Download VSEPR Theory

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 of center
Examples
• NO lone pairs
BeF2
Trigonal Planar
• Three atoms off center
• No lone pairs
Examples
BCl3
Tetrahedral
• Four atoms of center
• No lone pairs
Bond Angle
109.5°
Examples
CH4
Trigonal Pyramidal
• Three atoms off center
• ONE lone pair
Examples
NH3
Bent
• TWO atoms off center
• One OR two lone pairs
Examples
H2 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.
Group 5A and 6A do this
Group 5A
Trigonal Bipyramidal
• FIVE atoms of the center
• NO lone pairs
Examples
PCl5
Group 6A Octahedral
• SIX atoms off center
• NO lone pairs
Examples
SF6
• YOU MUST MEMORIZE THIS 
• Notice trigonal bypyramidal and octahedral are not on
this…you’re welcome
Practice
• Draw the lewis dot diagram for CF4
•
•
Determine the shape
Determine the bond angle
• Draw the lewis dot structure for PH3
• Determine the shape
• Determine the bond angle