Molecular Geometry

1
Molecular Geometry
The shape of molecules
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.
2
Lewis structures tell us nothing about how atoms
in a molecule are arranged in 3-dimensional space.
Could you have predicted the molecular geometry
of carbon tetrachloride from its Lewis structure?
3
A useful model for predicting the shape of
molecules is the
VSEPR Theory
Valence Shell Electron Pair Repulsion 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 (electrons in bonds and in
  lone pairs repel each other).

4
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.
5
The 5 Main Shapes
Linear 180
Trigonal planar 120
Tetrahedral 109.5
Octahedral 90, 180
Trigonal bipyramidal 120, 180
Molecules adopt a geometry that minimizes
electron-electron repulsions ? this occurs when
e- pairs are as far apart as possible.
6
Steps to determining molecular geometry

• Draw a Lewis structure
• Count the of bonds and of lone pairs around
  the central domain (electronic domains)
• -Single, double and triple bonds count as ONE
  domain
• -Each lone pair counts as ONE domain
• Use AXE chart to determine shape
• (the name of the molecular geometry is based on
  position of the atoms, not on the domains)

7
VSEPR Notation

• Also known as AXE notation
• A central atom
• X atoms bonded to the central atom
• E of lone pairs on central atom

Examples
CH4 is
AX4
NH3 is
AX3E
H2O is
AX2E2
8
Lets look at a few examples
9
Trigonal planar
bent
bent
Trigonal pyramidal
10
Note Lone pairs take up more space than bonding
pairs and thus decrease the predicted bond angle
CH4 Tetrahedral, 109.5
NH3
Trigonal pyramidal, 107
the bond angle has been reduced by the one lone
pair
H2O
Bent, 104.5
the bond angle has been reduced by the two lone
pairs
11
Hybrid Orbitals atomic orbitals formed by
blending different orbitals together
Just like with other hybrids, the characteristics
of the hybrid orbitals will depend upon the
traits of the parent orbitals.
12
Consider carbon
How many valence electrons does carbon have?
4
According to its electron dot structure, how many
unpaired electrons does carbon have?
What does carbons orbital filling diagram look
like?
Atomic carbon only has 2 upaired electrons!
In order to form chemical bonds, carbons atomic
orbitals must hybridize to form molecular
orbitals.
13

two sp hybrid orbitals (AX2)

three sp2 hybrid orbitals (AX3)

four sp3 hybrid orbitals (AX4)
14
Sigma bonding
When two atomic orbitals (hybridized or not)
overlap end-on, they form a single sigma bond.

s bond

2
sp3
AX2E2
15
Pi bonding
When two atomic orbitals overlap in a
side-to-side fashion, they form a pi bond.
Orbitals that form pi bonds are usually NOT
hybridized.
p bond

• One unhybridized p orbital results in one pi
  bond.
• A sigma and a pi bond form a double bond.

O
O
s bond
16
Multiple pi bonds
p bond
2
s
N
N
s bond
17
Resonance Structures
equivalent Lewis dot structures
Draw the stick structure for CO32-
AX3 so sp2 hybrid.
O
C
O
O
Draw the resonance structures for SO42-
AX4 so sp3 hybrid.
18
Effect of resonance structures
C6H6
Unhybridized p-orbitals
Localized p bonds
Delocalized p bonds
19
Polarity in bonding

• In covalent bonds, electrons arent always shared
  equally between the two nuclei.

• This is because some elements have a greater
  affinity for electrons than others.

Flashback!
Electronegativity Ability of an atom to attract
electrons when in a molecule.
20
(No Transcript)
21
Electronegativity differences greater than 1.7
typically results in the formation of an ionic
bond.
22
When electrons in a covalent bond are
concentrated near one of the nuclei, the bond is
POLAR.
?
?-
If the electrons are shared equally between the
nuclei, the bond is NONPOLAR.
23
Both bonds in water are polar. How does this
affect the polarity of the molecule?
O
H
H

• Use arrows along the bonds to indicate the
  direction of the pull on the electrons in the
  bond (the dipole moment).
• A plus-sign on the tail of the arrow shows that
  the bond is more positive at that end

• Add the two vectors (the arrows) together to find
  the net polarity (dipole moment) of the molecule

Because of the bent structure of water, water is
a polar molecule.
24
Cl
All of the bonds in CCl4 are polar. Is this
molecule polar?
Cl
Cl
Cl
Since the molecule is symmetrical, all of the
dipoles along the bonds cancel each other
out. CCl4 is a nonpolar molecule and has no net
dipole moment.
25
Are these planar molecules polar or nonpolar?
nonpolar
polar
nonpolar
Are these linear molecules polar or nonpolar?
polar
nonpolar
polar