Lecture 11 VSEPR and Molecular Geometries

Lecture 11 VSEPR and Molecular Geometries - VSEPR and...

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VSEPR and Molecular Geometries B. A. Rowland 53750/53760 VSEPR Model I We have thus far constructed 2-D representations of simple molecules. Unfortunately, this is not the correct picture We thus introduce the concept of the Valence Shell Electron Pair Repulsion model (VSEPR) in order to help us turn our 2-D structures into 3-D ones. Once we have 3-D pictures, we will be able to predict certain chemical properties (dipole moments, bond angles, bond orbitals). VSEPR Model II The main idea of the VSEPR model is that pairs of electrons repel, and they will want to minimize that repulsion as best they can in the molecule (i.e. they want to get as far away from each other as possible). The one caveat we must remember—lone pairs of electrons repel bonding pairs of electrons much more than bonding pairs can repel themselves. This means that in a molecule the lone pairs will require more “room” than bonding pairs. Using the VSEPR Model There is a demi-algorithm you can use to apply the VSEPR model to obtain molecular geometries, but thankfully this has been all worked out for us (see charts later in lecture notes under ‘molecular geometries’ as well as diagram in lecture slides). The algorithm would go like: 1. Draw the Lewis dot structure for the molecule in question. 2. Count electron pairs around central atom and arrange them in such a way as to minimize the repulsions (put them as far apart as possible). 3. Determine terminal atomic positions from the way the electron pairs are arranged. 4. Determine the steric number. 5. Name the electronic geometry from the steric number. 6. Name the molecular geometry from the positions of the atoms and lone pairs in the molecule. 7. Assign bond angles. VSEPR Examples Some examples as to the application of the VSEPR model include: BeCl 2 : Beryllium chloride has two bonding pairs of electrons and no lone pairs, so the way to best arrange the electron pairs is to put them on opposite sides of the beryllium atom, at 180 degrees apart. This leads to a linear geometry for BeCl 2 . Putting the lone pairs anywhere else on the beryllium atom would lead to greater electron pair repulsion, and hence is forbidden by VSEPR.
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BF 3 : The central boron in BF 3 has three bonding pairs and no lone pairs of electrons. Logically, the way to arrange these pairs so as to minimize the repulsions is to place them 120 degrees apart around the boron atom, thus yielding the trigonal planar geometry. Placing them in any other arrangement would violate VSEPR as the repulsions between
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This note was uploaded on 02/02/2009 for the course CH 53750 taught by Professor Rowland during the Spring '09 term at University of Texas.

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Lecture 11 VSEPR and Molecular Geometries - VSEPR and...

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