VSEPR Theory Molecular Geometry VSEPR Theory Valence Shell

VSEPR Theory Molecular Geometry

VSEPR Theory • Valence Shell Electron Pair Repulsion Theory • A theory for predicting molecular shapes that assumes that electrons like to be as far from each other as possible.

-You must be able to draw the lewis structure to determine the shape. -We determine the shape by looking at the center atom and what is coming off of it!

Linear • Two atoms off center Examples • NO lone pairs Be. F 2

Trigonal Planar • Three atoms off center • No lone pairs Examples BCl 3

Tetrahedral • Four atoms off center • No lone pairs Bond Angle 109. 5° Examples CH 4

Trigonal Pyramidal • Three atoms off center • ONE lone pair Examples NH 3

Bent • TWO atoms off center • One OR two lone pairs Examples H 2 O

But…But why do the lone pairs NOT go on opposite sides of the atom in a bent molecule. . don’t they repulse? • Yes they do repulse, but lone pairs (the ones that don’t bond) are also much closer to the nucleus than bonded pairs! So lone pairs are closer to each other and push the bonds away. AKA. THEY STAY TOGETHER LIKE YOUR ELECTRON DOT DIAGRAMS SHOW.

Sometimes the center atom can have more than 8 electrons around them. Groups 5 A and 6 A do this.

Group 5 A Trigonal Bipyramidal • FIVE atoms off the center • NO lone pairs Examples PCl 5

Group 6 A Octahedral • SIX atoms off center • NO lone pairs Examples SF 6

• YOU MUST MEMORIZE THIS • Notice trigonal bypyramidal and octahedral are not on this…you’re welcome

Practice • Draw the Lewis dot diagram for CF 4 • • Determine the shape Determine the bond angle

• Draw the lewis dot structure for PH 3 • Determine the shape • Determine the bond angle