.jovecore.com/CDNSource/lessons/1065/1065image_2.svg" style="height: 80px;" />
 
 En comparant les trois charges formelles, la structure de gauche peut être identifiée comme celle qui est préférable car elle ne comporte que des charges formelles de zéro. 
 
 Autre exemple, l'ion thiocyanate, un ion formé à partir d'un atome de carbone, d'un atome d'azote et d'un atome de soufre, pourrait avoir trois structures moléculaires différentes : NCS–, CNSn‐ ou CSN–. Les charges formelles présentes dans chacune de ces structures moléculaires peuvent servir à choisir la disposition la plus probable des atomes. Les structures de Lewis possibles ainsi que les charges formelles pour chacune des trois structures possibles pour l'ion thiocyanate sont : le carbone au centre avec des doubles liaisons, l'azote au centre avec des doubles liaisons, et le soufre au centre avec des doubles liaisons.  
 
 <img alt="Figure3" src="https://www.jovecore.com/CDNSource/lessons/1065/1065Image_3.svg" style="height: 95px;" />
 
 Notez que la somme des charges formelles dans chaque cas est égale à la charge de l'ion (–1). Cependant, la première disposition d'atomes avec le carbone au centre est préférable parce qu'elle a le plus petit nombre d'atomes ayant des charges formelles non nulles. En outre, elle permet de placer le moins d'atome électronégatif au centre et la charge négative sur l'élément le plus électronégatif. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chimie 2e, Chapitre 7.4 Charges formelles et résonance.</a>

Charge formelle

Dans certains cas, il semble y avoir plus d’une structure de Lewis valide pour les molécules et les ions polyatomiques. Le concept de charges formelles peut être utilisé pour ...

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lewis structures with formal charge

Lewis structures can sometimes show more than one reasonable arrangement for a molecule or polyatomic ion. Formal charge helps chemists choose the best Lewis structure when that happens. It is a useful way to compare possible structures in chemistry.

Formal charge is the hypothetical charge an atom would have if bonding electrons were shared evenly between the atoms. It can also be found by starting with the valence electrons of the neutral atom, subtracting the nonbonding electrons, and then subtracting the number of bonds on that atom in the Lewis structure. This is a bookkeeping tool, not the actual charge on the atom.

The formal charge of a full structure should also be checked as a whole. For a neutral molecule, the sum of all formal charges must be zero. For an ion, the sum of all formal charges should match the ion’s overall charge.

A common example is the interhalogen ion ICl4−. To assign formal charges, the bonding electron pairs are divided equally among the I–Cl bonds. Then lone pairs are assigned to their atoms. In this structure, each chlorine has seven electrons assigned to it, and iodine has eight. That gives iodine a formal charge of −1 and each chlorine a formal charge of 0, which adds up to the −1 charge of the ion.

Formal charge also helps predict molecular structure when several Lewis structures are possible. A structure with all formal charges equal to zero is preferred over one with nonzero charges. If nonzero charges are needed, the best choice has the smallest charges possible. Structures are also preferred when adjacent formal charges are zero or opposite in sign. When several choices are still possible, the negative charge should be on the more electronegative atom.

Carbon dioxide shows why these rules matter. One possible structure has carbon in the center with two double bonds. Other possibilities place carbon between a single and a triple bond, or place oxygen in the center with double bonds. The structure with carbon in the center and two double bonds is preferred because all of its formal charges are zero.

The thiocyanate ion, NCS−, gives another clear example. It can be written with carbon, nitrogen, or sulfur in the center. The formal charges in each possible Lewis structure help identify the most likely arrangement of atoms. The carbon-centered structure is preferred because it has the fewest atoms with nonzero formal charges. It also places the least electronegative atom in the center and the negative charge on the more electronegative element.

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Lewis Structures with Formal Charge

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