Molecular Geometry
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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.