Molecular Geometry – Shapes!

Why does shape matter? Properties of molecules depend on: Types of atoms present Arrangement around the central atom Polarity of Molecule is also determined by shape Polarity will affect the interaction between molecules

VSEPR Theory “Valence Shell Electron Pair Repulsion” # of shared pairs and lone pairs around CENTRAL atom used to determine overall shape Theory states: “Repulsion between the sets of valence – level electrons surrounding an atom causes these sets to be as far apart as possible.”

How do we determine shapes?? VSEPR utilizes an ABE formula A = central atom B = atoms bonded to central atom E = lone pairs of electrons on CENTRAL ATOM ONLY Example: H2O AB2E2 Oxygen is central atom 2 hydrogen bonded to it (B2) 2 lone pairs on oxygen (E2)

What do I do with the formula AB2E2 This shape is bent – from table on page 27A of your packet

Repulsion between two unshared pairs of electrons is greatest – push farthest apart Repulsion between a shared and unshared pair of electrons is intermediate Repulsion between two shared pairs of electrons is least

Follow the steps to get the ABE formula and shape Back of first page of packet!!! Draw the Lewis Dot structure for molecule Check to see if central atom is an exception Determine the number of atoms attached to central atom # = subscript on B Determine # of lone pairs on CENTRAL ATOM # = subscript on E Match the ABE formula you just determined to the shape on the chart (27A)

More Examples CH4 NH3 HBr (first one on your sheet)

Polarity

What is Polarity??? Polarity arises when one of the atoms has the electrons more than the other atom Unequal sharing Red end represents area of greater electron density (electrons are there more often)

What is the result?? Bond has a positive and negative end The end that is negative “sees” the electrons more than the positive side Atom that has greater electronegativity will “see” electrons more

How do we determine if a bond is polar? Difference in electronegativities If the difference is > than 0.3, the bond is POLAR covalent If the difference is < than 0.3, the bond is non polar covalent Example: O-H bond ΔEN = 3.5 – 2.1 = 1.4 POLAR!!!! -δ +δ O H *oxygen is more electronegative so it ‘sees’ the shared electrons more Points toward MORE EN atom

Other examples Are these polar? If so, indicate the positive and negative end. O F H Cl B Si

Molecules can be polar too! Molecules will be polar if: Bonds are polar AND Molecule is NOT symmetric *if a molecule has lone pair (nonbonding pairs) of electrons, automatically POLAR USE FLOW CHART IN NOTES!!!!!

ΔEN = 1.0, polar bonds CO2 – non polar, symmetrical OVERALL , this molecule is not polar. ΔEN = 1.4, polar bonds H2O – POLAR, 2 lone pairs OVERALL, this molecules IS polar

CCl4 ΔEN = 0.5, polar bonds Symmetrical NON POLAR!!! CH3F ΔEN = 1.5 and .4, polar bonds Non Symmetrical POLAR!!!

Both molecules are CH2Cl2. Which is Polar? VS. The molecule on the left is non symmetric (all negative pull is to one side) - POLAR