Name: predicting shape and molecular polarity
Uploaded: 2023-04-30T18:28:31.000Z
Duration: 2 min 36 s
Channel: JoVE

Die VSEPR-Theorie hilft bei der Bestimmung von Elektronenpaargeometrien und molekularen Geometrien.
Eine Reihe von Schritten wird verwendet, um die Geometrie und die Bindungswinkel von Molekülen, wie z. B. Phosphortrichlorid, vorherzusagen.
Der erste Schritt besteht darin, die Lewis-Struktur des Moleküls zu zeichnen.
Zählen Sie als Nächstes die Gesamtzahl der Elektronengruppen auf dem Zentralatom. Um Phosphor herum gibt es vier Elektronengruppen: drei Bindungspaare und ein einzelnes Paar.
Bestimmen Sie nun die Elektronenpaargeometrie. Die Elektronenpaar-Geometrie ist tetraedrisch. Aufgrund des einsamen Paares ist die Molekülgeometrie jedoch trigonal pyramidal. Das einsame Paar reduziert den Bondwinkel auf weniger als 109,5°.
Bindungselektronenpaare werden nicht immer gleichmäßig zwischen den beiden Bindungsatomen aufgeteilt.
Bei einer kovalenten Bindung wie der von Flusssäure werden die Elektronen in Richtung des elektronegativeren Atoms gezogen, was durch eine Teilladung angezeigt wird. Solche Bindungen werden als polare Bindungen bezeichnet.
Durch die Ladungstrennung entsteht ein Vektor, der als Bindungsdipolmoment bezeichnet wird und durch den griechischen Buchstaben μ angegeben wird. Sein Wert ist das Produkt aus der Größe der Teilladungen und dem Abstand zwischen ihnen.
Dipolmomente werden häufig in Debyes ausgedrückt. Ein Debye entspricht 3,336 × 10−30 Coulomb-Metern.
Der Vektor zeigt vom kleineren zum elektronegativeren Atom und zeigt das Bindungsdipolmoment an. Seine Länge ist proportional zur Größe der Elektronegativitätsdifferenz zwischen den beiden Atomen. Die meisten zweiatomigen Moleküle, die Atome verschiedener Elemente enthalten, haben Dipolmomente und sind daher polare Moleküle.
In mehratomigen Verbindungen wird das Nettodipolmoment durch die einzelnen Bindungsdipolmomente und die Geometrie der Verbindung bestimmt.
Stellen Sie sich ein Wassermolekül mit zwei polaren Bindungen vor. Es hat eine gebogene Form und ist ein polares Molekül.
Im Gegensatz dazu ist ein Kohlendioxidmolekül linear. Die beiden Kohlenstoff-Sauerstoff-Bindungen sind polar, aber in entgegengesetzte Richtungen ausgerichtet, wodurch sich das Dipolmoment des jeweils anderen aufhebt und das Gesamtmolekül unpolar wird.

molekulare geometrie und dipolmomente

Mit der VSEPR-Theorie können die Elektronenpaargeometrien und Molekülstrukturen wie folgt bestimmt werden:

<ol>
	<li>Schreiben Sie die Lewis-Struktur des Moleküls oder mehratomigen Ions auf.</li>
	<li>Zählen Sie die Anzahl der Elektronengruppen (Einzelpaare und Bindungen) um das Zentralatom. Eine Einfach-, Doppel- oder Dreifachbindung zählt als ein Bereich der Elektronendichte.</li>
	<li>Identifizieren Sie die Elektronenpaargeometrie anhand der Anzahl der Elektronengruppen.</li>
	<li>Verwenden Sie die Anzahl der freien Elektronenpaare, um die Molekülstruktur zu bestimmen. Wenn mehr als eine Anordnung von freien Elektronenpaaren und chemischen Bindungen möglich ist, wählen Sie diejenige, die die Abstoßungen minimiert.</li>
</ol>

Dipolmoment eines Moleküls

Wenn Atome mit unterschiedlicher Elektronegativität eine Bindung eingehen, werden die Elektronen zum elektronegativeren Atom gezogen, sodass ein Atom eine teilweise positive Ladung (δ+) und das andere Atom eine teilweise negative Ladung (δ–) erhält. Solche Bindungen werden polare kovalente Bindungen genannt und die Ladungstrennung führt zu einem Bindungsdipolmoment. Die Größe eines Bindungsdipolmoments wird durch den griechischen Buchstaben µ dargestellt und ergibt sich aus:

μ = Qr

Dabei ist Q die Größe der Teilladungen (bestimmt durch die Elektronegativitätsdifferenz) und r der Abstand zwischen ihnen.

Dipolmomente werden üblicherweise in Debye ausgedrückt, wobei ein Debye 3,336 × 10−30 C·m entspricht.

Das Bindungsdipolmoment ist ein Vektor, der durch einen Pfeil dargestellt wird, der entlang der Bindung vom weniger elektronegativen zum elektronegativeren Atom zeigt, mit einem kleinen Pluszeichen am weniger elektronegativen Ende.

Abhängig von seiner Molekülstruktur und der Polarität jeder seiner Bindungen kann es auch bei einem ganzen Molekül zu einer Ladungstrennung kommen. Solche Moleküle nennt man polar. Das Dipolmoment misst das Ausmaß der Nettoladungstrennung im gesamten Molekül. In zweiatomigen Molekülen bestimmt das Bindungsdipolmoment die Molekülpolarität.

Wenn ein Molekül mehr als eine Bindung enthält, muss die Geometrie berücksichtigt werden. Wenn die Bindungen in einem Molekül so angeordnet sind, dass die Vektorsumme ihrer Bindungsmomente gleich Null ist, dann ist das Molekül unpolar (z. B. CO2). Das Wassermolekül hat eine gebogene Molekülstruktur und die beiden Bindungsmomente heben sich nicht auf. Daher ist Wasser ein polares Molekül mit einem Nettodipolmoment.

Dieser Text wurde angepasst von  <a href="https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity">Openstax, Chemistry 2e, Abschnitt 7.6 Molecular Structure and Polarity</a>.

Molekulare Geometrie und Dipolmomente

Mit der VSEPR-Theorie können die Elektronenpaargeometrien und molekularen Strukturen wie folgt bestimmt werden:

Schreiben Sie die Lewis-Struktur des Moleküls oder ...

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Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Covalent Bonding and Structure

Kovalente Bindung und Struktur

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

Molecular Geometry and Dipole Moments

predicting shape and molecular polarity

Molecular shape and molecular polarity can be predicted with VSEPR theory and dipole moments. Start by writing the Lewis structure of the molecule or polyatomic ion. Then count the electron groups around the central atom. Lone pairs and bonds both count as electron groups, and a single, double, or triple bond each counts as one region of electron density.

Use that count to identify the electron-pair geometry. Next, use the number of lone pairs to determine the molecular structure. If more than one arrangement of lone pairs and bonds is possible, choose the one with the fewest repulsions. This step helps show how atoms arrange themselves in space.

Bond polarity depends on electronegativity, which is how strongly an atom pulls shared electrons. When atoms with different electronegativities form a bond, the electrons are pulled toward the more electronegative atom. That leaves one atom with a partial positive charge, written as δ+, and the other with a partial negative charge, written as δ−. These are polar covalent bonds.

The charge separation in a polar bond creates a bond dipole moment. Its magnitude is written with the Greek letter μ and follows the relation μ = Qr, where Q is the size of the partial charges and r is the distance between them. Dipole moments are usually measured in debyes. One debye equals 3.336 × 10^−30 C·m. A bond dipole is shown as an arrow pointing from the less electronegative atom toward the more electronegative atom, with a small plus sign on the less electronegative end.

A whole molecule can also have a dipole moment. That depends on both the polarity of its bonds and its molecular shape. In diatomic molecules, the bond dipole determines the molecular polarity. In molecules with more than one bond, the bond dipoles must be added as vectors. If the vector sum is zero, the molecule is nonpolar, as in CO2. Water has a bent molecular structure, so its bond moments do not cancel. As a result, water is a polar molecule with a net dipole moment.

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Learn how VSEPR theory predicts molecular shape, how bond polarity forms, and why some molecules like CO2 are nonpolar while water is polar.

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Predicting Shape and Molecular Polarity

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