1. Covalent Bonding Chapter 8 Gilbert N. Lewis 1875-1946.
Originated idea of shared electrons in chemical bonds.
First produced heavy water. Coined the term “photon.”

2. Cl- Cl Na AND: NaCl is an ionic compound
Most bonding discussed so far has been ionic bonding, i.e.,
bonding where electrons are transferred from one element
to another forming ions. For example, in NaCl: Na
gives electron to Cl
Which then become:
Cl-
Na+
AND:
NaCl is an ionic compound

3. Na Na Cl Cl + ion atom ion atom
Transfer is of valence or outer electron or electrons, so that an octet of electrons is achieved.
Na is: 1s22s22p63s1 (note: 3s1 is a valence e-)
1s22s22p6
Note octet.
The Na+ ion is:
Cl is:
1s22s22p63s23p5 (the 3s23p5 are valence e’s)
Note octet
The Cl- ion is:
1s22s22p63s23p6 + Na
ion Na
atom Cl
ion Cl
atom
(We normally “hide” the octet surrounding cations)

4. O Ne Mg Each Lewis dot represents a valence electron as Group number.
The number of valence electrons is the same
as Group number. Na
Na - Group 1- one valence electron
Mg - Group 2 - two valence electrons Mg
O - Group 6 - six valence electrons O
Ne - Group 8 - eight valence electrons Ne

5. Octet Rule This is an octet, or a configuration of s2p6 Mg+ Mg2+ O
Atoms tend to gain, lose or share electrons until
each is surrounded by 8 valence electrons.
This is an octet, or a configuration of s2p6
All inert gases (Group VIII) have s2p6 configuration.
Which of following have inert gas configuration?:
K K Mg
Mg Mg O
O N Cl-

6. Octet Rule Note that the most stable form of atoms or ions
from previous slide have an inert gas configuration K+
1s22s22p63s23p6
= [Ar]
Mg2+
1s22s22p6
= [Ne]
1s22s22p6
=[Ne]
O2-
N3-
1s22s22p6
=[Ne]
Mg2+ and O2- and N3- are isoelectronic

7. Octet Rule Na = [Ne]3s1 Al = [Ne]3s23p1 Cl = [Ne]3s23p5
Element electron configurations can be
represented as inert gas core plus valence e’s
Na = [Ne]3s1
Al = [Ne]3s23p1
Cl = [Ne]3s23p5
Ar = [Ne]3s23p6 (= [Ar])
K = [Ar]4s1

8. . . . . . . + . Cl . . Covalent Bonding Cl Cl Cl Cl
is the sharing of electrons to achieve octet
It usually involves non-metallic elements and hydrogen
For example, Cl2 is: Cl
. . .
Cl Cl
x x x
. . . Cl
x x x +
Can also use a line to represent a pair of electrons in a bond Cl . x
x x
. .

9. . . x H C H, Li, Be, B H H C H Exceptions to octet rule:
H can bond with only one other atom.
It can never be “sandwiched”
X-H
is o.k.
is not
X-H-X
. x x C . H
Methane, CH4 is:
H C H H
Or:

10. The Rules OH- SO42- CH3OH Examples 1. Add up all the valence electrons
2. With charged species (ions):
a) if negative, add electrons equal to ion charge
b) if positive, remove electrons equal to ion charge
3. Distribute around atoms so that octet rule is obeyed
(note exceptions for H, Li, etc..)
Examples
OH-
SO42-
CH3OH

11. . . . H O H Bonding and Nonbonding electron pairs Consider H2O
non bonding pair
H O H
. . .
bonding pair
bonding pair
non bonding pair
In water:
there are two bonding pairs and....
two non-bonding pairs of electrons

12. Multiple Bonds O O O O O O O O N N N N N N or or + +
Each oxygen has only
7 (not 8) electrons O + O O O O O or O O
A double bond (two pairs of electrons) is formed N N N N or N N +
A triple bond (three pairs of electrons) is formed

13. Polar Bonds H H Cl Cl Na Cl H Cl Covalent Bonds Ionic Bond
0 0 H H Cl Cl Na Cl
Covalent Bonds
Ionic Bond
(transfer of e’s)
(sharing of e’s)
+ -
+ < +1 = partial positive charge
- > -1 = partial negative charge H Cl
How do we know which atom
is + and which atom is - ?
Polar Covalent Bond
Electrons are shared, but not equally

14. Electronegativity F: =4.0 (most electronegative) to
Electronegativity is a measure of the tendency of an
atom in a molecule to attract bonding electrons to itself.
Atoms with large (positive) Ionization Energies and very
negative Electron Affinities are more electronegative.
Linus Pauling developed an arbitrary scale of
electronegativities () with values ranging from:
F: =4.0 (most electronegative) to
Cs: =0.7 (least electronegative)

15. Electronegativity

16. Electronegativity and Bond Polarity
(1) In a bond between two atoms, the atom with the higher
electronegativity () is partially negative (-).
(2) The larger the difference in electronegativities (), the
more polar the bond.
Which of the following bonds are the:
(a) most polar, and (b) least polar.
In each case, indicate the positive and negative ends of the bond.
Least Polar
Most Polar
(Ionic)
Atom  F O N C H Li
+ -
- +
- +
+ -
C-O
N-C
C-H
Li-F
=
=0.4
=
=1.0
=
=0.5
=
=3.0

17. Polar Bonds and Dipole Moments
In physics, a positive and negative charge separated
by a distance has a “dipole moment” ()
Since a polar diatomic molecule has a separated positive
and negative charge, it has a dipole moment.
+ -
H-Cl
=
=0.9
+ -
H-F
=
=1.9
Because HF has a more polar bond than HCl,
it has a larger dipole moment.
Alternative
Notation
H-Cl
H-F

18. Lewis Structures of Polyatomic Species
(Dot structures, once again)
1. Count number of valence electrons.
Remember to consider charge of polyatomic ions.
2. Arrange atoms and connect with single bonds.
For species of the form XYn, the X atom is central.
3. Complete octets on outer atoms (other than H).
4. Put remaining electrons on central atom.
Even if this creates greater than an octet.
5. If central atom has less than octet,
create multiple bonds.

19. Lewis Structures of Polyatomic SpeciesH Cl
CH2Cl2
1. # val e- = 1x4 + 2x1 + 2x7 = 20 e-
2. Arrange atoms and put in single bonds.
Single bonds have used up 8 e-, leaving 20-8 = 12 e-.
3. Complete octets on Cl atoms.
This uses remaining 12 e-.
4. No e- remaining to put on central atom.
5. Carbon has an octet of 8 e-. No need for multiple bonds.

20. Lewis Structures of Polyatomic Species
PF3 P F
1. # val e- = 1x5 + 3x7 = 26 e-
2. Arrange atoms and put in single bonds.
Single bonds have used up 6 e-, leaving 26-6 = 20 e- remaining.
3. Complete octets on F atoms.
This uses 18 e-, leaving 2 e- remaining .
4. Put remaining 2 e- on central atom.
5. Phosphorus has an octet of 8 e-. No need for multiple bonds.

21. FC = # valence e’s - # assigned e’s
Which of the following is correct structure for NCS- ? C S N C S N C S N OR OR
Don’t they all have correct number of valence e’s?
Don’t they all obey the octet rule?
Which is the preferred structure?
Use the concept of FORMAL CHARGE.
Formal Charge (FC) is charge atom would have if all atoms had the same electronegativity, i.e., if all bonding e’s were shared equally by all atoms.
FC = # valence e’s - # assigned e’s O O N C O O
4-5= -1
6-6= 0
6-5= +1
6-6= 0
5-5= 0

22. Formal Charge = # val e- - # assigned e-
Which of the following is correct structure for NCS- ? C S N C S N C S N
# val e
# assigned e FC
Formal Charge = # val e- - # assigned e-
# val e- = # valence e- in isolated atom
# assigned e- = # owned e- + (1/2) # shared e-
Rules:
(1) Lewis structure with smaller (closer to zero) formal charges is more realistic structure.
2) If any atoms must have negative formal charges,
they should be the more electronegative atoms.

23. Formal Charge C O C O Rules: Formal Charge = # val e- - # assigned e-
# val e- = # valence e- in isolated atom
# assigned e- = # owned e- + (1/2) # shared e-
Rules:
(1) Lewis structure with smaller formal charges is
more realistic structure.
(2) If any atoms must have negative formal charges,
they should be the more electronegative atoms.

24. Resonance Structures O O O O Ozone (O3) 1. 18 val e- 2. 14 e- left
4. No e- left OR
5. Make double bond
Resonance Structures O
“Average” Structure

25. The Structure of Ozone O O O Resonance Structure “Average” Structure
BO:
BO: O O
BL:  
BL:  
This is the actual measured
ozone Bond Length
Actual Structure O
117o
BO = Bond Order
BL = Bond Length

26. Resonance Structures
Examples
NO SO3
NO3- H C
C C H H H H H

27. Exceptions to the Octet Rule
Odd Number of Electrons
Examples:
NO: # val e- = = 11 e-
NO2: # val e- = 5 + 2x6 = 17 e-
ClO2: # val e- = 7 + 2x6 = 19 e-
Molecules with odd numbers of electrons are comparatively
rare, and are often unstable

28. Exceptions to the Octet Rule
Some Boron and Beryllium Compounds
BeF2: # val e- = 2 + 2x7 = 16 e- Be F X Be F
FC:
FC:
BF3: # val e- = 3 + 3x7 = 24 e-
FC:
FC: B F X B F
One of 3 resonance
structures

29. Exceptions to the Octet Rule
Expanded Valence
PCl5 P Cl 2
val e- 4
e- left 10 6
3. All gone 8
10 e- around P
More than an octet
Where do the extra electrons go??

30. Expanded Valence PCl5 3d 3p 3s x o x x x 5 e- from P: o
Phosphorus
Z=15
1s22s22p63s23p3 x
Where can
we sit? 3d o x x x 3s 3p
5 e- from P: o
5 e- from Cl’s: x

31. Expanded Valence NCl5 ?? 1s22s22p3
Nitrogen
Z=7
1s22s22p3
NCl5 ??
There are no empty 2d orbitals to accept
the extra electrons.
Expanded Valence Compounds
(1) Central atom is 3rd. period or below
(2) Outer atoms are usually halogens or oxygen

32. Expanded Valence Additional Examples
I3- AsF6-
SF XeF4

33. Lewis Structures of Organic Molecules
Guidelines:
X-H
(1) Hydrogen atoms have 1 bond
-C- C= C
(2) Carbon atoms have 4 bonds N ..
Nitrogen atoms have 3 bonds and 1 e- lone pair
(4) Oxygen atoms have 2 bonds and 2 e- lone pairs O ..
These guidelines result from the preference of the
above atoms to have a formal charge of zero in
organic molecules

34. Bond Enthalpy H-H(g) --> H(g) + H(g) Cl-Cl(g) --> Cl(g) + Cl(g)
The Bond Enthalpy (aka Bond Dissociation Enthalpy) is
H to break a bond in a gas phase molecule.
H-H(g) --> H(g) + H(g)
H = D(H-H) = 436 kJ/mol
Cl-Cl(g) --> Cl(g) + Cl(g)
H = D(Cl-Cl) = 242 kJ/mol
H-F(g) --> H(g) + F(g)
H = D(H-F) = 567 kJ/mol
H(g) + F(g) --> H-F(g)
H =
???
= - D(H-F) = -567 kJ/mol
When a bond is formed, H is the negative of
the Bond Enthalpy.

35. Bond Enthalpies and Bond Lengths – criteria
The greater the bond order (triple vs. double vs. single bonds), the stronger and shorter the bonds.
The closer the two bonding atoms, the stronger the bond (compare H-H vs. C-C).
The greater the difference in electronegativity, the shorter the bond (compare O-H vs. C-H).

36. Bond Order (BO), Bond Enthalpy (BE), Bond Length (BL)
Bond BO BE BL
C-C
C=C
CC
H-H
H-C
H-O 1 2 3
348 kJ
614
839
436
413
460
1.54 
1.34
1.20
0.74
1.09
0.96
N-N
N=N
NN
C-O
C=O
CO 1 2 3
163
418
941
359
806
1083
1.47
1.24
1.10
1.43
1.20
1.13