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

K12 Standards

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North Carolina K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

PS.Chm.3.1

Electronegativity

Nichtmetalle bilden kovalente Bindungen, indem sie sich Elektronen teilen. Aber werden diese Elektronen gleichmäßig auf beide Atome verteilt, oder zieht ein Atom die Elektronen mehr als das andere an? Das Lewis-Modell stellt alle kovalenten Bindungen als zu gleichen Teilen geteilte Elektronen dar;dies ist jedoch nicht immer der Fall.Wenn beispielsweise gasförmiger Stickstoff in ein elektrisches Feld gebracht wird, richtet er sich gleichmäßig zwischen den Polen aus. Wenn jedoch gasförmiger Chlorwasserstoff, ein neutrales Molekül, in ein elektrisches Feld gebracht wird, richtet sich der Wasserstoff zur Kathode und das Chlor zur Anode aus, was anzeigt, dass Wasserstoff eine teilweise positive Ladung und Chlor eine teilweise negative Ladung hat. Die Fähigkeit eines Atoms, Elektronen anzuziehen, wird Elektronegativität genannt.Chlor soll also elektronegativer als Wasserstoff sein, indem es die gemeinsamen Elektronen zu sich selbst zieht, während es sich dem Ablösen seiner eigenen Elektronen widersetzt. Dadurch wird die Bindung jedoch nicht ionisch. Bei einer ionischen Bindung werden Elektronen von Metallen auf Nichtmetalle übertragen, während bei HCl die Elektronen ungleichmäßig verteilt sind.Die Elektronendichte ist am Chlor höher als am Wasserstoffatom, wodurch eine polare kovalente Bindung entsteht. Je größer der Unterschied in der Elektronegativität zwischen zwei Atomen ist, desto polarer ist die Bindung. Daher finden sich neben unpolaren kovalenten oder ionischen Bindungen auch polare kovalente Bindungen in einer Vielzahl von Verbindungen.Der amerikanische Chemiker Linus Pauling untersuchte die Energien, die erforderlich sind, um Bindungen in Molekülen wie zweiatomigem Chlor oder Wasserstoff aufzubrechen. Er stellte auf der Grundlage thermochemischer Daten eine Elektronegativitätsskala auf, die zur Vorhersage von Bindungstypen beiträgt. Die Elektronegativität ist mit der Ionisationsenergie und der Elektronenaffinität der Atome verbunden.Im Periodensystem nehmen die Elektronegativitätswerte von links nach rechts zu Metalle sind im Vergleich zu Nichtmetallen weniger elektronegativ, mit Ausnahme von Übergangsmetallen. Zusätzlich nehmen die Elektronegativitätswerte in der Spalte nach unten und mit zunehmender Atomgröße ab, da die Atome weniger in der Lage sind, Elektronen an sich zu ziehen. Fluor, das elektronegativste Element, hat den willkürlich zugewiesenen Elektronegativitätswert von 3, 98.Francium hingegen ist das am wenigsten elektronegative Element mit einem Elektronegativitätswert von 0, 7. Die Elektronegativität hat keine Einheit;sie kann nicht experimentell bestimmt werden.

elektronegativität

Ob eine Bindung unpolar oder polar kovalent ist, wird durch eine als Elektronegativität bezeichnete Eigenschaft der Bindungsatome bestimmt.  Die Elektronegativitätswerte der Elemente wurden von einem der berühmtesten Chemiker des 20. Jahrhunderts ausgearbeitet: Linus Pauling. Pauling untersuchte die Energien, die zum Aufbrechen von Bindungen in heteronuklearen Molekülen wie Wasserstoff und Fluorid erforderlich sind. Basierend auf den Werten schlug er vor, dass die zum Aufbrechen einer Bindung erforderliche Energie dem Durchschnitt der Bindungsenergien von H2 (436 kJ/mol) und F2 (155 kJ/mol) entsprechen würde, also 296 kJ/mol. Die experimentell erhaltene Bindungsenergie von HF beträgt jedoch 565 kJ/mol, was viel höher ist als der vorhergesagte Wert. Um diesen Unterschied zu erklären, schlug Pauling vor, dass die Bindung einen ionischen Charakter haben muss, der durch das Konzept der Elektronegativität erklärt wird.  Elektronegativität ist ein Maß für die Tendenz eines Atoms, Elektronen (oder Elektronverteilung) anzuziehen.  Die Elektronegativität bestimmt, wie die gemeinsamen Elektronen in einer Bindung zwischen den beiden Atomen verteilt werden. Je stärker ein Atom die Elektronen in seinen Bindungen anzieht, desto größer ist seine Elektronegativität. Elektronen in einer polaren kovalenten Bindung werden zum elektronegativeren Atom verschoben; Somit ist das elektronegativere Atom dasjenige mit der partiell negativen Ladung. Je größer der Unterschied in der Elektronegativität ist, desto polarisierter ist die Elektronenverteilung und desto größer sind die Partialladungen der Atome. <h4>Elektronegativität und das Periodensystem</h4> <ul> <li>Die Elektronegativität nimmt in einer Periode im Periodensystem von links nach rechts zu und in einer Gruppe nach unten hin ab. </li> <li>Die von Pauling abgeleiteten Elektronegativitätswerte folgen diesem beschriebenen Muster, wobei die höheren Elektronegativitäten oben rechts im Periodensystem liegen.</li> <li>Daher weisen die Nichtmetalle, die sich ebendort befinden, tendenziell die höchsten Elektronegativitäten auf, wobei Fluor das elektronegativste Element von allen ist (EN = 4,0). </li> <li>Metalle neigen dazu, weniger elektronegative Elemente zu sein und die Metalle der Gruppe 1 weisen die niedrigsten Elektronegativitäten auf. </li> <li>Edelgase sind von der Elektronegativitätsliste ausgeschlossen, da diese Atome normalerweise keine Elektronen mit anderen Atomen teilen, da sie eine vollständige Valenzschale haben. (Obwohl Edelgasverbindungen wie XeO2 existieren, können sie nur unter extremen Bedingungen gebildet werden und passen daher nicht wirklich in das allgemeine Modell der Elektronegativität.)</li> </ul> <h4>Elektronegativität versus Elektronenaffinität</h4> Es ist darauf zu achten, Elektronegativität und Elektronenaffinität nicht miteinander zu verwechseln. Die Elektronenaffinität eines Elements ist eine messbare physikalische Größe, nämlich die Energie, gemessen in kJ/mol, die freigesetzt oder aufgenommen wird, wenn ein isoliertes Gasphasenatom ein Elektron aufnimmt. Elektronegativität hingegen beschreibt, wie stark ein Atom Elektronen in einer Bindung anzieht. Es handelt sich um eine dimensionslose Größe, die berechnet und nicht gemessen wird. Pauling leitete die ersten Elektronegativitätswerte ab, indem er die Energiemengen verglich, die zum Aufbrechen verschiedener Bindungstypen erforderlich waren. Er wählte eine willkürliche relative Skala von 0 bis 4. Dieser Text wurde adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding">Openstax, Chemistry 2e, Section: 7.2 Covalent Bonding.</a>

Elektronegativität

Ob eine Bindung unpolar oder polar kovalent ist, wird durch eine Eigenschaft der Bindungsatome bestimmt, die als Elektronegativität bezeichnet wird. 

Elektronegativitätswerte von ...

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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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