Molecular Geometry and Valence
Bond / Molecular Orbital Theory
Chapter 10
Molecular Shapes
• Sugars and sweeteners taste sweet because
they fit into spots on the tongue that trigger
that response
• Artificial or ‘no calorie’ sweeteners fit in the
same place but do not get metabolized as
quickly or at all… meaning they pass through
fast and do not absorb
VSEPR Theory
• The Valence Shell Electron Pair Repulsion
(VSEPR) Theory
– States that electron pairs (defined as lone pairs or
single, double, triple bonds, and even single e-’s)
repel one another and want to push as far away as
possible
– Affects the shape
(molecular geometry)
Shapes
• The preferred shape of a molecule is the one
that allows maximum distance between
electrons (bonding and lone pairs)
• Depends on
1. How many come off the central atom(s)
2. How many lone pairs / bonding pairs
TWO electron Groups
• LINEAR: Maximizes separations by 180⁰
• Draw out BeCl2
• CO2
THREE electron Groups
• Trigonal planar geometry: maximum
separation is ~120⁰
• Draw BF3
• Draw CH2O
– Notice the
bond angles (ok
to round to 120⁰
FOUR electron Groups
• Tetrahedron geometry – maximum bond
angle is 109.5⁰
• Draw CH4
FIVE electron Groups
• Trigonal Bipyramidal Geometry
– 2 bond angles for maximum separation: 90 ⁰ & 120⁰
• Draw out PCl5
• “Axial chlorines” (top and bottom) vs the three
“equatorial Chlorines”
SIX electron Groups
• Six electron groups around a central atom
assume an octahedral geometry
• Draw (think about) SF6
• Maximum bond angle
is ALL 90⁰
Practice
• Determine the molecular geometry of NO3– Draw it out first
– Resonance ?
– How many bonding sites??
• Three… what does this tell about its shape?
Bonded vs Un-bonded electrons
• The previous shapes were all “bonded” electron
groups
• Think about what happens with un-bonded
electron pairs (lone electron pairs)
• VSEPR theory says these are also greatly (-) and
will push away and repel from one another in a
similar fashion
– Only they create different shapes because there is not
an atom to “see” on the lone pairs
• Nonbonding electrons
still push (very, very
negative)
• They still have repulsions
and according to the
VSEPR theory they will
migrate as far away as
possible to alleviate this
stress
REPULSIONS
Lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
Most Repulsive
Least Repulsive
• Think about how this can affect bond angles
• The more repulsive (the greater they push
away) the greater the bond angle
FOUR electron groups with
ONE LONE PAIR
• Think about ammonia
– There are 4 electron groups (3 bonded and 1 lone)
• So, push them apart just like a tetrahedron…
but there is nobody bonded on one side
• This ‘push’ creates a pyramidal shape
(trigonal pyramidal)
Trigonal Pyramidal
FOUR electron groups with
TWO LONE PAIRS
• Think about Water
– There are 4 electron groups (2 bonded and 2 lone)
• So, push them apart just like a tetrahedron…
but there is nobody bonded on two sides
• This ‘push’ creates a BENT shape
Bent Molecular Geometry
FIVE electron groups with
ONE LONE PAIR
• Draw out / Consider SF4
• In order to get these as far away as possible:
SEESAW
Geometry
FIVE electron groups with
TWO LONE PAIRS
• Draw out / Consider BrF3
• In order to get these as far away as possible:
T-SHAPED
Geometry
FIVE electron groups with
THREE LONE PAIRS
• Draw out / Consider XeF2
• In order to get these
as far away as possible:
LINEAR
Geometry
SIX electron groups with
ONE LONE PAIR
• Think about BrF5
– There are 6 electron groups (5 bonded and 1 lone)
• So, push them apart just like a octahedron… but
there is nobody bonded to one of the sides
• This ‘push’ creates a SQUARE PYRAMIDAL
MOLECULAR GEOMETRY
SIX electron groups with
TWO LONE PAIRS
• Think about XeF4
– There are 6 electron groups (4 bonded and 2 lone)
• So, push them apart just like a octahedron
(AGAIN)… but there is nobody bonded to TWO
of the sides
• This ‘push’ creates a SQUARE PLANAR
MOLECULAR GEOMETRY
Summary of VSEPR Theory
• The geometry of a molecule is determined by the
number of electron groups on the central atom(s)
• The number of e- groups can be determined from
the Lewis structure of the molecule. If the
molecule has resonance structures, use one of
them – doesn’t matter which one
• Each of the following counts: lone pair, single
bond, double bond, triple bond, or a single
electron
Summary (cont’d)
• The geometry of the e- groups is determined
by minimizing their repulsions:
Lone pair-lone pair > lone pair-bonding pair >
bonding pair-bonding pair
• Bond angles will vary, the presence of lone
pairs will usually make bond angles smaller
than the ideal angle of the particular
geometry
Page 414-415
Copy this table
On a piece of
Computer paper
(provided)
Review sheet /
Study guide /
Summary for
Molecular
geometry
Quick Practice
• Which of the following statements is always true
according to VSEPR theory?
a) The shape of a molecule is determined by
repulsions among bonding electron groups.
b) The shape of a molecule is determined by
repulsions among nonbonding electron groups
c) The shape of a molecule is determined by the
polarity of its bonds
d) The shape of a molecule is determined by
repulsions among all electron groups on the
central atom
VSEPR Theory: Predicting Shapes
1. Draw the Lewis structure
2. Determine the total number of electron
groups around the central atom
3. Determine the number of bonding groups
and the number of lone pairs around the
central atom
4. Use Table 10.1 to determine the electron
geometry and molecular geometry
Polarity
• As we discussed in chapter 9, bonds can be
polar (unequal sharing of electrons, EN)
• Entire molecules can be POLAR!!
Polarity
• Bonds are easy… just look at the differences in
electronegativity
• For molecules, there must be a ‘pull’ in
different directions, but not equal and
opposite
WE STILL NEED TO USE OUT TABLE
FROM CHAPTER 9!
Nonpolar Molecules
• Think about two kids playing tug-o-war…
– If they are equal in strength, there will be no net
dipole moment
– They cancel out
– Go nowhere
Polar Molecules
• Consider the same
two kids pulling… if
now instead, one kid
pulls up and one kid
pulls to the left…
there WILL be a net
movement (they will
not cancel out
In summary, to determine whether a
molecule is polar:
• Draw a Lewis structure, determine the
molecular geometry
• Determine whether the molecule contains
polar bonds
• Determine whether the polar bonds add
together to form a net dipole moment (add up
vectors) – what is a vector???
Vector Addition
• Refer to page 420 for VECTOR ADDITION
• One dimension = two kids pulling left and
right (left / left OR right/ right)
• Two dimensions = pulling up and to the right
(in different directions, off of the same plane)
Page 421:
Practice
• Determine whether NH3 is polar
1. Draw the Lewis Structure
2. Determine where the molecule has polar bonds
3. Determine whether the bonds add together to
form a net dipole moment
• The three dipole moments sum to a net
dipole moment. The molecule is POLAR!
PRACTICE #2
• Determine whether CF4 is polar.
Interactions
• Nonpolar and Polar molecules DO NOT MIX
– Think about water (polar) and oil (nonpolar)
– They will separate and keep their distance
• Polar molecules interact STRONGLY with other
polar molecules
– So when you mix P and NP together, the P will
group and bunch up while the NP stay away and
will not penetrate the group
Page 422
• Chemistry in your day
– How Soap Works
• Read through this excerpt and answer the
question in your notes
– Consider the detergent molecule below (in your
book). Which end do you think is polar? Which
end is nonpolar?
Hybridization
• Hybridization is a mathematical preocedure in
which the standard atomic orbitals are
combined to form new atomic orbitals called
Hybrid Orbitals
• It is essentially adding two orbitals together
SP3 hybridization  ___ ___ ___ ___
Pi and Sigma bonds
• A pi (π) bond is the overlap between the halffilled p orbitals overlap side-by-side
• A sigma (σ) bond is the overlap end-to-end
• A pi (π) bond is formed after the sigma bond
• A sigma (σ) bond is always the FIRST BOND
formed
https://www.youtube.com/watch?v=Y
cSPPKESpwc
• Short unit…
– Molecular shapes through the pi / sigma bonds