Lecture 31 11/18/05

Electron Pair Geometries

Molecules with only single covalent bonds around central atom.

Axial atoms Equatorial atoms

Repulsion

Lone pair – Lone pair

Lone pair – Bonding pair

Bonding pair – Bonding pair

Highest repulsion

Geometries for Four Electron Pairs

Molecular Geometries for Five Electron Pairs

Molecular Geometries for Six Electron Pairs

Example: What is the geometry of NH3?

1. Draw Lewis structure2. Determine # of bonds and lone pairs around central atom3. Electron-pair geometry4. Molecular geometry

Formaldehyde, CH2O

••

C HH

O••

••

CHH

O••

1. Draw Lewis structure

2. Determine # of bonds and lone pairs around central atom

3. Electron-pair geometry

4. Molecular geometry

Molecular polarity dependent on two things: Bond polarity Geometry

Dipole Moment Measurement of the molecular polarity

Molecular Polarity

CO2 and H2O

Which one is polar?

Intermolecular Forces (IMF)Noncovalent interactions

Influence propertiesMelting pointBoiling point

Greater the IMF, higher the boiling pointSolubility

Like dissolves likeStructure of complex molecules like

proteins

Beta-carotene

Dipole-Dipole attractionPolar / Polar interaction

More polar molecules greater IMF

5 – 25 kJ/mol (150-1000 KJ/mol – covalent bond)

Polar molecules can induce a dipole in non-polar molecules Polarization

Polarizability Degree to which electron cloud can be distorted to form dipole Higher the molar mass, greater the polarizibility

Dipole / Induced Dipole ForcesPolar / non-polar interaction