What’s coming up???
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Oct 25
Oct 27
Oct 29
Nov 1
Nov 3,5
Nov 8,10
Nov 12
Nov 15
Nov 17
The atmosphere, part 1
Midterm … No lecture
The atmosphere, part 2
Light, blackbodies, Bohr
Postulates of QM, p-in-a-box
Hydrogen and multi – e atoms
Multi-electron atoms
Periodic properties
Periodic properties
Ch. 8
Ch. 8
Ch. 9
Ch. 9
Ch. 9
Ch.9,10
Ch. 10
Ch. 10
• Nov 19
Valence-bond; Lewis structures Ch. 11
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VSEPR
Hybrid orbitals; VSEPR
MO theory
MO theory
bonding wrapup
Review for exam
Nov 22
Nov 24
Nov 26
Nov 29
Dec 1
Dec 2
Ch. 11
Ch. 11, 12
Ch. 12
Ch. 12
Ch. 11,12
400
Cl
F
-ELECTRON AFFINITY
300
S
200
C
H
100
Si
O
Li
Na
B
0
-100
0
1
2
He
3
4
Be
Al
5
6
7
8
9
10
11
N
12
13
14
P
15
16
17
18
Mg
Ne
-200
ATOMIC NUMBER
Ar
19
THE OCTET RULE
Atoms combine to form compounds in an
attempt to obtain a stable noble gas electron
configuration with 8 electrons in the valence
shell
A stable electronic configuration can be attained
in two ways…….
ELECTRON TRANSFER
ELECTRON SHARING
IONIC BONDING
COVALENT BONDING
LEWIS MODEL OF BONDING
IONIC BONDING
electron transfer
Cl
Na
1s22s22p63s23p5
1s22s22p6 3s1
Lewis Symbol
Na
Cl
symbol for element
and a dot for each
valence electron

Na+
1s22s22p6
Cl
1s22s22p63s23p6
COVALENT BONDING
electron sharing
Atoms go as far as possible toward
completing their octets by
sharing electron pairs
Consider F2
F
F
The electronic configuration of F is 1s22s22p5
Lewis Symbols…..
F
F
These are combined to form F2
COVALENT BONDING
electron sharing
Atoms go as far as possible toward
completing their octets by
F 1s22s22p5 +
F
+
F
bonding
pair of
electrons
sharing electron pairs
F 1s22s22p5
F
F
or
F
F
non-bonding, or
lone pair of
electrons
The simplest molecule
IONIC OR COVALENT?
The type of bond that forms depends on the
electronegativity difference between the two
atoms involved in the bond
IONIC VERSUS COVALENT
BONDS
Compounds composed of elements with a large
difference in
ELECTRONEGATIVITY tend to have
significant ionic character in their bonding
B has a greater share
A
B
HYDROGEN FLUORIDE
Fluorine is more electronegative than hydrogen.
+
H
+
F
H
F
LEWIS BONDING MODEL
Lewis structures are based on a
localized electron model
Electrons are always localized in one of two ways:
As lone pairs on a specific atom
OR
As bonding pairs between two specific atoms
Drawing Lewis Structures
Lewis structures consider only valence electrons
Water
H2O
H
bonding pair of
electrons
O
H
non-bonding, or lone pair
of electrons
How do we draw these?????
Building Lewis (ELECTRON DOT)
Structures of Molecules.
HCN as an example...
Step 1. Count the total number of valence electrons
H has 1
Total of 10
N has 5
Step 2. Place one e pair between each BONDED atom
C has 4
H
C
N
We have 6 e left
All atoms must have an octet or duet
Step 3. Add electrons to terminal atoms first
to get an octet or duet.
Building Lewis (ELECTRON DOT)
Structures of Molecules.
Add remaining electrons to terminal
atoms first
Add 6 electrons in pairs to give the N an octet.
Step 3.
H
C
N
Step 4. Add any electrons left over to central atom
We have none left!
Step 5. Check for an acceptable Lewis Structure
Do all atoms have an octet?
IN THIS CASE
Building Lewis (ELECTRON DOT)
Structures of Molecules.
Step 5. Check for an acceptable Lewis Structure
bring electron pairs from outer N atom to form
shared pairs to give C its octet!!!
H
Still no octet on C
H
C
C
N
Do it again!!!!
N
H
C
N
three electron pairs between the C and N………...
Building Lewis (ELECTRON DOT)
Structures of Molecules.
H
C
N
H
C
N
Another possible structure is….
H
N
C
How can we choose?
FORMAL CHARGE
FORMAL CHARGE
gives an indication of the extent to which atoms
have gained or lost electrons in the process of covalent
bond formation.
Each atom is assigned all of its lone electrons and
half of the electrons bonded to it.
Formal
#valence _
charge = electrons
{
#unshared
electrons
_
} {
1/2#shared
electrons
Structures with the lowest formal charges are likely to
have the lowest energy.
}
All possible Lewis structures with stable
electronic configurations for HCN and HNC.
H
C
Formal
#valence _
charge = electrons
N
{
H
#unshared
electrons
N
_
} {
Calculate formal charge for this one H
FC on C
=4 -0
- 1/2 (8)
=0
FC on N
=5 -2
- 1/2 (6)
=0
Hydrogen is zero
C
1/2#shared
electrons
C
}
N
All possible Lewis structures with stable
electronic configurations for HCN and HNC.
H
0
C
0
N
0
H
0
N
+1
C
-1
WE CHOOSE THE STRUCTURE WITH THE
FORMAL CHARGES CLOSEST TO ZERO
AND ANY NEGATIVE FORMAL CHARGES ON THE
MOST ELECTRONEGATIVE ELEMENTS
Example: H2CO
Total number of valence electrons = 6+4+1+1=12
Put carbon in the middle…..
H
Place electrons between atoms
Now add remaining electrons to O
C
O
H
Now O has an octet but C does not….
Share a pair between C and O
Now we have a double bond between C and O
Example: H2CO
Total number of valence electrons = 6+4+1+1=12
H
C
H
O
C
Give C an octet
H
We can write this….
H
H
C
H
O
O
Let’s look at the nitrate anion NO3
-
Count up valence electrons
N has five
1s22s22p3
O has six
1s22s22p4
Plus one extra for negative charge
Valence electrons =
5 + 3 x 6 + 1 = 24
Nitrate anion NO3 Put a pair between each atom
O
O
N
O
nitrogen does not have noble gas structure!!!
form a double bond by sharing a pair from one
of the oxygen atoms……….
FORM A DOUBLE BOND BETWEEN O AND N
-
Here is one
O
O
N
O
Here is another!
Here is another!
-
-
O
O
N
O
O
O
N
O
Experiment shows all three bonds are the same.
All bond lengths 128 pm
O
N
All bond angles 120 0
O
O
Any one of the structures suggests one is different!
O
O
N
Double Bond
Single Bond
O
Should be different!
So…….
RESONANCE
We use a double headed arrow between the
structures..
O
O
O
O
N
N
N
O
O
O
O
The electrons involved are said to be
DELOCALIZED over the structure.
The blended structure is a
RESONANCE HYBRID
O
SO2
LEWIS STRUCTURE
S
O
S
O
O
O
Experiment shows that both S-O bonds are
equivalent.
We say that the real SO2 molecule is a hybrid of the
two resonance forms.
EXCEPTIONS TO THE OCTET RULE…….
Molecules with more than 8 electrons
around central atom.
Molecules with less than an octet around
central atom
Molecules with unpaired electrons.
Lets do SF6……..
Elements in rows 3 and following can exceed the
octet rule:
When it is necessary to exceed the octet rule the extra
electrons go on the central third row element.
F
F
F
S … 12
S
SF6
F
F
F
I3 -
I
I
I
Central I … 10
FREE RADICALS
Molecules which have unpaired electrons.
NO2
Is a free radical
Total number of valence electrons = 5+6+6 = 17
O
N
O
Form double bond to get N close to octet
O
N
O
O
RESONANCE
N
O