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

VSEPR theory helps to determine electron-pair geometries and molecular geometries.
A series of steps is used to predict the geometry and bond angles of molecules, such as phosphorus trichloride.
The first step is to draw the Lewis structure of the molecule.
Next, count the total number of electron groups on the central atom. Around phosphorus, there are four electron groups: three bonding pairs and one lone pair.
Now, determine the electron-pair geometry. The electron-pair geometry is tetrahedral. However, because of the lone pair, the molecular geometry is trigonal pyramidal. The lone pair reduces the bond angle to less than 109.5&#176;.
Bonding electron pairs are not always shared equally between the two bonding atoms.
In a covalent bond like that of hydrofluoric acid, the electrons are pulled toward the more electronegative atom, indicated by a partial charge. Such bonds are called polar bonds.
The charge separation creates a vector called the bond dipole moment, which is indicated by the Greek letter &#181;. Its value is the product of the magnitude of the partial charges and the distance between them.
Dipole moments are commonly expressed in debyes. One debye is equal to 3.336 &#215; 10&#8722;30 coulomb-meters.
The vector points from the less to the more electronegative atom and indicates the bond dipole moment. Its length is proportional to the magnitude of the electronegativity difference between the two atoms. Most diatomic molecules containing atoms of different elements have dipole moments and therefore are polar molecules.
In polyatomic compounds, the net dipole moment is determined by the individual bond dipole moments and geometry of the compound.
Consider a water molecule with two polar bonds. It has a bent shape and is a polar molecule.
In contrast, a carbon dioxide molecule is linear. The two carbon&#8211;oxygen bonds are polar but are oriented in opposite directions, canceling out each other&#39;s dipole moment and making the overall molecule nonpolar.

moleculaire geometrie en dipoolmomenten

De VSEPR-theorie kan worden gebruikt om de geometrieën van elektronenparen en moleculaire structuren als volgt te bepalen:

<ol style="padding-left: 35px; margin: 0;">
 <li>
 Schrijf de Lewis-structuur van het molecuul of polyatomisch ion.
 </li>
 <li>
 Tel het aantal elektronengroepen (alleenstaanden paren en bindingen) rond het centrale atoom. Een enkele, dubbele of drievoudige binding telt als één gebied van elektronendichtheid.
 </li>
 <li>
 Identificeer de geometrie van de elektronenparen op basis van het aantal elektrongroepen.
 </li>
 <li>
 Gebruik het aantal alleenstaanden paren om de moleculaire structuur te bepalen. Als er meer dan één opstelling van alleenstaande paren en chemische bindingen mogelijk is, kies dan degene die de afstoting minimaliseert.
 </li>
</ol>

 Dipoolmoment van een molecuul


 Wanneer atomen met verschillende elektronegativiteiten een binding vormen, worden de elektronen naar het meer elektronegatieve atoom getrokken, waardoor het ene atoom een gedeeltelijke positieve lading (δ+) en het andere atoom een gedeeltelijke negatieve lading (δ−) heeft. Dergelijke bindingen worden polaire covalente bindingen genoemd, en de scheiding van lading veroorzaakt een dipoolmoment van de binding. De grootte van een van de binding wordt weergegeven door de Griekse letter µ en wordt gegeven door:


 
 μ = Qr
 


 waar Q de grootte van de gedeeltelijke ladingen is (bepaald door het elektronegativiteitsverschil), en r de afstand tussen hen is. Dipoolmomenten worden doorgaans uitgedrukt in debyes, waarbij één debye gelijk is aan 3,336 × 10⁻³⁰ C·m.


 Het bindingdipoolmoment is een vector die wordt weergegeven door een pijl die langs de binding wijst van het minder elektronegatieve naar het meer elektronegatieve atoom, met een klein plusteken aan het minder elektronegatieve uiteinde.


 Een geheel molecuul kan ook een scheiding van lading hebben, afhankelijk van zijn moleculaire structuur en de polariteit van elk van zijn bindingen. Dergelijke moleculen worden als polariteit beschouwd. Het dipoolmoment meet de mate van netto scheiding van lading in het molecuul als geheel. In diatomische moleculen bepaalt het dipoolmoment van de binding de moleculaire polariteit.


 Wanneer een molecuul meer dan één binding bevat, moet de geometrie in overweging worden genomen. Als de bindingen in een molecuul zo zijn gerangschikt dat de vectoroptelling van hun bindingmomenten gelijk is aan nul, is het molecuul niet-polair (bijvoorbeeld CO₂). Het watermolecuul heeft een gebogen moleculaire structuur, en de twee bindingsmomenten heffen elkaar niet op. Daarom is water een polair molecuul met een netto dipoolmoment.


 
 Deze tekst is aangepast van
 
 <a href="https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity">
 
 Openstax, Chemistry 2e, Section 7.6 Molecular Structure and Polarity
 
 </a>

 Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/7-6-molecular-structure-and-polarity">Openstax, Chemistry 2e, Section 7.6 Molecular Structure and Polarity</a>.

Moleculaire geometrie en dipoolmomenten

The VSEPR theory can be used to determine the electron pair geometries and molecular structures as follows:

	Write the Lewis structure of the molecule or ...

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Covalent Bonding and Structure

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BELANGRIJKE TERMEN EN CONCEPTEN

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.

Predicting Shape and Molecular Polarity

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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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