Ch 6.5 Molecular Geometry

VSEPR Theory Use VSEPR theory Valence-Shell-Electron-Pair Repulsion =the repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible. Determined by number of valence electrons of the central atom 3-D shape is a result of bonded pairs and lone pairs of electrons

Bonding and Shape of Molecules Number of Bonds Number of Unshared Pairs Covalent Structure Shape Examples 2 3 4 1 AB2 Linear Trigonal planar Tetrahedral Pyramidal Bent or Angular BeCl2 BF3 CH4, SiCl4 NH3, PCl3 ONF AB3 AB4 AB3E AB2E

Number of Unshared Pairs Number of Bonds Number of Unshared Pairs Covalent Structure Shape Example 2 AB2E2 Bent or Angular H2O 5 AB5 Trigonal Bipyramidal PF5 6 AB6 Octahedral SF6

The VSEPR Model .. .. .. O S O S F N O C Linear Bent Trigonal planar pyramidal F P AB6 F F S Students often confuse electron-domain geometry with molecular geometry. You must stress that the molecular geometry is a consequence of the electron domain geometry. The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. F C Tetrahedral Trigonal bipyramidal Octahedral

Methane CH4 C 109.5o H Tetrahedral geometry C H

N H N H H H Ammonia- NH3 .. N H .. N H Trigonal Pyramidal geometry

Water –H2O .. .. .. .. SO2 .. O H O H H Bent geometry

Boron trifluoride - BF3 B F B F 120o Trigonal planar

Carbon dioxide – CO2 O C O O C Linear

N 107o H .. .. C 109.5o H O .. H 104.5o H Bent Tetrahedral Pyramidal

Learning Check Determine the molecular geometry: A. CCl4 B. H2CO

Learning Check Determine the molecular geometry: A. CCl4 Tetrahedral B. H2CO

Learning Check Determine the molecular geometry: A. CCl4 Tetrahedral B. H2CO Trigonal Planar

Intermolecular Forces Forces of attraction between molecules Very weak compared to ionic and metallic bonding. The strongest occur between polar molecules.

Intermolecular Forces Dipole: One end slightly positive and one end slightly negative. Can show the polar nature with an arrow pointing to the more electronegative atom. O H H H Cl

Just because a molecule has a polar bond does not make it a polar molecule. The bond polarities cancel if they are in opposite directions. Example: CO2

Attractions Between Molecules The two weakest attractions between molecules. Dipole Interactions When polar molecules are attracted to one another. Dispersions Forces Caused by the motion of electrons Weakest of all interactions.

Hydrogen Bonds A strong attraction found in hydrogen-containing molecules. A hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom. Example Water

Homework 6.5 page 210 #33-34, 37-38 NOT 37-42!!