Molecular Geometry
• Bond length: the distance between two
atoms held together by a chemical bond
– Bond length decreases as the number of
bonds between two atoms increases.
• Single bond is the longest.
• Triple bond is the shortest.
2
Molecular Geometry
• Bond angle: the angle made by the
“lines” joining the nuclei of the atoms in a
molecule
O
H
H
104.5o
3
Who cares about molecular
shape?
The shape of a molecule plays a very important
role in determining its properties.
Properties such as smell, taste, and proper
targeting (of drugs) are all the result of
molecular shape.
Molecular Shape
Lewis structures don’t show us how atoms in a
molecule are arranged in 3-dimensional space.
Could you have predicted the arrangement of atoms on
the right from just seeing it’s Lewis structure?
So how do we find the
shape of a molecule?
A useful model for predicting the shape of
molecules is the…
Main Premise of
VSEPR Theory
Molecules will adopt a 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
methane, CH4
H C H
You might think this is the
farthest that the hydrogens
can get away from each other
H
109.5°
But if you think in 3 dimensions, this shape
actually causes less repulsion between the
bonding pairs of electrons
90°
The 5 Main Shapes:
•Molecules adopt a geometry (shape)
that minimizes e – e repulsions g this
occurs when e- pairs are as far apart
as possible.
Here are the STEPS for
determining molecular
geometry:
• 1. Draw a Lewis structure
• 2. Count the # of bonds and # of lone pairs
around the central atom (these are called
“domains”)
– Single, Double and Triple bonds count as ONE
domain
– Each lone pair counts as ONE domain
• 3. Use CHART to determine shape
– Name of molecule shape is based on position of
atoms (not domains)
What the heck is an electron
domain?
• It’s a region in space where electrons are
likely to be
– So it’s an area of high electron “concentration”
– A domain is wherever there is a bond or a
lone pair around an atom
• Bonds = Bonding Domains
• Lone Pairs = Nonbonding Domains
How many “domains” around
each atom?
Remember the BIG PICTURE?
• These “domains” 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 the lowest
possible energy state for the molecule.
Geometries
Let’s take a look at some
molecular shapes…
VSEPR Example
VSEPR Example
VSEPR Example
NOTE:
• Lone pairs take up more space than
bonding pairs and thus push atoms farther
away from each other (decrease the
predicted bond angles).
• Let’s take a look at this phenomena…
H
H C H
methane, CH4
H
109.5°
less repulsion between the
bonding pairs of electrons
90°
H
H C H
..
H N H
ammonia
H
H
NH3
You might predict the planar shape
in the middle, but the lone pair
forces the hydrogens downward
H
H C H
..
H N H
H
H
109.5°
107°
..
H O
.. H
water, H2O
104.5°
For tetrahedral earrangements:
# lone
pairs
0
1
2
Bond
angle
109.5
107
104.5
CH4
NH3
H2O
Example
Molecular Geometry
• Many of the molecules we have discussed have
central atoms surrounded by 2 or more identical
atoms:
ABn
where A = central atom
B = outer atoms
n = # of “B” atoms
Examples: CO2, H2O, BF3, NH3, CCl4
26
H
..
H C H
H N H
H
H
..
H O
.. H
H
C H
H
H
N H
H
H
H
O
H
Now, Let’s Combine VSEPR
with Polarity…..
Review
• If two atoms share electrons, but don’t
share them equally, we call it a polar
covalent bond
– Electrons are pulled closer to the more
electronegative atom
– One end of the molecule develops a partial
negative charge because it has a higher
electron density
Dipole Moments
• A molecule such as H-Cl that has an area of
partial positive charge and an area of partial
negative charge is said to have a “dipole
moment”
• The dipolar character is represented by an
arrow pointing toward the (-) end of the
molecule
Dipole Moments
• ALL diatomic molecules with a polar bond have
a dipole moment
– Ex. H-Cl
• SOME polyatomic molecules with polar bonds
have a dipole moment
– Ex. water (does); carbon dioxide (does NOT)
– It depends on the molecular geometry!!!
Water is Polar
• It has polar H-O bonds
• Its geometry is “bent”
• So the molecule is polar
– It has a dipole
BF3 is Non-polar
• It has polar B-F bonds
• Its geometry is “trigonal planar”
• So the molecule is non-polar
– It does NOT have a dipole
– All the fluorines “cancel”
Polarity of Molecules
• Examples:
35
A parting thought…
• “Like dissolves Like”
– Polar molecules dissolve in polar
solvents
– Nonpolar molecules dissolve in nonpolar
solvents
• Polar and nonpolar solutions will not mix
– Ex. Water (polar) and Oil (nonpolar)
Hydrogen bonds
• Attractions between water molecules
• Hydrogen bonding always involves
hydrogen
• Affects water
Hydrogen bonds
• Yes, hydrogen bonds are the strongest of
the intermolecular forces
• Properties of water:
– Pulls water into droplets
– High boiling point
– Surface tension
Network Solid
• Solids in which all the atoms are
covalently bonded together.
• Very strong bonds
• Very high melting points
• Diamond is an example
Hybridization
•
•
•
•
•
When s and p orbitals overlap
Types of bonds in sp orbitals
sp = triple bond
sp2 = double bond
sp3 = single bond
Pi () Bonds
• Pi bonds are
characterized by
– Side-to-side overlap.
– Electron density
above and below the
internuclear axis.