Valence Shell Electron Pair Repulsion Theory Planar triangular Valence Shell Electron Pair Repulsion Theory Tetrahedral Trigonal pyramidal Bent

Molecular Shape VSEPR theory assumes that the shape of a molecule is determined by the repulsion of electron pairs.

Lone pairs repel more strongly than bonding pairs!!! A. VSEPR Theory Types of e- Pairs Bonding pairs - form bonds Lone pairs - nonbonding e- Lone pairs repel more strongly than bonding pairs!!!

VSEPR Theory Based on Electron Dot (Lewis structures) 06/10/99 Based on Electron Dot (Lewis structures) Theory predicts shapes of compounds abbreviated VSEPR VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves) Electrons around central nucleus repel each other. So, structures have atoms maximally spread out

VSEPR overview Names of Shapes: tetrahedral trigonal pyramidal Bent 06/10/99 Each shape has a name (you will have to know these) Names of Shapes: tetrahedral trigonal pyramidal Bent Linear trigonal planar

Models Tetrahedral Triangular Planar Bent or V Linear Trigonal pyramidal

methane, CH4 Tetrahedral 109.5° Bonds are all evenly spaced electrons

ammonia NH3 .. .. .. .. Trigonal Pyramidal Less repulsion between the bonding pairs of electrons

.. .. water, H2O 109.5° (109.5°) 109.5° (107°) 109.5° (104.5°)

.. ..

Bent or V 2 unshared pairs of e’s at top of O repel bonds and force them to bend

molecular geometry. H2CO

molecular geometry. H2CO

Number of electron pairs SHAPE Molecule Lewis Structure Number of electron pairs CH4 NH3 4 Tetrahedral Trigonal Pyramidal 4 (3 shared 1 lone pair)

Number of electron pairs Molecule Lewis Structure Number of electron pairs H2O SHAPE Bent or V 4 (2 shared 2 lone pairs) Linear CO2 2

Number of electron pairs SHAPE Molecule Lewis Structure Number of electron pairs BeCl2 BF3 Linear 2 Trigonal Planar 3

Fluorine is the Base of Comparison

Which atom attracts e- more? electronegativities 2.1 3.0 H ― Cl δ+ δ- 2.5 3.5 H 2.1 ― C = O ― 2.1 H O = C = O

POLAR MOLECULES = uneven distribution of charge. 1 side of molecule is negative ; one side is slightly positive. * Creating poles. NON-POLAR MOLECULES = no difference in charge on outside of molecule. Electrons are evenly distributed. Uniform charge on outside of molecule.

Predict the polarity of CH4 (methane) Step 1: Determine polarity of bonds Bonds are evenly spaced out. Step 2: Determine polarity of molecule If bonds making up a molecule are non-polar, then the molecule is non-polar. Therefore, CH4 is a non-polar molecule.

Which atom attracts electrons more? Carbon dioxide. Step 1: Determine polarity of bonds O = e- pulled toward Which atom attracts electrons more? Step 2: Determine polarity of Molecule (shape is linear) If bonds making up a molecule are polar, then the molecule may be polar or non-polar, depending on its shape.

The center of the positive charges in located on the carbon atom The center of the negative charge is also located on the carbon atom. Since center of both the positive and negative charge are located in the same spot in the molecule, there is no difference in overall charge) so the molecule is non-polar.

Look at sulfur dioxide. Step 1: Determine polarity of bonds Which atom attracts more e-(s)? Center of positive charge is on the sulfur atom. While the center of negative charge is located ½ way between the two oxygen atoms. Since polarity of the bonds and shape of the molecule result in an uneven distribution of charge – SO2 is a polar molecule.

H2O PH3 CCl4 Ammonia (NH3) SO3 CH3Cl Now that you have seen how to apply the two steps to determine the polarity of molecules, see if you can predict the polarity of the following: H2O PH3 CCl4 Ammonia (NH3) SO3 CH3Cl

H2O (Water) Step 1: Polarity of bonds Based on electronegativity difference between H and O, bond is polar Step 2: Shape of molecule Based on VSEPR theory, water is bent. Center of positive charge is between the two hydrogen, and center of negative charge on oxygen. WATER is a POLAR molecule.

PH3 Step 1: Polarity of bonds Based on electronegativity difference between H and P, bonds are polar Trigonal Pyramidal 1 unshared pair around central Atom 3 shared bond Polar Molecule…

NH3 (Ammonia) Step 1: Polarity of bonds Based on electronegativity difference between H and N, bond is polar Step 2: Shape of molecule Based on VSEPR theory, ammonia has a trigonal pyramidal shape. Center of positive charge is between hydrogen atoms, and center of negative charge on oxygen. AMMONIA is a POLAR molecule.

CCl4 (carbon tetrachloride) Step 1: Polarity of bonds Based on electronegativity difference between C and Cl, bonds are polar Step 2: Shape of molecule Based on VSEPR theory, CCl4 has a tetrahedral shape. Center of positive charge is on carbon, and center of negative is also on the carbon. No separation of charge. Carbon tetrachloride is a NON- POLAR molecule.

SO3 (Sulfur trioxide) Step 1: Polarity of bonds Based on electronegativity difference between S and O, bond is polar Step 2: Shape of molecule Based on VSEPR theory, SO3 is trigonal planar. Center of positive charge is on the sulfur, and center of negative charge is between the oxygen atoms (also on S). SO3 a NON- POLAR molecule.

CH3Cl (Chloromethane) Step 1: Polarity of bonds C-H bonds are non-polar, C-Cl bon is polar Step 2: Shape of molecule Based on VSEPR theory, CH3Cl is tetrahedral. Cl end of bond is negative , while C end of bond is positive. There is a net separation of charge so molecule is POLAR.