Name: lewis structures with formal charge
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 42 s
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Formal Charges

Einige Moleküle oder polyatomare Ionen können durch mehrere Lewis-Strukturen dargestellt werden, aber wie kann man entscheiden, welche davon die dominierende Struktur ist? Durch Berechnung der formalen Ladungen der Atome kann die der tatsächlichen Struktur des Moleküls am ähnlichsten Lewis-Struktur bestimmt werden. Jedem Atom wird eine hypothetische Ladung zugeordnet, die als formale Ladung bezeichnet wird.Dies wäre die Ladung des Atoms, wenn alle anderen Atome im Molekül dieselbe Elektronegativität hätten. Es wird angenommen, dass jedes Bindungselektron zu gleichen Teilen von den beiden Atomen geteilt wird. Betrachten Sie Chlorwasserstoff.Um die formale Ladung auf jedem Atom zu bestimmen, addieren Sie zunächst die Anzahl der nicht-bindenden Elektronen zur Hälfte der bindenden Elektronen und subtrahieren dann den erhaltenen Wert von der Anzahl der Valenzelektronen. Die Summe aller formalen Ladungen in einem Molekül oder Ion ist gleich der Nettoladung des Moleküls oder Ions. Zum Beispiel kann Distickstoffmonoxid durch drei mögliche Lewis-Strukturen dargestellt werden eine mit zwei Doppelbindungen, eine mit einer Dreifachbindung zwischen den Stickstoffatomen und eine mit einer Dreifachbindung zwischen dem Stickstoff und dem Sauerstoff-die alle die Oktett-Regel erfüllen.Die beste Lewis-Struktur wird durch formale Ladungsberechnungen ermittelt. Stickstoff hat fünf Valenzelektronen, und Sauerstoff hat sechs Valenzelektronen. Die Berechnung auf der Grundlage der Anzahl der nicht-bindenden Elektronen und der halben Anzahl der bindenden Elektronen ergibt die formale Ladung für jede Struktur.Da Distickstoffmonoxid ein neutrales Molekül ist, muss die Summe aller formalen Ladungen Null ergeben. Im Allgemeinen liegen die formalen Ladungen an den einzelnen Atomen in einer dominanten Lewis-Struktur nahe bei Null. Daher kann die dritte Struktur mit der höheren formalen Ladung ignoriert werden.Zusätzlich sollte eine negative formale Ladung, falls vorhanden, vom elektronegativsten Atom getragen werden. Da Sauerstoff elektronegativer als Stickstoff ist, wird die zweite Struktur mit der negativen formalen Ladung des Sauerstoffs als die dominierende Struktur für Distickstoffmonoxid identifiziert. Formale Ladungen sind nicht die tatsächlichen Ladungen von Molekülen oder Atomen, sondern eine buchhalterische Konvention.Die tatsächliche Ladung des Moleküls hängt von mehreren Faktoren ab, einschließlich des Unterschieds in der Elektronegativität zwischen den teilnehmenden Atomen.

formalladungen

In einigen Fällen gibt es scheinbar mehr als eine gültige Lewis-Struktur für Moleküle und mehratomige Ionen. Das Konzept der Formalladungen kann verwendet werden, um die am besten geeignete Lewis-Struktur vorherzusagen, wenn mehr als eine sinnvolle Struktur existiert. <h4>Berechnung der Formalladungen</h4> Die Formalladung eines Atoms in einem Molekül ist die hypothetische Ladung, die das Atom hätte, wenn die Elektronen in den Bindungen gleichmäßig zwischen den Atomen verteilt wären. Alternativ ergibt sich die Formalladung, wenn von der Anzahl der Valenzelektronen eines neutralen Atoms zunächst die nichtbindenden Elektronen reduziert werden, gefolgt von der Subtraktion der Anzahl der Bindungen, die mit diesem Atom in der Lewis-Struktur verbunden sind. Somit errechnet sich die Formalladung wie folgt:   <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065_Equations-01.png" style="height: 60px;"> Die angestellte Berechnung der Formalladung kann noch einmal überprüft werden, indem die Summe der Formalladung für die gesamte Struktur ermittelt wird. Die Summe der Formalladung aller Atome in einem Molekül muss Null sein; Die Summe der Formalladungen in einem Ion sollte der Ladung des Ions entsprechen. Bedenken Sie, dass die für ein Atom berechnete Formalladung nicht die tatsächliche Ladung des Atoms im Molekül ist. Die Formalladung ist nur ein nützliches Verfahren zur Balancierung; es weist nicht auf das Vorhandensein tatsächlicher Ladung hin. <h4>Berechnung der formalen Ladung aus Lewis-Strukturen</h4> Um jedem Atom im Interhalogenion ICl4− seine Formalladung zuzuordnen, sollten die folgenden SChritte befolgt werden. Schritt 1. Teilen Sie die bindenden Elektronenpaare gleichmäßig für alle I-Cl-Bindungen auf: <img alt="Figure1" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065image_1.svg" style="height: 120px;"> Schritt 2. Ordnen Sie den Atomen ihre freie Elektronenpaare zu. Jedem Chloratom sind nun sieben Elektronen zugeordnet, dem Jodatom acht. Schritt 3. Subtrahieren Sie diese Zahl von der Anzahl der Valenzelektronen für das neutrale Atom: Jod: 7 – 8 = –1 Chlor: 7 – 7 = 0 Die Summe der Formalladung aller Atome beträgt –1, was mit der Ladung des Ions (–1) identisch ist. <h4>Verwendung der Formalladung zur Vorhersage der Molekülstruktur</h4> Die Anordnung der Atome in einem Molekül oder Ion wird als Molekülstruktur bezeichnet. In vielen Fällen kann das Befolgen der Schritte zum Schreiben von Lewis-Strukturen zu mehr als einer möglichen Molekülstruktur führen – zum Beispiel zu mehreren unterschiedlichen Bindungen oder Platzierung einzelner Elektronenpaare oder zu sich unterscheidenden Anordnungen von Atomen. Einige Richtlinien welche der möglichen Strukturen für ein bestimmtes Molekül oder Ion am wahrscheinlichsten ist, können bei der Entscheidung hilfreich sein: <ol> <li>Eine Molekülstruktur, in der alle Formalladungen Null sind, ist einer Struktur vorzuziehen, in der einige Formalladungen nicht Null sind.</li> <li>Wenn die Lewis-Struktur Formalladungen ungleich Null aufweisen muss, ist die Anordnung mit den wenigsten und kleinsten Formalladungen ungleich Null vorzuziehen.</li> <li>Lewis-Strukturen sind vorzuziehen, wenn benachbarte Formalladungen Null sind oder das entgegengesetzte Vorzeichen haben.</li> <li>Bei der Auswahl aus mehreren Lewis-Strukturen mit ähnlicher Verteilung der Formalladungen ist die Struktur mit den negativen Formalladungen an den elektronegativeren Atomen vorzuziehen.</li> </ol> Um zu sehen, wie diese Richtlinien anzuwenden sind, betrachten wir einige mögliche Strukturen für Kohlendioxid, CO2. Es ist bereits bekannt, dass das weniger elektronegative Atom typischerweise die zentrale Position einnimmt, aber die Formalladungen helfen dabei zu verstehen warum dies von Bedeutung ist. Für CO2 lassen sich drei mögliche Strukturen zeichnen: Kohlenstoff im Zentrum mit zwei Doppelbindungen, der Kohlenstoff im Zentrum mit einer Einfach- und Dreifachbindung und Sauerstoff im Zentrum mit Doppelbindungen.  <img alt="Figure2" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065image_2.svg" style="height: 80px;"> Beim Vergleich der drei Formalladungen kann die Struktur links als die vorzuziehende identifiziert werden, da sie nur formale Ladungen von Null aufweist. Als weiteres Beispiel könnte das Thiocyanat-Ion, ein aus einem Kohlenstoffatom, einem Stickstoffatom und einem Schwefelatom gebildetes Ion, drei verschiedene Molekülstrukturen haben: NCS–, CNS– oder CSN–. Die in jeder dieser Molekülstrukturen vorhandenen formalen Ladungen können dabei helfen, die wahrscheinlichste Anordnung der Atome auszuwählen. Mögliche Lewis-Strukturen und die Formalladungen für jede der drei möglichen Strukturen für das Thiocyanat-Ion sind: Kohlenstoff im Zentrum mit Doppelbindungen, Stickstoff im Zentrum mit Doppelbindungen und Schwefel im Zentrum mit Doppelbindungen.  <img alt="Figure3" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065Image_3.svg" style="height: 95px;"> Zu beachten ist, dass die Summe der Formalladungen jeweils gleich der Ladung des Ions (–1) ist. Trotzdem wird die erste Anordnung der Atome mit Kohlenstoff im Zentrum bevorzugt, weil sie die geringste Anzahl von Atomen mit Formalladungen ungleich Null aufweist. Außerdem wird das am wenigsten elektronegative Atom in der Mitte platziert und die negative Ladung auf das elektronegativste Element. Dieser Text wurde adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chemistry 2e, Chapter 7.4 Formal Charges and Resonance.</a>

Formalladungen

In einigen Fällen gibt es scheinbar mehr als eine gültige Lewis-Struktur für Moleküle und mehratomige Ionen. Das Konzept der formellen Gebühren kann verwendet werden, um ...

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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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