Chapter 7
Covalent Bonds and Molecular Architecture
Octet rule: Main group elements tend to undergo reactions
that leaves them with either 2 or 8 electrons in their outer
or valence shell achieved by sharing electrons.
Exceptions to the rule includes all elements that have d
levels close in energy. However, much of their chemistry
can still be explained by the octet rule.
It is important to realize that this is a model that can also
help us understand molecular structure.
Main Group
Elements
Covalent bonding and Lewis structures
If H would form a covalent bond, how many bonds would it
form?
Octet rule: Elements tend to undergo reactions to form covalent
bonds that leaves them with either 2 or 8 electrons in their outer or
valence shell.
Valence electrons are the electrons with the highest principle
quantum number.
molecular orbitals
atomic orbitals
H2 has an electronic environment similar to He
One way chemists have of indicating a sharing of two electrons is to
use a line to connect the two atoms, H2 is drawn H-H or H:H. These
are called Lewis structures.
If Cl would form a covalent bond with H, how many bonds would it
form?
chlorine is 1 electron short of a filled 3p level
hydrogen is 1 electron short of a filled 1s level
How many valence electrons does each atom posses?
(valence electrons are the electrons with the highest principle
quantum number)
How many electrons does H need to share to have an outer shell that
resembles an inert gas? Which inert gas?
How many does Cl need to share to have an outer shell that
resembles an inert gas? Which inert gas?
H
Cl
How many valence electrons does C posses?
4
If C forms covalent bonds, how many bonds would it form to satisfy the
octet rule? 4
If H would form a covalent bond with C, how many bonds would it
form?
H
H-C-H
H
Lewis structure
How many valence electrons does N posses?
5
If N forms covalent bonds with H, how many bonds would it form
to satisfy the octet rule?
3
If H would form a covalent bond with N, how many bonds would it
form?
1
H-N-H
H
How many valence electrons does O possess?
6
If O forms covalent bonds with H, how many bonds would it form to
satisfy the octet rule? 2
If H would form a covalent bond with O, how many bonds would it
form? 1
H
‫׀‬
:O:
‫׀‬
H
If B would form a covalent bond, how many bonds would it form?
If H would form a covalent bond with B, how many bonds would it
form?
How many valence electrons does each atom posses?
How many does B need to share to have an outer shell that resembles
an inert gas? Which inert gas?
Boron is a hopeless mess; much of its chemistry is very different
from other elements in that the only way it can obtain an octet of
electrons is by forming charged complexes; similarly with
aluminum
H
-1
H-B-H
‫׀‬
‫׀‬
Na+
H-B-H
H
‫׀‬
H
Geometry of molecules
Suppose two groups were attached at a single point in space, and
suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their repulsion
( how would they arrange themselves to minimize repulsion)?
Suppose two groups were attached at a single point in space, and
suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their repulsion (
how would they arrange themselves to minimize repulsion)?
Suppose three groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their repulsion?
Suppose three groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their repulsion?
.
Suppose four groups were attached at a single point in space,
and suppose these groups repelled each other, what
geometric arrangement would these group chose to
minimize their repulsion?
Suppose four groups were attached at a single point
in space, and suppose these groups repelled each other,
what geometric arrangement would these group chose
to minimize their repulsion?
.
Suppose four groups were attached at a single point
in space, and suppose these groups repelled each other,
what geometric arrangement would these group chose
to minimize their repulsion?
.
Suppose five groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their repulsion?
Suppose five groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their repulsion?
Suppose five groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their repulsion?
Suppose six groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their
repulsion?
Suppose six groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their
repulsion?
Suppose six groups were attached at a single point in space,
and suppose these groups repelled each other, what geometric
arrangement would these group chose to minimize their
repulsion?
octahedral geometry
http://www.jcrystal.com/steffenweber/POLYHEDRA/p_10.html
Let’s define a group as either an atom or a pair of valence electrons not
involved in bonding, ignore bonding electrons and electrons in inner shells
draw the Lewis structures, and predict the geometry of the following
molecules
H2O
What is the central atom?
O
How many valence electrons around O?
6
.
How many groups around the central atom?
4
What is the geometry of the molecule?
.
:O:
The geometry of a molecule is determined only by location of the
nuclei. The electrons can not be located because of the uncertainty
principle
CCl4; CH4
What is the central atom?
C
How many valence electrons around C?
4
What is the Lewis structure?
How many groups around the central atom?
What is the geometry of the molecule?
tetrahedral
4
NH3
What is the central atom?
N
5
How many valence electrons around N?
What is the Lewis structure?
How many groups around the central atom?
What is the geometry of the molecule?
4
pyramidal
Draw Lewis structures and predict the shape of the following
compounds:
1. SiCl4
2. CH5N
3. CH2O
4. C2H2Cl2
5. C3H4
PCl5
What is the central atom?
P
SF6
What is the central atom?
S
Remember that we can only locate the position of a heavy atom; the
position of electrons is not determined.
We have partially explained the geometry observed when atoms
combine to form molecules. The geometry of the molecule is
determined locally by the central atom. How do we identify central
atoms?
Central atoms are determined by the number of bonds needed to
complete the octet.
H, Halogens are seldom central atoms
B, C, N O in the first row
Al, Si, P, S in the second row …
Shapes of molecules
1. linear
http://intro.chem.okstate.edu/1314F00/Lecture/Chapter10/VSEPR.
html
2. trigonal planar
3. bent
4. tetrahedral
5. trigonal pyramidal
6. trigonal bipyramidal
7. seesaw
8. T shaped
9. octahedral
10. square pyramidal
11. square planar
What experimental evidense is there that CH4 is tetrahedral and not
square planar?
Consider CH2Cl2:
H
Cl
H
C
Cl
H
H
C
Cl
Cl
Are there two compounds with the formula CH2Cl2?
No
Consider CHBrClF
Are these the same?
Consider CHBrClF
Are these the same?
Consider CHBrClF
Are these the same?
You recall that we were able to explain atomic structure using s, p , d, f
orbitals. Can we explain the structure of molecules using these same
orbitals?
Remember the shape of these orbitals:
s: spherically symmetric
p: 3 orbitals each with two lobes touching each other at the nucleus
and oriented 90 ° to each other.
d: 5 orbitals with a more complicated structure.
d
s
p
Chemists like to think that the electrons are the glue that hold atoms
together. Therefore, can the structure of the molecules we just
described, for example, CH4, PCl5, be explained using the hydrogen
atom atomic orbitals just shown?
Schroedinger Equation is a differential equation. :
Properties of a differential equation:
1.
the equation may have more than one solution.
2.
any combination of solutions (sum or difference) is also a
solution
2s = 1/4(1/2a3).5(2-r/a)(2.718)r/2a
2p = 1/4(1/2a3).5(r/a)(2.718)r/2acos 
…
Linus Pauling: hybridization of atomic orbitals
What were to happen if we combined ½ of a 2s orbital with one of
the ½ 2p orbitals mathematically?
These hybrid orbitals are
directional, pointing 180°
2s
2p
away from each other and are
called sp hybrid orbitals
a 2s + b 2p
+
+
+
a 2s - b 2p
-
+
+
-
+
Combining a 2s orbital with 2 2p
orbital can result in 3 sp2 hybrid
orbital that point at 120 ° to each
other
Combining a 2s orbital with 2 2p orbital results in 3 sp2
hybrid orbital that point at 120 ° to each other; note that one p
orbital remains unchanged by these mathematics.
A summary of the types of hybridization necessary to product
maximum electron density in the necessary direction as dictated by
experimental geometries
sp hybridization:
2 orbitals pointing 180 ° to each other;
2 atomic p orbitals remain unchanged
sp2 hybridization:
3 orbitals pointing 120 ° to each other;
1 atomic p orbitals remains unchanged
sp3 hybridization:
4 orbitals pointing to the corners of a
regular tetrahedron; all atomic p orbitals used
dsp2 hybridization:
4 orbitals pointing to the corners of a
square; 4 d orbitals, 1 p orbital unchanged
dsp3 hybridization:
5 orbitals pointing to the corners of a
trigonal bipramid; 4 d orbitals unchanged
d2sp3 hybridization:
6 orbitals pointing to the corners of a
octahedron; 3d orbitals unchanged
sp2
d2sp3
Shapes of the hybrid
orbitals
sp3
Draw the Lewis structure of C2H4 so that every carbon has a
filled octet and each hydrogen has a He configuration
How many groups around each carbon?
3
What is the geometry at each carbon?
trigonal
Atomic p orbitals on each C
sp2 hybrid orbitals
Draw the Lewis structure of C2H2 so that every carbon has
a filled octet and each hydrogen has a He configuration
How many groups around each carbon?
2
What is the geometry about each carbon?
digonal: 180 °
H-CC-H
Bond lengths
C-C 1.54 *10-10 m
C=C 1.34*10-10 m
CC 1.2* 10-10 m
How do we explain the formation of CH4 using the electronic
configuration of C
Valence electrons of C
_____
____ 2s
promote 1 electron: requires
investment of energy
add electrons from H: allows the
formation of 4 bonds instead of 2
and satisfies the octet rule
hybridize
_____ _____2p
How do we explain the structure of molecules such as SF4?
3d
____
_____ _____ ______ ____
3p ___ ____ ____
promote an electron
3s ____
3d ____
3p ___ ____ ____
3s ____
_____ _____ ______ ____
hybridize
____ ___
____
3d _____ _____ ______ ____
____ _____ dsp3
add electron from F
How do we explain the structure of molecules such as SF6?
3d
3p ___
3s ____
____
_____ _____ ______ ____
____
promote 2 electrons
3d
3p ___
3s ____
____
____
____
_____ _____ ______ ____
____
hybridize
add electron from F
3d _____ _____ ______
____ ___ ____ ____ _____ _____
How good are Lewis structures at explaining molecular properties?
For compounds of carbon: excellent very few exceptions
For other elements: very good, some exceptions
Consider the Lewis structures of the diatomic molecules of the
elements
H2,
O2
N2
F2
Any exceptions?
How good is this model at explaining molecular properties?
For compounds of carbon: excellent very few exceptions
For other elements: very good, some exceptions
Consider the Lewis structures of the diatomic molecules of the
elements
H2,
O2
N2
F2
Any exceptions? O2 is paramagnetic
Molecular orbital Theory
Basic tenets: molecules are formed by combining atomic orbitals
on each atom that have the proper orientation.
Whenever two atomic orbitals combine to form a molecular orbital,
one combination is obtained by mathemetically adding the two
together; this orbital goes down in energy relative to the atomic
level. The other, obtained by the mathematical difference between
the two orbitals, goes up in energy.
If a level goes down in energy relative to the atomic level, it is
referred to as a bonding molecular orbital
If a level goes up in energy relative to the atomic level, it is called
an antibonding level
If a level is not affected relative to the atomic level it is called a
non-bonding orbital
H2
This model predicts that H2- should have some stability; the total
energy of He2 is the same as two isolated He atoms so nothing
keeps the molecule together and it falls apart in two He atoms.
O2
F2
N2
Draw the Lewis structure of ozone: O3
: :
Draw the Lewis structure of ozone
-
:
:O:
:O: :O:
+
Resonance structures: two structures that are identical except for
the location of the electrons
Symbol chemists use to denote resonance structures
H
H
H
H
C
H
H
C
H
C
C
C
C
C
C
C
C
C
H
H
H
C
H
H
Draw the structure of sulfuric acid; H2SO4
Draw the structure of the sulfate ion; SO4-2
Why do covalent bonds form?
Covalent bonds: the sharing of electrons
Electronegativity: generally meant to identify the unevenness in
sharing electrons in a covalent bond