COVALENT BONDING
Lewis Dot
Diagram
Valence Bond Theory
Electrons in a covalent bond reside in a
region that is the
overlap of individual atomic orbitals.
For example, the covalent bond in molecular
hydrogen can be thought of as the overlap of
two hydrogen 1s orbitals.
Problem with VB Theory
What is the predicted bond H-O-H angle in water?
HYBRID ORBITALS
To account for the shape (and resulting
behavior) of molecules, chemists have
devised hybrid orbitals.
If you recall, s, p, d, and f orbitals exist in
atoms. However, when atoms form covalent
bonds, some of these orbitals “merge” or
“fuse” to form hybrid orbitals.
They have undergone hybridization.
http://www.youtube.com/watch?v=SJdllffWUqg
Carbon Atom in CH4
The four orbitals
(2s, 2px, 2py, and 2pz)
merge to form four sp3
orbitals of equal energy.
VSEPR THEORY
Lewis dot diagrams gives us a good ideas about
which atoms are connected in a molecule.
However, because it is a two dimensional
representation of a three dimensional molecule,
it has limitations.
It does not give us molecular geometry, which is
the spatial arrangement of the atoms.
The Valence Shell Electron Pair Repulsion
(VSEPR) model, is used to explain molecular
geometry.
VSEPR Theory states that electron pairs (shared
or lone) tend to arrange themselves around an
atom in such a way that repulsion between pairs
is minimized.
In other words, electron groups (domains)
spread out from each other as much as possible.
sp3 Tetrahedron or tetrahedral
Bond angle
between any two
sp3 orbital is
109.5 degrees.
Common
examples: CH4,
NH3, H2O
Lone Pairs Effect on
Bond Angles
109.5º
107º
104.5º
Why would a lone pair occupy more
space than a shared pair?
3
sp hybridization
The four sp3 orbital repel each other as much as
possible and form the “tetrahedral” shape.
sp3 Shape of Molecules
Lone pairs play a vital
role in electron
repulsion.
However, lone pairs
are "invisible" as far
as the geometry of
the atom is
concerned.
Only bonded atoms are considered when naming the shape
of the molecule.
http://www.youtube.com/watch?v=oDotFuloZTg&feature=related
Shapes associated with
3
sp
Tetrahedron
Bent
Trigonal
pyramid
Not Quite the Octet Rule
sp2 hybridization
Shapes associated with
Trigonal (triangular)
planar
Bent
2
sp
The unchanged porbital is capable of
making a pi bond.
Results in the
trigonal planar
arrangement.
Bond angle is
120 degrees.
Common
examples:
BF3, CH2O
Even Less than Not Quite the
Octet Rule
sp hybridization
Beyond the Octet Rule
Elements with n values equal to or greater than 3, can
have more than 8 electrons around it.
3
sp d
hybridization
Bond angles Associated with
sp3d
Shapes associated with
Trigonal
bipyramidal
3
sp d
Linear
T-shaped
See-saw
3
2
sp d hybridization
Shapes associated with
Octahedral
3
2
sp d
Linear
Square planar
T-shaped
Square pyramidal
Ethyne (Acetylene) – C2H2
The 2s and one of the 2p orbital merge to form the sp
hybrid orbitals – 180 degree bond angle.
The two remain 2p orbitals from each carbon atom
merge together to create the double and triple bond.
Multiple Bonds:
Sigma vs Pi Bonds
Single bonds (the first
bonds that form) are
always sigma (σ)π.
The second or third bonds that form in
double or triple bonds are always pi (π).
Bond Length
becomes shorter with
each pi bond.
Bond Strength
increases with each
pi bond.
The molecule
becomes more rigid
with each pi bond.
Ethene C2H4
What is the
hybridization of each
carbon atom?
What is the bond
angle?
Impact of Multiple Bonds on
Bond Angles
122º
116º
122º
122º
122º
116º
Why would multiple bonds occupy
more space than a single bond?
Ethane – sp3
Ethene – sp2
Ethyne – sp
REVIEW
http://cost.georgiasouthern.edu/chemistry/general/molecule/quiz/frame4b.htm
NITRATE ION
Draw the Lewis Dot Structure of the NITRATE ION (NO3-1).
If the above Lewis structure for nitrate were correct,
the nitrate ion would have one bond that is shorter
and stronger than the other two.
UPON CLOSER EXAMINATION…
Laboratory analyses show all three of the bonds in the
nitrate ion to be the same strength and the same length.
The behavior of the bonds suggests they are longer than
double bonds and shorter than single bonds. They are
also stronger than single bonds but not as strong as
double bonds.
HOW DO WE EXPLAIN THIS PHENOMENON?
RESONANCE
The double bond can occur between any oxygen
atom and the central nitrogen atom.
Each of these structures is called a
resonance structure
NITRATE ION
The nitrate ion is not really changing from one resonance
structure to another as previously thought.
The ion behaves as if it were a blend of the three
resonance structures.
OTHER RESONANCE STRUCTURE
Carbonate ion (CO3-2)
Thiocyanate ion (SCN-)
THE MOST FAMOUS RESONANCE
STRUCUTRE
BENZENE – C6H6
or
or
DELOCALIZED ELECTRONS
CARBON NANOTUBES