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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.
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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 isgreatest– 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)
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? • The molecule on the left is non symmetric (all negative pull is to one side) - POLAR VS.