from the Lewis electron dot structure using

1
PREDICTING THE SHAPES OF MOLECULES
from the Lewis electron dot structure using
the principle that electron pairs stay as far
apart as possible.
BONDED PAIRS
Electron pairs
LONE PAIRS
2
VALENCE SHELL ELECTRON PAIR REPULSION
VSEPR
Based on the idea that all electron pairs repel
each other.
The bonding and lone pairs push apart as far as
possible..
This means that atoms bound to a central atom are
as far apart as possible.
we can find the molecular shape!
Lets see how it works...
3
Linear Molecules

• With two bonds (or pairs of bonds), no lone
  pairs.
• A---B---A
• Ex. BeCl2

Ex. CO2
4
Trigonal Planar Molecules

• With 3 bonds (single or double), no unpaired
  electrons.
• Ex. BF3

Ex. CO32-
5
SeO2
ELECTRON PAIR GEOMETRY (EPG)
THREE ELECTRON PAIRS AROUND THE SELENIUM ATOM.
VSEPR treats double bonds like a single bond
EPG TRIGONAL PLANAR
Now place the oxygen atoms
6
SeO2
Electron Pair Geometry is trigonal planar
ADD OXYGENS
SeO2 molecular geometry is V-SHAPED, OR BENT
7
There are four electron pairs around the carbon
atom.
CH4
If molecule was square planar, bond angles would
only be 90 degrees.
8
There is a better arrangement for four electron
pairs
H
TETRAHEDRAL
109.5
H
H
H
tetrahedral EPG
4 electron pairs
The shape of CH4 is tetrahedral.
Ex. NH4
9
NH3 The electron pair geometry around the
nitrogen is tetrahedral
There are four electron pairs around the nitrogen
atom.
PUT ON THE 3 H ATOMS
N
H
H
H
The shape of NH3 is trigonal pyramidal.
10
H2O The electron pair geometry around the oxygen
is tetrahedral
There are four electron pairs around the oxygen
atom.
H
O
H
PUT ON THE 2 H-ATOMS
O
H
H
The molecular shape of H2O is V-shaped or bent.