Chemical Bonding II: VESPR Chapter 10 Chemical Bonding II: VESPR

What is the VSEPR theory? Valence shell electron pair repulsion (VSEPR) theory: electron groups (lone pairs, single, double, triple bonds, or even single eectrons) have a repulsive property This theory deals with the 3-D shape of molecules. Electron groups want to be as far away from one another as possible. There are 5 basic shapes

Two electron groups Draw the Lewis dot diagram for BeCl2 This molecule is weird because there are no lone pairs on Be. Actually, this type of molecule is pretty rare. The geometry would be LINEAR

What about CO2? 2 electron groups (2 double bonds) around central atom, no lone pairs. LINEAR!

Three electron groups Draw a Lewis dot diagram for BF3 How can you maximize distance between the electron groups? Trigonal Planar geometry: tri = 3, plane = flat Since the bonds around B are all the same, equal repulsion, equal distance. Bonds are at 120 degree angles

Draw the lewis dot diagram What about H2CO? Draw the lewis dot diagram Double bond is slightly more repulsive than each of the single bonds (more electrons involved), so double bond pushes single bonds away. Angle of bond is not exactly 120 degrees. (actually angles are 121.6, 121.6, and 116.2)

Question: Why do we only consider electron groups on the central atom???

Other shapes: Tetrahedral geometry: think of a pyramid with a triangular base (tetrahedron): 4 electron groups. Try methane, CH4 Trigonal Bipyramidal Geometry: 5 electron groups. Try PCl5 Octahedral Geometry: 6 electron groups. Try SF6

Let’s try it: What is the molecular geometry of CCL4? Homework: Pg 453 29-34, try 37 and 38 (tricky)

VSEPR theory: effect of lone pairs Lone pairs around the center atom have a slightly more repulsive force than a bond. Two lone pairs = most repulsive Bonding pair – bonding pair = least repulsive

4 electron groups with lone pairs: NH3: Draw the lewis dot diagram. Predict the geometry H2O: Draw the lewis dot diagram. Predict the geometry

How to predict the geometries? Summary of VSEPR theory: 1. Geometry of a molecule is determined by the number of electron groups on the central atom 2. The number of electron groups can be determined from the Lewis dot structure (any resonance structure will work) 3. Each of the following counts as a single electron group: a lone pair, a single bond, a double bond, a triple bond, or a single electron 4. The geometry is determined by minimizing the repulsion: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair 5. bond angles can vary from ideal because double and triple bonds are more repulsive than single bonds and lone pairs are even more repulsive.

Great. Now what? Now you PREDICT! 1. Lewis dot diagram 2. count electron groups on central atom 3. determine number of bonding groups and lone pairs 4. use table 10.1 on pg 414 in book to predict geometry

Larger molecules You can do larger molecules too! Just consider geometry of each centralized atom Try CH3OH Start with lewis dot diagram

Sketching them (yuck) When you sketch these molecules… Straight line = bond in plane of paper Hatched wedge = bond going into the page (back) Solid wedge = bond coming out of page (front)

Molecular shape and Polarity Determining if a bond is polar is one thing, but this individual polarity can lead to BIGGER things! Whole molecules can be polar!!!! This is basically the most important thing in chemistry. Ever.

Determining molecular polarity 1. Draw a Lewis dot structure and use table (pg 414) to determine geometry 2. determine the polarity of each individual bond 3. Consider all of the bonds together with geometry. Is there a net dipole moment? (Overall pull?) Then it is POLAR! Take a look at 421 in your book

Let’s try it! Draw the Lewis dot structure, determine the geometry, sketch the 3D model and determine the polarity of: NH3 CF4

More Practice! Quiz Tomorrow! Pg 418, practice 10.4 Pg 453, 1-7 Pg 454 39-50

Valence Bond Theory We use various theories to describe the observable properties of the elements. Different theories meet different needs. For example, Lewis dot theory allows us to predict bonds and compounds, but isn’t a true representation of the electrons (dots). Valence bond theory: electrons reside in quantum mechanical orbitals on individual atoms (sometimes s,p,d,f…sometimes a combo or ‘hybrid’ of two or more)

When 2 atoms approach each other, the electrons and nucleus of both interact. ENERGY!!! If the energy is lowered by this interaction, bond occurs!!!! Think about what you already know: energy is minimized when orbitals are filled, soooo when 2 atoms with partially filled orbitals come near one another, the orbitals overlap/fill This is a closer representation of what is actually happening. Electrons aren’t dots connected by a line (as in Lewis). The exist in space and certain configurations are more stable

Hybridization This is only part of the story. Think of H2S. Complete the electron configuration: You can see how the overlap satisfies the needs of both atoms. Easy. But…. What about C and H? Complete the electron config: How can we resolve this?

Orbitals in a MOLECULE are not necessarily the same as orbitals in an ATOM!!!!! 1. the number of standard orbitals = number of hybrid orbitals formed (orbitals are conserved) 2. particular combinations of standard orbitals determines shapes and energies of hybrid orbitals formed 3. the particular type of hybridization that occurs is the one that yields the overall LOWEST ENERGY for the molecule.

Hybridization requires energy to occur, but the energy ‘payback’ from forming a bond is generally large

Types of hybridization sp3 hybridization: Think of hydrogen and carbon again! sp2 hybridization: Think of H2CO sp hybridization: think of C2H2

Types of bonds Sigma bond: first bond formed. ALWAYS SIGMA! (hybridized sp). CAN rotate! Pi bond: any other bonds formed (left over p). CANNOT ROTATE!

So, what now? Everything is still the same, EXCEPT now you can identify as sigma or pi bonds. I like you, so I won’t make you sketch these 

Homework Pg 453 11-13, 15