Download The Theories: Molecular Geometry

The Theories: Molecular
Geometry
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Atoms oriented in very well defined relative positions in the molecule.
Molecular Geometry = general shape of the molecule as determined
by the relative positions of the atomic nuclei.
Theories Describing the structure and bonding of molecules are:
– VSEPR = considers mostly electrostatics in determining the
geometry of the molecule.
– Valence Bond Theory = considers quantum mechanics and
hybridization of atomic orbitals.
– Molecular Orbital Theory = claims that upon bond formation new
orbitals that are linear combinations of the atomic orbitals are
formed.
1. What does VSEPR stand for?
2. What does hybridization mean?
Geometrical Shapes
5 charge clouds,
trigonal bipyramidal
3 charge clouds,
trigonal planar
2 charge clouds,
linear
4 charge clouds,
tetrahedral
6 charge clouds,
Octahedral
3.What is the difference between bonding and
lone electron pairs? 4. How do you find out
how many electrons are in a molecule?
5.Use your text,
add two more
examples of
each type of
molecular
geometry.
6.Justify why PCl5
has a different
geometry than
IF5.
How to predict?
• Lewis dot structure determines the total # of electrons
around the central atom. Multiple bonds (double and
triple) count as one.
• The number of bonding and nonbonding electron
pairs determines the geometry of electron pairs and
the molecular geometry.
• Lone e Pairs affect geometry more than bonding
pairs. 7. WHY?
• Multiple bonds have larger affect on geometry than
single bonds. 8. WHY?
Polarity of Molecules
• Bond dipole a positive charge next to a negative charge.
• Dipole moment,  the magnitude of the net bond dipole of a
molecule  = Qxr Q = the net charge separation; r = the
separation distance. Units: debyes (D) where 1 D = 33.36x1030
Cm.
• A polar bond forms when two atoms of between two atoms
involved in a bond have significantly different electronegativities.
– Most electronegative substance will have a slight negative
charge (represented as )
– The positive (electron poor) side of the bond is represented
as + or
–  points in direction of the negative charge.
• Net polarity (dipole moment) of a molecule is obtained using the
vector sum of polarities of individual bonds.
9.Determine if NH3, H2O, CO2 have dipole moments.