What

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Whats coming up???

• Oct 25 The atmosphere, part 1 Ch. 8
• Oct 27 Midterm No lecture
• Oct 29 The atmosphere, part 2 Ch. 8
• Nov 1 Light, blackbodies, Bohr Ch. 9
• Nov 3,5 Postulates of QM, p-in-a-box Ch. 9
• Nov 8,10 Hydrogen and multi e atoms Ch. 9
• Nov 12 Multi-electron atoms Ch.9,10
• Nov 15 Periodic properties Ch. 10
• Nov 17 Periodic properties Ch. 10
• Nov 19 Valence-bond Lewis structures Ch. 11
• Nov 22 VSEPR Ch. 11
• Nov 24 Hybrid orbitals VSEPR Ch. 11, 12
• Nov 26 MO theory Ch. 12
• Nov 29 MO theory Ch. 12
• Dec 1 bonding wrapup Ch. 11,12
• Dec 2 Review for exam

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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
IONIC BONDING
COVALENT BONDING
ELECTRON SHARING
LEWIS MODEL OF BONDING
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IONIC BONDING
electron transfer
Cl
1s22s22p63s23p5
1s22s22p6 3s1
Lewis Symbol
symbol for element
and a dot for each
valence electron
?
Na
1s22s22p63s23p6
1s22s22p6
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COVALENT BONDING
electron sharing
Atoms go as far as possible toward
sharing electron pairs
completing their octets by
Consider F2
The electronic configuration of F is 1s22s22p5
Lewis Symbols..
These are combined to form F2
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COVALENT BONDING
electron sharing
Atoms go as far as possible toward
sharing electron pairs
completing their octets by
F 1s22s22p5
F 1s22s22p5


or
non-bonding, or lone pair of electrons
bonding pair of electrons
The simplest molecule
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IONIC OR COVALENT?
The type of bond that forms depends on the
electronegativity difference between the two
atoms involved in the bond
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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
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HYDROGEN FLUORIDE
Fluorine is more electronegative than hydrogen.


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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
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Drawing Lewis Structures
Lewis structures consider only valence electrons
H2O
Water
non-bonding, or lone pair of electrons
bonding pair of electrons
How do we draw these?????
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Building Lewis (ELECTRON DOT) Structures of
Molecules.
HCN as an example...
Count the total number of valence electrons
Step 1.
H has 1
Total of 10
C has 4
N has 5
Place one e- pair between each BONDED atom
Step 2.
H
C
N
We have 6 e- left
All atoms must have an octet or duet
Add electrons to terminal atoms first
Step 3.
to get an octet or duet.
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Building Lewis (ELECTRON DOT) Structures of
Molecules.
Add remaining electrons to terminal atoms first
Step 3.
Add 6 electrons in pairs to give the N an octet.
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
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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
C
N
Still no octet on C
Do it again!!!!
three electron pairs between the C and N...
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Building Lewis (ELECTRON DOT) Structures of
Molecules.
Another possible structure is.
How can we choose?
FORMAL CHARGE
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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 charge

valence electrons
_
unshared electrons
1/2shared electrons

Structures with the lowest formal charges are
likely to have the lowest energy.
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All possible Lewis structures with stable
electronic configurations for HCN and HNC.
Calculate formal charge for this one
- 1/2 (8)
FC on C
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- 0
0
5
- 2
- 1/2 (6)
0
FC on N
Hydrogen is zero
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All possible Lewis structures with stable
electronic configurations for HCN and HNC.
0 1 -1
0 0 0
WE CHOOSE THE STRUCTURE WITH THE
FORMAL CHARGES CLOSEST TO ZERO
AND ANY NEGATIVE FORMAL CHARGES ON THE MOST
ELECTRONEGATIVE ELEMENTS
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Example H2CO
Total number of valence electrons
641112
Put carbon in the middle..
Place electrons between atoms
Now add remaining electrons to O
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
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Example H2CO
Total number of valence electrons
641112
Give C an octet
We can write this.
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Lets 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
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Nitrate anion NO3-
Put a pair between each atom
nitrogen does not have noble gas structure!!!
form a double bond by sharing a pair from one of
the oxygen atoms.
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FORM A DOUBLE BOND BETWEEN O AND N
Here is one
Here is another!
Here is another!
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Experiment shows all three bonds are the same.
All bond lengths 128 pm
N
All bond angles 120 0
Any one of the structures suggests one is
different!
Double Bond
Single Bond
Should be different!
So.
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RESONANCE
We use a double headed arrow between the
structures..
The electrons involved are said to be DELOCALIZED
over the structure.
The blended structure is a RESONANCE HYBRID
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LEWIS STRUCTURE
SO2
Experiment shows that both S-O bonds are
equivalent.
We say that the real SO2 molecule is a hybrid of
the two resonance forms.
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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..
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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.
S 12
SF6
Central I 10
I3-
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FREE RADICALS
Molecules which have unpaired electrons.
NO2
Is a free radical
Total number of valence electrons 566 17
O
N
O
Form double bond to get N close to octet
RESONANCE