Chemical Bonding I:
Basic Concepts
Chapter 9
Valence electrons are the outer shell electrons of an
atom. The valence electrons are the electrons that
particpate in chemical bonding.
Group
e- configuration
# of valence e-
1A
____
1
2A
ns2
__
____
ns2np1
3
4A
ns2np2
4
5A
______
5
6A
ns2np4
6
___
ns2np5
7
9.1
9.1
The Ionic Bond
Li + F
1 22s22p5
1s22s1s
e- +
Li+ +
Li+ F [He]
1s
1s2[2Ne]
2s22p6
Li
Li+ + e-
F
F -
F -
Li+ F -
9.2
Electrostatic (Lattice) Energy
Lattice energy (E) is the energy required to completely _______
one mole of a solid ______ compound into ________ ions.
Q+Q E=k
r
Q+ is the charge on the _______
Q- is the charge on the ________
r is the distance between the ______
Lattice energy (E) _______
as Q ______ and/or
as r ______.
cmpd
MgF2
MgO
LiF
LiCl
lattice energy
2957
Q= ???
3938
1036
853
9.3
Born-Haber Cycle for Determining Lattice Energy
o
DHoverall
= DHo1 + DHo2 + DHo3 + DHo4 + DHo5
9.3
9.3
A ___________is a chemical bond in which two or more
electrons are ________ by two atoms.
Why should two atoms share electrons?
F
+
7e-
F
F F
7e-
8e- 8e-
Lewis structure of F2
_______covalent bond
____ pairs
F
F
_____ pairs
______covalent bond
______ pairs
F F
______ pairs
9.4
Lewis structure of water
H
+
O +
H
single covalent bonds
H O H
or
H
O
H
2e-8e-2e_______bond – _________ share two pairs of electrons
O C O
or
O
O
C
double bonds
- 8e8e- 8ebonds
double
_______ bond – two atoms share________ of electrons
N N
triple
bond
8e-8e
or
N
N
triple bond
9.4
Lengths of Covalent Bonds
Bond
Type
Bond
Length
(pm)
C-C
154
CC
133
CC
120
C-N
143
CN
138
CN
116
Bond Lengths
_____ bond < ________ Bond < _____ Bond
9.4
9.4
____________________ or _________ is a covalent
bond with _____________ electron ______ around
one of the two atoms
electron _____
region
H
electron _____
region
F
e- ????
H
d+
e- ???
F
d-
9.5
______________ is the ability of an atom to ______
toward itself the ______ in a chemical bond.
Electron Affinity _______, Cl is ______
X (g) + e-
X-(g)
Electronegativity - _______, F is ________
9.5
9.5
9.5
Classification of bonds by difference in electronegativity
Difference
Bond Type
0-.5
Covalent
2
0 < and <1.7
Ionic
Polar Covalent
Increasing difference in electronegativity
Covalent
_____
________
partial transfer of e-
_____
transfer e-
9.5
Classify the following bonds as ionic, polar covalent,
or covalent: The bond in CsCl; the bond in H2S; and
the NN bond in H2NNH2.
Cs – 0.7
Cl – 3.0
3.0 – 0.7 = 2.3
H – 2.1
S – 2.5
2.5 – 2.1 = 0.4
N – 3.0
N – 3.0
3.0 – 3.0 = 0
______
_________
________
9.5
Writing Lewis Structures
1. Draw __________ of compound showing what
atoms are bonded to each other. Put
_______electronegative element in the center.
2. Count _____________of valence e-. Add 1 for
each ________ charge. Subtract 1 for each
positive charge.
3. Complete an _____ for all atoms except
hydrogen
4. If structure contains too many electrons, form
____________bonds on ______ atom as
needed.
9.6
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – __ is less electronegative than___, put___ in center
Step 2 – Count _______electrons N - ______ and F ______
????
Step 3 – Draw _____bonds between N and F atoms and complete
_____ on N and F atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
___ single bonds +____ lone pairs _____= ?
F
N
F
F
9.6
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is ______ electronegative than O, put ____ in center
Step 2 – Count valence electrons -_____________________
????? = 24 valence electrons
Step 3 – Draw ______ bonds between C and O atoms and complete
______ on C and O atoms.
Step 4 - Check, are # of e- in structure ____ to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons
Step 5 - Too many electrons, form _____bond and re-check # of e-
O
C
O
2 ____ bonds (2x2) = 4
1 _____bond = 4
8 ______ pairs (8x2) = 16
Total = 24
O
9.6
Two possible skeletal structures of formaldehyde (______)
H
C
O
H
H
C
H
O
An atom’s __________ is the difference between the number
of _______electrons in an isolated atom and the number of
electrons assigned to that atom in a Lewis structure.
formal charge
on an atom in
a Lewis
structure
=
total number
total number
of valence
of nonbonding
electrons in electrons
the free atom
-
1
2
(
__________
)
The ____________charges of the atoms in a molecule or
ion must _______ the ______ on the molecule or ion.
9.7
H
-1
+1
C
O
formal charge
on an atom in
a Lewis
structure
H
=
C – 4 eO – 6 e2H – 2x1 e12 e-
2 single bonds (2x2) = 4
1 double bond = 4
2 lone pairs (2x2) = 4
Total = 12
total number
total number
of valence
of nonbonding
electrons in electrons
the free atom
-
1
2
(
total number
of bonding
electrons
)
formal charge
= 4 -2 -½ x 6 = -1
on C
formal charge
= 6 -2 -½ x 6 = +1
on O
9.7
H
H
0
C
formal charge
on an atom in
a Lewis
structure
0
O
=
C – 4 eO – 6 e2H – 2x1 e12 e-
2 single bonds (2x2) = 4
1 double bond = 4
2 lone pairs (2x2) = 4
Total = 12
total number
total number
of valence
of nonbonding
electrons in electrons
the free atom
-
1
2
(
total number
of bonding
electrons
)
______charge
= 4 - 0 -_______ = 0
on C
_____charge
=6on O
_______ = 0
9.7
Formal Charge and Lewis Structures
1. For ______ molecules, a Lewis structure in which there
are_______________ is _______to one in which formal
charges are present.
2. Lewis structures with ______ formal charges are
less_______ than those with _____ formal charges.
3. Among Lewis structures having similar distributions of
formal charges, the most plausible structure is the one in
which ________ formal charges are placed on the more
____________atoms.
Which is the most likely Lewis structure for CH2O?
H
-1
+1
C
O
H
H
H
0
C
0
O
9.7
A ___________________is one of two or more Lewis
structures for a single molecule that cannot be represented
accurately by only _______ Lewis structure (_____ formal
charge has been determined!).
More possible structures gives the overall structure more
validity.
The true structure is
an___________ of all the
possible structures.
9.8
Ozone
O
O
+
O
O
+
O
O
What are the resonance structures of the
carbonate (CO32-) ion?
9.8
Violations of the Octet Rule
Usually occurs with B and elements of higher periods and
most nonmetals. Common exceptions are: Be, B, P, S, Xe,
Cl, Br, and As.
How do you know if it’s an EXPANDED octet?
– More than ______ bonds
– ????????? doesn’t work out
with just 8
____: 4
____: 6
_____: 8 OR 10
BF3
____: 8, 10, OR 12
____: 8, 10, OR 12
SF4
Exceptions to the Octet Rule
The Incomplete Octet
Be – 2eBeH2
H
Be
H
4e-
B – 3eBF3
-
24e-
F
B
F
3 single bonds (3x2) = 6
Total = 24
F
9.9
Exceptions to the Octet Rule
_____________Molecules
NO
N – 5eO – 6e11e-
N
O
The __________ (central atom with principal quantum number n > 2)
SF6
S – 6e6F – 42e-
F
F
F
6 single bonds (6x2) = 12
S
F
F
F
Total = 48
9.9
The _______ change required to break a particular bond in
one mole of ________ molecules is the ___________
Bond Energy
DH0 = 436.4 kJ
H2 (g)
H (g) + H (g)
Cl2 (g)
Cl (g) + Cl (g) DH0 = 242.7 kJ
HCl (g)
H (g) + Cl (g) DH0 = 431.9 kJ
O2 (g)
O (g) + O (g) DH0 = 498.7 kJ
O
O
N2 (g)
N (g) + N (g) DH0 = 941.4 kJ
N
N
Bond Energies
_____ bond <______ bond < ______ bond
9.10
Bond Energies (BE) and Enthalpy changes in reactions
Imagine reaction proceeding by breaking all bonds in the
reactants and then using the gaseous atoms to form all the
bonds in the products.
DH0 = _______________________________
endothermic
exothermic
9.10
H2 (g) + Cl2 (g)
2HCl (g)
2H2 (g) + O2 (g)
2H2O (g)
9.10
Use bond energies to calculate the enthalpy change for:
H2 (g) + F2 (g)
2HF (g)
Type of
bonds broken
H
H
F
F
Type of
bonds formed
H
F
Number of
bonds broken
Bond energy
(kJ/mol)
Energy
change (kJ)
1
1
436.4
156.9
436.4
156.9
Number of
bonds formed
Bond energy
(kJ/mol)
Energy
change (kJ)
2
568.2
1136.4
9.10