Structure of molecules
Objectives:
•To understand molecular structure and bond angles
•To learn to predict molecular geometry from the number of electron pairs
•To learn to apply the VSEPR model to molecules with double bonds
3-D Molecular
Structure
• Lewis structures =
arrangement of valence e• Also want to talk about the 3-D
geometric structure of
molecules
• Example: H2O
• “Bent” shape with 104.5° angle
between the two hydrogens
3-D Molecular
Structure
• Linear structure
• All atoms in a line
• 180° angle between the oxygen
atoms
3-D Molecular
Structure
• Trigonal planar
• Example: BF3
• Flat (planar) with 120° angles
between fluorine atoms
3-D Molecular
Structure
• Tetrahedral
• Example: CH4
• Tetrahedron: A four-sided
pyramid
• 109.5° angles between Hs in
methane
3-D VSEPR Model
• Molecular shape determines molecular
properties!!!!!!!
• Valence Shell Electron Pair Repulsion
(VSEPR) Model allows us to make
predictions about molecular shape
• Pairs of electrons surrounding an atom
repel each other
• Atoms in molecule are positioned to
minimize repulsion (stable state)
• Bonding e-s and lone pairs are positioned
as far apart as possible
VSEPR: 2 pairs of
electrons
• BeCl2
• First, Lewis diagram:
• Be (2 valence e-) + 2 Chlorine (7
valence e-) = 16 valence e• Beryllium is exception to octet rule
- often forms “electron deficient
molecules”
• Cl - Be - Cl
• Arrange remaining e- (16-4 = 12 e-)
around the Chlorines
VSEPR: 2 pairs of
electrons
• BeCl2 has two pairs of electrons
surrounding it
• What is best way to arrange two
pairs of electrons to minimize
repulsions?
• Put electron pairs 180° degrees apart
• Now we can specify 3-D molecular
structure: linear
• Whenever you have 2 pairs of
electrons surrounding an atom,
they should always be placed at
180°
VSEPR: 3 pairs of
electrons
• BF3 - Boron trifluoride
• Doesn’t follow octet rule
• Boron tends to form “electron
deficient” configurations
• Boron (3 valence e-) + 3 Fluorine
(7 valence e-) = 24 valence e• Boron has 3 e- pairs around it
• Distribute remaining e- (24-6= 18
e-) around the fluorines
VSEPR: 3 pairs of
electrons
• BF3 - Boron trifluoride
• How to minimize repulsion
between 3 pairs of electrons?
• 120° angles between electron
pairs
• Trigonal planar structure
• Whenever 3 pairs of e- around
atom, should always be placed at
corners of triangle, with 120°
angle between...
VSEPR: 4 pairs of
electrons
• CH4 - Methane
• Lewis Diagram
• Carbon (4 valence e-) + 4
Hydrogen (1 valence e-) = 8
valence e• Follows duet (hydrogen) and
octet (carbon) rules
VSEPR: 4 pairs of
electrons
• CH4 - Methane
• How to minimize repulsion
between 4 pairs of electrons?
• 109.5° angles between electron pairs
• Tetrahedral shape (like a square
pyramid)
• Whenever 4 pairs of e- around atom,
should always be placed at 4 corners
of tetrahedron, with 109.5° angle
between...
• Whenever you have 4 pairs of earound atom: tetrahedral geometry
Try it with Ammonia
• NH3 - Ammonia
1. Draw Lewis structure
★H - N - H
|
H
★Nitrogen (5 valence e-) + 3 Hydrogen (1 valence e-) = 8 valence e(6 already accounted for by lines)
★Follow octet and duet rules
★
..
H-N-H
|
H
Try it with Ammonia
•
..
H-N-H
|
H
• How many electron pairs?
• 4
• How to reduce repulsions?
• Tetrahedral geometry
• Angles?
• 109.5° - but angle between hydrogens actually
smaller - lone pair more repulsive than bonded
pair
• We call this TRIGONAL PYRAMIDAL
Steps for predicting
molecular geometry with
VSEPR
1.Draw Lewis structure for molecule
2.Count electron pairs and arrange them in a way that minimizes
repulsions: put them as far apart as possible
3.Determine positions of atoms from the way the electron pairs are
shared
4.Determine name of molecular structure (linear, trigonal planar,
tetrahedral, bent, trigonal pyramid)
VSEPR Model of Water
• First - Lewis structure
• H-O-H
• Oxygen (6 valence e-) + 2 Hydrogen (2 valence e-) = 8 valence e- (4
accounted for by lines)
• Follow octet and duet rules
•
..
H-O-H
··
VSEPR Model of
Water
•
..
H-O-H
··
• How many electron pairs?
• 4
• How to reduce repulsions?
• tetrahedral geometry
• Angles?
• 109.5° angles (actually less, since lone pairs
repulse more than bonded pairs)
• We call this BENT shape
VSEPR and
Molecules with Double
Bonds
• Carbon Dioxide
• 2 electron pairs: linear structure
(180° bond angle)
• We know that there are 4 e- pairs
around carbon, but not
tetrahedral
• 2 pairs of electrons between
Carbon and each Oxygen
• Each double bond should be
treated as single bond for
geometry purposes
Hybrid Orbitals
• If an s and a p orbital overlap,
what happens?????
• You get a linear sp orbital - a
hybrid combination of the s
and p orbitals
• This is the type of hybridization
you get with linear molecular
geometry!
Hybrid Orbitals
• If an s and 2 p orbitals
overlap, what happens?????
• You get a trigonal sp2 orbital a hybrid combination of the s
and 2 p orbitals
• This is the type of hybridization
you get with trigonal
molecular geometry!
Hybrid Orbitals
• If an s and 3 p orbitals
overlap, what happens?????
• You get a tetrahedral sp3
orbital - a hybrid combination
of the s and 3 p orbitals
• This is the type of hybridization
you get with tetrahedral
molecular geometry!
Guide to hybrid orbitals:
Geometry
Hybridization
Example
linear
sp
BeH2, HCN
trigonal
sp2
BF3
tetrahedral
sp3
CH4, H2O
Questions
• What is hybridization of CH4?
• Procedure - count regions of
electron density around the
atom and match this number
with the corresponding
hybridization