Chapter 2 MOLECULAR GEOMETRY AND CHEMICAL BONDING THEORY Done By Mrs. Fatima Itani

INTRODUCTION A) Lewis structures do not indicate the three dimensional shape of a molecule. They do not show the arrangement space of the atoms. B) Molecules have definite shapes and the shape of a molecule controls some of its chemical and physical properties.

What is the VSEPR Theory? The VSEPR Theory is used to predict the shapes of molecules based on the repulsion of the bonding and non-bonding electrons in the molecule. The shape is determined by the number of bonding and non-bonding electrons in the molecule. In order to determine the shape, the Lewis diagram must be drawn first. When determining the shape of a molecule with multiple bonds, treat the multiple bonds as if they were single bonds (i.e. one bonding pair)

II. Valence Shell Electron Pair Repulsion Theory - VSEPR - predicts the shapes of a number of molecules and polyatomic ions. A) Assumptions of VSEPR Theory 1) Electron pairs in the valence shell of an atom tend to orient themselves so that the total energy is minimized. This means that: the electrons will take positions as far away from each other as possible to minimize _______________.

2) Because lone pairs of electrons are spread out more broadly than bond pairs, repulsions are greatest between two lone pairs, intermediate between a lone pair and a bond pair, and weakest between two bonding pairs of electrons. 3) Repulsive forces decrease rapidly with increasing interpair angle - greatest at 90 o, much weaker at 120 o, and very weak at 180 o.

V alence S hell E lectron P air R epulsion Theory Planar triangular Tetrahedral Trigonal pyramidal Bent

VSEPR Theory 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

1) The central atom is called A. 2) All the outer atoms are designated with an X. 3) Any lone pair electrons are designated with an E.

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

1) Two regions of high electron density AX 2 molecules a) Look at the example of the BeF 2(g) molecule. The Lewis Structure is:

b) CO 2 - The Lewis Structure is: Focus on the C atom. How many regions of high electron density? What is the geometry of the molecule? c) HCN - The Lewis Structure is: What is the geometry of the molecule?

2 pairs as far apart as possible are at 180 o from each other. This gives a linear molecule.

2) Three regions of high electron density AX 3 molecules a) Look at the example of BF 3 molecules. The Lewis Structure is

If the number of bond pairs of electrons around the central atom is 3, the arrangement of the pairs is: 3 pairs as far apart as possible are at 120 o from each other. This gives a trigonal planar molecule.

3) Four regions of high electron density AX 4 molecules The name of the geometry is TETRAHEDRAL and the angle is approximately o.

4) 3 bond pairs and 1 lone pair: EXAMPLE IS: NH 3. AX 3 E molecule The name of the geometry is TRIGONAL PYRAMID.

5) AX 2 E 2 Two bonding regions and 2 lone pairs: EXAMPLE IS H 2 O. The name of the geometry is ANGULAR OR BENT.

Tetrahedral pyramidal bent Electron geometry is tetrahedral for all

Triangular PlanarTetrahedral Trigonal pyramidal Linear Bent or V Models

methane, CH 4 Bonds are all evenly spaced electrons 109.5° Tetrahedral

Less repulsion between the bonding pairs of electrons.. ammonia NH 3.. Trigonal Pyramidal

° (109.5°) 109.5° (107°) 109.5° (104.5°) water, H 2 O

..

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

molecular geometry. H 2 CO

molecular geometry. H 2 CO

VSEPR shape tetrahedral planar linear

tetrahedral Methane (CH 4 ) simple hydrocarbon. Shape is not planar but tetrahedral.

C H H H H tetrahedral

? EX : Roth pond

Methane gas, CH 4

MoleculeLewis StructureNumber of electron pairs CH 4 NH 3 SHAPE Tetrahedral Trigonal Pyramidal 4 4 (3 shared 1 lone pair)

MoleculeLewis StructureNumber of electron pairs H2OH2O CO 2 SHAPE Bent or V 4 (2 shared 2 lone pairs) 2 Linear

MoleculeLewis StructureNumber of electron pairs BeCl 2 BF 3 SHAPE 2 3 Linear Trigonal Planar

Fluorine is the Base of Comparison

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

POLAR MOLECULES POLAR MOLECULES = uneven distribution of charge. negativeslightly positive Creating poles. 1 side of molecule is negative ; one side is slightly positive. * Creating poles. NON-POLAR MOLECULES 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 CH 4 (methane) bonds Step 1: Determine polarity of bonds Bonds Bonds are evenly spaced out. If bonds making up a molecule are non-polar, then the molecule is non-polar. Therefore, CH 4 is a non-polar molecule. molecule Step 2: Determine polarity of molecule

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

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. non-polar. 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 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 – SO 2 is a polar molecule. Which atom attracts more e-(s)?

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: H 2 O PH 3 CCl 4 Ammonia (NH 3 ) SO 3 CH 3 Cl

H 2 O (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.

PH 3 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…

NH 3 (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.

CCl 4 (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, CCl 4 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.

SO 3 (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, SO 3 is trigonal planar. Center of positive charge is on the sulfur, and center of negative charge is between the oxygen atoms (also on S). SO 3 a NON- POLAR molecule.

CH 3 Cl (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, CH 3 Cl 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.

Solids Review