November 11, 2009
•E X A M # 3 C o r r e c t i o n - C h e c k O W L
•C h a p t e r 8
•Homework
posted
•Today’s Topic: Lewis Structures (Sec. 8.2)
Bond Properties (Sec. 8.3)
Why does a covalent bond form?
1.
2.
3.
4.
The valence electrons on two atoms are
attracted to one another
The nuclei of two atoms are attracted to
one another
The valence electrons on one atom are
attracted to the nucleus of another atom
The repulsive forces between the
protons in two nuclei and between the
electrons in two atoms are strong
48%
35%
12%
5%
1
2
3
4
Last time…
 Bonds form when the balance of forces favors bond
formation
 Valence electrons are those in the outermost shell
(highest n-level)
 Lewis structures are used to show the valence
electrons



Elemental symbol = core (nucleus + inner electrons)
Lines = bonding electrons
Dots = electrons not involved in a bond
Rules for Drawing Lewis Structures
1.
2.
3.
4.
5.
6.
7.
Write the skeletal structure
Add up the total # of valence electrons
Draw a bond between the central atom and each
surrounding atom
Add lone pairs to the outer atoms to complete their octets
Add remaining electrons to central atom
If central atom does not have an octet, “borrow” electrons
from other atoms (make double/triple bonds)- DO NOT
ADD ELECTRONS (F and Cl do not form multiple bonds;
C, N, O, P, and S do)
Sometimes you can’t complete an octet (B and Be)
How many total bonding electrons
does an acetate ion have?
1.
2.
3.
4.
5.
6.
7.
7
8
10
12
13
14
15
55%
26%
8%
8%
3%
1
2
1%
3
4
5
0%
6
7
Examples
Try this
one at
home!
Draw the Lewis structure for SO3. How many lone pairs does it have?
1.
2.
3.
4.
5.
2
4
6
8
10
64%
14%
3%
1
14%
4%
2
3
4
5
What does the Lewis Structure for N2 look like?
1.
:N

N
:
79%

2.
:NN:

3.
4.
: NN:



:NN:
15%
1%
1
2
4%
3
4
Exceptions to the Octet Rule
 H (2)
 Be (4)
 B (6)
 Large elements with available d orbital
Organic Structures
 CH3CH3
 CH3CN
 CHCCHCH2