Geometry of Molecules
Significance?
Enzymes (metabolic reactions)
Drugs
Eyesight
Sense of smell
VSEPR
 Valence Shell Electron Pair Repulsion
 Based on the notion that electron groups will orient
themselves as far away from one another as possible,
making bond angles as large as possible.
Structure of water
Water has two lone pairs.
This causes a compression of
the bond angle to 105.
Water’s bonds are polar and
is asymmetrical in shape so
the bond polarities do not
cancel. This also makes the water
molecule polar, giving it a
net positive charge on one side
and a net negative charge on
the other.
The Unusual Properties of
Water
 Unusually
high
boiling
point
 Compared
to other
compounds
in Group 16
Two Types of Geometries
 Electron-Pair Geometry – based on the arrangement of
electron groups around the central atom. (multiple
bonds treated as one group)
 Molecular Geometry – based on the arrangement of
atoms bonded to the central atom. (must differentiate
between bonding and nonbonding electron groups)
 Molecular shape determined by bond angles.
 Molecular size determined by bond length
Electron-Pair Geometries and
Approximate Bond Angles
Nonbonding or “lone” pairs spread out more and
can cause bond angles to compress. In ammonia,
NH3, the electron-pair geometry predicts a bond angle
of 109.5 but the lone pair causes a slight compression
of the angle to 107.
Linear (straight line)
Ball and stick
model
Space filling
model
EX. CO2
Bent
Ball and stick
model
Space filling
model
EX. H2O
Trigonal pyramid
Ball and stick
model
EX. NH3
Space filling
model
Tetrahedral
Ball and stick
model
Ex. CH4
Space filling
model
Molecular Polarity
 Molecules may be polar or nonpolar.
 Molecules that are polar are said to have a dipole or
possess a dipole moment.
 Dipole Moment is a value that indicates the degree of
polarity of the molecule.
Carbon dioxide is nonpolar
Water is polar
Intermolecular attractions
 Attractions between
molecules
 van der Waals forces

Weak attractive
forces between
non-polar
molecules
 Hydrogen “bonding”

Strong attraction
between special
polar molecules
Intermolecular Forces of Attraction
Attraction between a hydrogen atom on one molecule and a very
electronegative atom on an adjacent molecule. Only occurs when
hydrogen is bonded to F, O, or N. (HF, H2O, NH3)
Force of a attraction between nonpolar molecules caused by
the motions of electrons and instantaneous dipoles.
Forces of attraction between polar molecules and ions.
Forces of attraction between polar molecules.
Dipole-dipole
Ion-dipole
Ion-dipole

Molecules will be polar if:
1. There are polar bonds in the molecule.
2. The molecule is not symmetrical.*
*If the molecule is symmetrical, the bond polarities will
cancel out leaving no net dipole in the molecule. In
this case the molecule is nonpolar or does not
possess a dipole moment.
Determining type of IFA
Hybridization
Occurs when atomic orbitals mix
to form a particular bonding set
around a central atom.
2p2
Hybrid orbitals
energy
H
2s2
H
C
H
methane
H
1s2
In methane the carbon is sp3 hybrid.
Hybridization in Methane
‘sp’ Hybrid Orbitals
Sigma bonds/overlap
Sigma overlap occurs on the internuclear axis and between the two bonded atoms.
All single bonds are sigma bonds.
pi bonds
Pi bonds occur when the overlap is on opposite sides of the internuclear axis.
Only regular ‘p’ orbitals can form pi bonds.
Double bonds are 1 sigma and 1 pi bond
Triple bonds are 1 sigma and 2 pi bonds
Double bond = one sigma and one pi bond
Triple Bond
One sigma and two pi bonds
How many sigma and pi bonds are in these molecules?
2-propanol
Alpha amino acid
Do you know?
1. Are the bonds ionic or covalent in the following?
(a) KCl (b) ZnS (c) PCl3 (d) Na2O (e) HCl
2. Which of the following bonds would have the most
ionic character?(a) F-F (b) F-O (c) F-N (d) F-C
3. How many valence electrons each of the following
have? (a) C (b) Si (c) Mg (d) P
4. Which of the following has the greatest lattice energy?
NaF or KCl
Why?
5. Draw the Lewis structure for SO32-. What is its e—pair
geometry, molecular geometry, bond angles, and
hybridization of the central atom?