Transcript Molecular Geometry

VSEPR Theory
 Area
of chemistry that deals with the
shape of molecules.

Shape determines many properties about a
molecule:
Reactivity
 Smell
 Taste

 Lewis
Structures do not reveal anything
about the 3-D arrangement of atoms in a
molecules.
 Could
you have predicted the 3-D shape of
CCl4 from the Lewis structure on the left?
 Valence
Shell Electron Pair Repulsion Theory
States that molecules will take on shape that is
lowest in energy.
 A low energy shape is one that minimizes the
valence shell electron pair repulsion (VSEPR)
between adjacent atoms.
 Atoms in a molecule try to spread out from one
another as much as possible to reduce the
“like charge repulsion” between their outer
electrons.

H
H C H
 Methane,
CH4

You might think this is the farthest that the
hydrogens can get away from each other.

But if you think in 3 dimensions, this shape
actually causes less repulsion between the
bonding pairs of electrons.
H

1. Draw proper Lewis structure.

2. Count the # of bonded atoms and # of lone
pairs around the central atom.

3. Use CHART to determine name of shape.

4. Redraw shape with proper geometry.
 1.
Draw proper Lewis Structure.
Count the # of bonded atoms and # of
lone pairs around the central atom.
 2.
4 Bonded Atoms
 0 Lone Pairs

 3.
Use CHART to determine name of
shape.

Tetrahedral
 4.
Redraw shape with proper
geometry.
 1.
Draw proper Lewis Structure.
Count the # of bonded atoms and # of
lone pairs around the central atom.
 2.
3 Bonded Atoms
 1 Lone Pairs

 3.
Use CHART to determine name of
shape.

Trigonal Pyramidal
 4.
Redraw shape with proper
geometry.
 These
bonds (shared electrons) and lone
pairs of electrons are all negatively charged
so they want to spread out from each other
as much as possible within a molecule.

This minimizes “like charge repulsion” between
electrons and represents for the molecule.
 Lone
pairs take up more space than bonding
pairs and thus push atoms farther away from
the lone pairs, but closer to each other.

This decreases the predicted bond angles.